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Maternal and perinatal outcomes in programmed versus natural vitrified–warmed blastocyst transfer cycles

  • Reeva Makhijani
    Affiliations
    Center for Advanced Reproductive Services, Division of Reproductive Endocrinology and Infertility, University of Connecticut School of Medicine, 2 Batterson Park Road, Farmington CT, USA
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  • Chantal Bartels
    Affiliations
    Center for Advanced Reproductive Services, Division of Reproductive Endocrinology and Infertility, University of Connecticut School of Medicine, 2 Batterson Park Road, Farmington CT, USA
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  • Prachi Godiwala
    Affiliations
    Center for Advanced Reproductive Services, Division of Reproductive Endocrinology and Infertility, University of Connecticut School of Medicine, 2 Batterson Park Road, Farmington CT, USA
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  • Alison Bartolucci
    Affiliations
    Center for Advanced Reproductive Services, Division of Reproductive Endocrinology and Infertility, University of Connecticut School of Medicine, 2 Batterson Park Road, Farmington CT, USA
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  • John Nulsen
    Affiliations
    Center for Advanced Reproductive Services, Division of Reproductive Endocrinology and Infertility, University of Connecticut School of Medicine, 2 Batterson Park Road, Farmington CT, USA
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  • Daniel Grow
    Affiliations
    Center for Advanced Reproductive Services, Division of Reproductive Endocrinology and Infertility, University of Connecticut School of Medicine, 2 Batterson Park Road, Farmington CT, USA
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  • Claudio Benadiva
    Affiliations
    Center for Advanced Reproductive Services, Division of Reproductive Endocrinology and Infertility, University of Connecticut School of Medicine, 2 Batterson Park Road, Farmington CT, USA
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  • Lawrence Engmann
    Correspondence
    Corresponding author.
    Affiliations
    Center for Advanced Reproductive Services, Division of Reproductive Endocrinology and Infertility, University of Connecticut School of Medicine, 2 Batterson Park Road, Farmington CT, USA
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Published:March 21, 2020DOI:https://doi.org/10.1016/j.rbmo.2020.03.009

      Abstract

      Research question

      Do maternal and perinatal outcomes differ between natural and programmed frozen embryo transfer (FET) cycles?

      Design

      Retrospective cohort study at a university-affiliated fertility centre including 775 patients who underwent programmed or natural FET cycles resulting in a singleton live birth using blastocysts vitrified between 2013 and 2018.

      Results

      A total of 384 natural and 391 programmed FET singleton pregnancies were analysed. Programmed FET resulted in higher overall maternal complications (32.2% [126/391] versus 18.8% [72/384]; P < 0.01), including higher probability of hypertensive disorders of pregnancy (HDP) (15.3% [60/391] versus 6.3% [24/384]; P < 0.01), preterm premature rupture of membranes (2.6% [10/391] versus 0.3% [1/384]; P = 0.02) and caesarean delivery (53.2% [206/387] versus 42.8% [163/381]; P = 0.03) compared with natural FET. After controlling for potential confounders, including age, body mass index, parity, smoking status, history of diabetes or chronic hypertension, infertility diagnosis, number of embryos transferred and use of preimplantation genetic testing, the adjusted odds ratio for HDP was 2.39 (95% CI 1.37 to 4.17) and for overall maternal complications was 2.21 (95% CI 1.51 to 3.22) comparing programmed with natural FET groups. The groups did not significantly differ for any perinatal outcomes analysed, including birth weight (3357.9 ± 671.6 g versus 3318.4 ± 616.2 g; P = 0.40) or rate of birth defects (1.5% [6/391] versus 2.1% [8/384]; P = 0.57), respectively.

      Conclusion

      Vitrified–warmed blastocyst transfer in a programmed cycle resulted in a twofold higher probability of HDP compared with transfer in a natural cycle. Natural FET cycle should, therefore, be recommended as first line for all eligible patients undergoing FET to reduce the risk of HDP.

      Keywords

      Introduction

      Assisted reproductive technologies (ART) have been associated with an increased risk of obstetric and perinatal complications, including hypertensive disorders of pregnancy (HDP), antepartum haemorrhage, preterm delivery, low birthweight and congenital anomalies compared with spontaneous conception (
      • Helmerhorst F.M.
      • Perquin D.A.M.
      • Donker D.
      • Keirse M.J.N.C.
      Perinatal outcome of singletons and twins after assisted conception: A systematic review of controlled studies.
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      • Jackson R.A.
      • Gibson K.A.
      • Wu Y.W.
      • Croughan M.S.
      Perinatal outcomes in singletons following in vitro fertilization: A meta-analysis.
      ;
      • Pandey S.
      • Shetty A.
      • Hamilton M.
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      Obstetric and perinatal outcomes in singleton pregnancies resulting from ivf/icsi: A systematic review and meta-analysis.
      ;
      • Hansen M.
      • Kurinczuk J.J.
      • Milne E.
      • de Klerk N.
      • Bower C.
      Assisted reproductive technology and birth defects: A systematic review and meta-analysis.
      ;
      • Tandberg A.
      • Klungsøyr K.
      • Romundstad L.B.
      • Skjærven R.
      Pre-eclampsia and assisted reproductive technologies: Consequences of advanced maternal age, interbirth intervals, new partner and smoking habits.
      ;
      • Sunderam S.
      • Kissin D.M.
      • Zhang Y.
      • Folger S.G.
      • Boulet S.L.
      • Warner L.
      • Callaghan W.M.
      • Barfield W.D.
      Assisted Reproductive Technology Surveillance - United States, 2016.
      ). Hypertensive disorders of pregnancy refer to a spectrum of hypertensive disorders in pregnancy ranging from gestational hypertension to eclampsia. The higher incidence of multiple pregnancy after ART and proportion of women of advanced maternal age undergoing IVF may partly explain the increased occurrence of these complications (
      • Gardner M.O.
      • Goldenberg R.L.
      • Cliver S.P.
      • Tucker J.M.
      • Nelson K.G.
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      The origin and outcome of preterm twin pregnancies.
      ;
      • Tandberg A.
      • Klungsøyr K.
      • Romundstad L.B.
      • Skjærven R.
      Pre-eclampsia and assisted reproductive technologies: Consequences of advanced maternal age, interbirth intervals, new partner and smoking habits.
      ;
      • Qin J.-B.
      • Sheng X.-Q.
      • Wu D.
      • Gao S.-Y.
      • You Y.-P.
      • Yang T.-B.
      • Wang H.
      Worldwide prevalence of adverse pregnancy outcomes among singleton pregnancies after in vitro fertilization/intracytoplasmic sperm injection: a systematic review and meta-analysis.
      ;
      • Sunderam S.
      • Kissin D.M.
      • Zhang Y.
      • Folger S.G.
      • Boulet S.L.
      • Warner L.
      • Callaghan W.M.
      • Barfield W.D.
      Assisted Reproductive Technology Surveillance - United States, 2016.
      ). Laboratory procedures, such as cryopreservation methods, extended blastocyst culture and use of preimplantation genetic testing (PGT), however, may also contribute to such adverse obstetric and perinatal outcomes (
      • Palomba S.
      • Homburg R.
      • Santagni S.
      • La Sala G.B.
      • Orvieto R.
      Risk of adverse pregnancy and perinatal outcomes after high technology infertility treatment: A comprehensive systematic review.
      ). In fact, compared with fresh embryo transfers, pregnancies after frozen embryo transfer (FET) cycles are at higher risk of HDP, placenta accreta, infants who are large for gestational age or have birth defects (
      • Roque M.
      • Lattes K.
      • Serra S.
      • Solà I.
      • Geber S.
      • Carreras R.
      • Checa M.A.
      Fresh embryo transfer versus frozen embryo transfer in in vitro fertilization cycles: A systematic review and meta-analysis.
      ;
      • Wennerholm U.
      • Henningsen A.A.
      • Romundstad L.B.
      • Bergh C.
      • Pinborg A.
      • Skjaerven R.
      • Forman J.
      • Gissler M.
      • Go K.
      • Tiitinen A.
      Perinatal outcomes of children born after frozen-thawed embryo transfer : a Nordic cohort study from the CoNARTaS group.
      ;
      • Ishihara O.
      • Ph D.
      • Araki R.
      • Ph D.
      • Kuwahara A.
      • Ph D.
      Impact of frozen-thawed single-blastocyst transfer on maternal and neonatal outcome : an transfer cycles from 2008 to 2010 in Japan.
      ;

      Centers for Disease Control and Prevention, 2016. 2016 Assisted Reproductive Technology Fertility Clinic Success Rates Report.

      ;
      • Maheshwari A.
      • Raja A.
      • Ph D.
      • Bhattacharya S.
      • D M.
      Obstetric and perinatal outcomes after either fresh or thawed frozen embryo transfer : an analysis of recorded in the Human Fertilisation and Embryology Authority anonymized dataset.
      ;
      • Luke B.
      Pregnancy and birth outcomes in couples with infertility with and without assisted reproductive technology: with an emphasis on US population-based studies.
      ;
      • Robles A.
      • Vidal M.
      • Checa M.A.
      • Prat M.
      Perinatal outcomes in children born after fresh or frozen embryo transfer : a Catalan cohort study based on 107.
      ). Given the growing utilization of FET cycles, which account for more than one-half of all the IVF cycles carried out in the USA, it is vital to identify the cause underlying the association between FET cycles and higher rates of such complications to improve patient safety and implement preventative measures.
      Previous studies have suggested that particular aspects of FET cycle protocols may affect obstetric and perinatal outcomes. For example, a recent study demonstrated no difference in HDP between fresh transfer and natural FET cycles in ovulatory women (
      • Shi Y.
      • Sun Y.
      • Hao C.
      • Zhang H.
      • Wei D.
      • Zhang Y.
      • Zhu Y.
      • Deng X.
      • Qi X.
      • Li H.
      • Ma X.
      • Ren H.
      • Wang Y.
      • Zhang D.
      • Wang B.
      • Liu F.
      • Wu Q.
      • Wang Z.
      • Bai H.
      • Li Y.
      • Zhou Y.
      • Sun M.
      • Liu H.
      • Li J.
      • Zhang L.
      • Chen X.
      • Zhang S.
      • Sun X.
      • Legro R.S.
      • Chen Z.J.
      Transfer of fresh versus frozen embryos in ovulatory women.
      ) whereas the same research group previously showed a higher prevalence of HDP after programmed FET cycles compared with fresh IVF cycles in women with PCOS (
      • Chen Z.
      • Shi Y.
      • Sun Y.
      • Zhang B.
      • Liang X.
      • Cao Y.
      • Yang J.
      • Liu J.
      • Wei D.
      • Weng N.
      • Tian L.
      • Hao C.
      • Yang D.
      • Zhou F.
      • Shi J.
      • Xu Y.
      • Li J.
      • Yan J.
      • Qin Y.
      • Zhao H.
      • Zhang H.
      • Legro R.S.
      Fresh versus Frozen Embryos for Infertility in the Polycystic Ovary Syndrome.
      ). This difference in results may be attributed to the method of endometrial preparation used by the disparate patient cohorts in the two studies. This is further supported by other studies that have shown that the risk of HDP does seem to be increased in programmed FET cycles compared with natural or modified natural cycles (
      • Ernstad E.G.
      • Wennerholm U.
      • Khatibi A.
      • Petzold M.
      • Bergh C.
      Neonatal and maternal outcome after frozen embryo transfer: Increased risks in programmed cycles.
      ;
      • Saito K.
      • Kuwahara A.
      • Ishikawa T.
      • Morisaki N.
      • Miyado M.
      • Miyado K.
      • Fukami M.
      • Miyasaka N.
      • Ishihara O.
      • Irahara M.
      • Saito H.
      Endometrial preparation methods for frozen-thawed embryo transfer are associated with altered risks of hypertensive disorders of pregnancy, placenta accreta, and gestational diabetes mellitus.
      ;
      • Von Versen-höynck F.
      • Narasimhan P.
      • Seda E.
      • Tierney S.
      • Martinez N.
      • Conrad K.P.
      • Baker V.L.
      • Winn V.D.
      Absent or Excessive Corpus Luteum Number Is Associated With Altered Maternal Vascular Health in Early Pregnancy.
      ). Additionally, the corpus luteum, which is present in natural and absent in programmed cycles, has been identified as possibly protective against the development of HDP (Arthur et al., 1996; Strauch et al., 2018;
      • Von Versen-höynck F.
      • Schaub A.M.
      • Chi Y.
      • Chiu K.
      • Liu J.
      • Lingis M.
      • Williams R.S.
      • Rhoton-vlasak A.
      • Nichols W.W.
      • Fleischmann R.R.
      • Zhang W.
      • Winn V.D.
      • Segal M.S.
      • Conrad K.P.
      • Baker V.L.
      Preeclampsia Increased Preeclampsia Risk and Reduced Aortic Compliance With In Vitro Fertilization Cycles in the Absence of a Corpus Luteum.
      ).
      The nature of IVF treatment is rapidly changing, and it is challenging to obtain data that reflect new techniques and are generalizable to one’s patient population. The two largest studies that have compared the difference in obstetric and perinatal outcomes between programmed and natural FET cycles were conducted internationally, limiting their generalizability to the US population (
      • Ernstad E.G.
      • Wennerholm U.
      • Khatibi A.
      • Petzold M.
      • Bergh C.
      Neonatal and maternal outcome after frozen embryo transfer: Increased risks in programmed cycles.
      ;
      • Saito K.
      • Kuwahara A.
      • Ishikawa T.
      • Morisaki N.
      • Miyado M.
      • Miyado K.
      • Fukami M.
      • Miyasaka N.
      • Ishihara O.
      • Irahara M.
      • Saito H.
      Endometrial preparation methods for frozen-thawed embryo transfer are associated with altered risks of hypertensive disorders of pregnancy, placenta accreta, and gestational diabetes mellitus.
      ). Furthermore, these studies included both cleavage- and blastocyst-stage embryo transfer cycles and one included both vitrification and slow freezing protocols for cryopreservation, although multicellular embryo transfer and slow freezing are no longer widely used (
      • Ernstad E.G.
      • Wennerholm U.
      • Khatibi A.
      • Petzold M.
      • Bergh C.
      Neonatal and maternal outcome after frozen embryo transfer: Increased risks in programmed cycles.
      ;
      • Saito K.
      • Kuwahara A.
      • Ishikawa T.
      • Morisaki N.
      • Miyado M.
      • Miyado K.
      • Fukami M.
      • Miyasaka N.
      • Ishihara O.
      • Irahara M.
      • Saito H.
      Endometrial preparation methods for frozen-thawed embryo transfer are associated with altered risks of hypertensive disorders of pregnancy, placenta accreta, and gestational diabetes mellitus.
      ). The only study using a US cohort included mainly blastocyst transfer and a few cleavage stage embryo transfer; however, it was limited by sample size and the cryopreservation technique used was unclear (
      • Von Versen-höynck F.
      • Narasimhan P.
      • Seda E.
      • Tierney S.
      • Martinez N.
      • Conrad K.P.
      • Baker V.L.
      • Winn V.D.
      Absent or Excessive Corpus Luteum Number Is Associated With Altered Maternal Vascular Health in Early Pregnancy.
      ;
      • Von Versen-höynck F.
      • Schaub A.M.
      • Chi Y.
      • Chiu K.
      • Liu J.
      • Lingis M.
      • Williams R.S.
      • Rhoton-vlasak A.
      • Nichols W.W.
      • Fleischmann R.R.
      • Zhang W.
      • Winn V.D.
      • Segal M.S.
      • Conrad K.P.
      • Baker V.L.
      Preeclampsia Increased Preeclampsia Risk and Reduced Aortic Compliance With In Vitro Fertilization Cycles in the Absence of a Corpus Luteum.
      ).
      Therefore, it is important to examine the effect that method of endometrial preparation has on obstetric and perinatal outcomes in a cohort of patients who underwent FET cycles using only previously vitrified blastocysts resulting in singleton birth to both reduce the effect of other cofounding variables and reflect current practice in US fertility centres. Therefore, we sought to determine whether maternal and perinatal outcomes differed between programmed and natural FET cycles involving transfer of previously vitrified blastocysts that resulted in a singleton live birth.

      Materials and methods

       Study design

      This was a retrospective cohort study conducted at a single university-affiliated fertility centre comparing the difference between maternal and perinatal outcomes after programmed versus natural FET carried out between March 2013 and October 2018. Only cycles in which previously vitrified blastocyst(s) derived from autologous oocytes were transferred and resulted in a singleton live birth were included. Only one cycle per patient was included in the analyses. Exclusion criteria included donor oocyte cycles, FET cycles resulting in multiple births, cycles in which cleavage stage embryos were transferred or cycles in which the embryos that were transferred had been cryopreserved using slow freezing. In addition, modified natural FET cycles involving ovulation induction with letrozole, clomiphene citrate or gonadotrophins, as well as those requiring HCG triggering to induce ovulation, were excluded. This study was approved by our university’s institutional review board on 15 February 2019.

       Treatment protocol

      IVF stimulation protocols have been previously described and were decided based on patient factors and physician preference (
      • Diluigi A.J.
      • Maier D.
      • Engmann L.
      • Schmidt D.W.
      • Benadiva C.
      • Nulsen J.N.
      A randomized trial of microdose leuprolide acetate protocol versus luteal phase ganirelix protocol in predicted poor responders.
      ;
      • Johnston-MacAnanny E.B.
      • DiLuigi. A.J.
      • Engmann L.
      • Maier D.
      • Benadiva C.
      • Nulsen J.
      Selection of first in vitro fertilizztion (IVF) cycle stimulation protocol for good prognosis patients: GnRH antagonist vs. agonist protocols.
      ). Gonadotrophin dosing was adjusted according to patient response. Trigger for final oocyte maturation was administered when three or more follicles reached 18 mm in diameter or above. Either HCG 3300–10000 IU SC (Pregynl, Merck, Kenilworth, NJ, USA; Novarel, Ferring Pharmaceuticals, Parsippany, NJ, USA), gonadotrophin releasing hormone (GnRH) agonist 1 mg (leuprolide acetate, Abbott Laboratories, Abbott Park, IL, USA) or a combination was used for trigger based on physician preference, protocol type and risk of ovarian hyperstimulation syndrome. Transvaginal ultrasound-guided oocyte retrieval was carried out 35 h after trigger injection. Oocytes were fertilized with either intracytoplasmic sperm injection or conventional insemination.
      Only good-quality blastocysts according to Gardner criteria (3BB or higher) (
      • Gardner D.K.
      • Schoolcraft W.B.
      Culture and transfer of human blastocysts.
      ) were vitrified. If PGT was being used, trophectoderm biopsy was carried out, embryos vitrified and either array comparative genomic hybridization or next-generation sequencing was used for aneuploidy testing. Vitrification was by rapid exposure to a cryoprotectant solution consisting of 15% ethylene glycol, 15% dimethylsulphoxide, 20% dextran and 0.5 mmol/l sucrose in a Cryolock® device (Irvine Scientific, Santa Ana, CA, USA). Before vitrification, a laser pulse of 300 µs (constant 0.9 J) was applied to collapse the blastocoel. Before embryo transfer, all blastocysts were rapidly warmed in solutions of 0.5 and 0.2 mmol/l sucrose and rinsed through HEPES-buffered human tubal fluid with 12 mg/ml human serum albumin. To allow time for re-expansion, embryos were warmed and equilibrated in culture 1–2 h before transfer.
      As previously described (
      • Kaye L.
      • Marsidi A.
      • Rai P.
      • Thorne J.
      • Nulsen J.
      • Engmann L.
      • Benadiva C.
      Frozen blastocyst transfer outcomes in immediate versus delayed subsequent cycles following GnRH agonist or hCG triggers.
      ), FET was carried out in either a natural or programmed cycle. The decision to use a programmed or natural FET cycle was based on the patient’s ovulatory status and physician preference. Programmed cycles consisted of downregulation with a GnRH agonist in the luteal phase of the preceding cycle followed by increasing doses of oral or transdermal oestradiol after menses. Intramuscular progesterone was started when endometrial thickness measured about 8 mm. On the sixth day of intramuscular progesterone, FET was carried out. In a natural FET cycle, patients were monitored with daily bloodwork beginning on cycle day 10. A transvaginal ultrasound was also carried out at this time to ensure a follicle had been recruited and that the endometrial stripe measured about 8 mm. Six days after detecting the LH surge (LH ≥20 IU/l), FET was carried out and the luteal phase supplemented with vaginal progesterone (Crinone, Merck, Kenilworth, NJ, USA; and Endometrin, Ferring Pharmaceuticals, Parsippany, NJ, USA), commencing 2 days after the LH surge (
      • Bartels C.B.
      • Ditrio L.
      • Grow D.R.
      • O’Sullivan D.M.
      • Benadiva C.A.
      • Engmann L.
      • Nulsen J.C.
      The window is wide: flexible timing for vitrified-warmed embryo transfer in natural cycles.
      ).

       Outcome measures

      Demographic and baseline cycle information were collected from the electronic medical records at our fertility centre, including age, body mass index (BMI), smoking status, parity, history of pre-existing diabetes or chronic hypertension (HTN), primary infertility diagnosis, mean number of embryos transferred and use of PGT. Outcome data, including mode of delivery and occurrence of maternal or perinatal complications, was self-reported and sent to the Society of Assisted Reproductive Technology. These data were collected by nurses who telephoned each patient after delivery to obtain delivery details and information on any complications.
      The primary outcomes were overall rate of maternal complications, particularly HDP, which included the diagnoses of pre-eclampsia, gestational hypertension, the syndrome of haemolysis, elevated liver enzymes and low platelet count or eclampsia. Secondary outcomes included maternal delivery characteristics and rate of other obstetric complications, e.g. mode of delivery, gestational diabetes, postpartum haemorrhage, preterm premature rupture of membranes (PPROM), chorioamnionitis, placenta accreta, placenta previa, postpartum haemorrhage and placental abruption, as well as perinatal outcomes (rate of birth defects, birth weight, gestational age at birth, newborn intensive care unit admission and fetal loss.) Birth defects were classified as major and minor as described in previous studies (
      • Bonduelle M.
      • Liebaers I.
      • Deketelaere V.
      • Derde M.P.
      • Camus M.
      • Devroey P.
      • Van Steirteghem A.
      Neonatal data on a cohort of 2889 infants born after ICSI (1991-1999) and of 2995 infants born after IVF (1983-1999).
      ). A major congenital anomaly was defined as a condition that reduces the viability or compromises the quality of life and requires medical treatment or a condition that causes functional impairment or requires surgical correction.

       Statistical analysis

      IBM SPSS© Statistics version 26.0 was used for statistical analysis. Student’s t-test or Mann–Whitney U was used to analyse continuous variables depending on whether the variable was normally distributed. Chi-squared or Fischer’s exact test was used for categorical variables and, where indicated, a post-hoc analysis involving pairwise comparisons was conducted if there were three or more independent groups, using z-test of two proportions with a Bonferroni correction. Continuous variables were presented as mean ± SD, and categorical variables presented as percentage and count. A binary logistic regression was conducted to control for potential covariates that may be associated with an increased risk of HDP, including age, BMI, and number of embryos transferred as continuous variables and smoking status, parity, history of diabetes mellitus or HTN, primary infertility diagnosis and use of PGT as categorical variables (
      • Vidaeff A.
      • Pettker C.M.
      • Simhan H.
      Gestational Hypertension and Preeclampsia ACOG PRACTICE BULLETIN.
      ;
      • Zhang W.Y.
      • von Versen-Höynck F.
      • Kapphahn K.I.
      • Fleischmann R.R.
      • Zhao Q.
      • Baker V.L.
      Maternal and neonatal outcomes associated with trophectoderm biopsy.
      ). All the variables were entered using a forced entry method, and all the predictor variables were tested in one block to assess their predictive ability while controlling for other predictors in the model. Both crude and adjusted odds ratios with 95% confidence intervals were calculated. In addition, subgroup analyses were conducted stratifying groups by age (<35 and ≥35 years), BMI (≤30 and >30 kg/m2) and use of PGT. A two-sided P-value of less than 0.05 was considered statistically significant. Bonferroni correction, however, was used for multiple comparisons. For the 10 maternal outcomes assessed, P <0.005 would be considered statistically significant. For the 15 perinatal outcomes assessed, P < 0.003 would be considered statistically significant.

      Results

       Baseline characteristics

      The baseline cycle and demographic information for each FET group of patients is presented in Table 1. A total of 775 FET cycles comprising 384 natural and 391 programmed FET cycles were included for analysis. Women who underwent natural FET cycles were significantly older (35.0 ± 3.7 versus 33.9 ± 3.9 years; P < 0.01) and had lower BMI (25.9 ± 5.7 versus 27.1 ± 6.2kg/m2; P < 0.01) than those who underwent programmed FET, respectively. Women who underwent natural FET cycles were also more likely to have used PGT (P = 0.01) and transferred fewer embryos (P < 0.01). Infertility diagnosis was significantly different between the two groups (Table 1). To determine which groups were significantly different, a post-hoc analysis was conducted involving pairwise comparisons using the z-test of two proportions with a Bonferonni correction. This showed that a significantly higher proportion of women with the diagnosis of anovulation or polycystic ovary syndrome used a programmed FET protocol compared with a natural cycle. In contrast, more women with unexplained or male factor infertility underwent a natural FET protocol. A comparable number of women in both groups were smokers and primigravida at time of conception. No differences were observed between groups in the proportion of women with history of diabetes mellitus and HTN.
      Table 1Baseline characteristics for natural and programmed frozen embryo transfer groups
      Baseline characteristicsNatural FET (n = 384)Programmed FET (n = 391)P-value
      Age, years, mean ± SD35.0 ± 3.733.9 ± 3.9<0.01
      Age group, % (n)<0.01
      <35 years49.7 (191/384)62.9 (246/391)
      ≥35 years50.3 (193/384)37.1 (145/391)
      BMI (kg/m2), mean ± SD)25.9 ± 5.727.1 ± 6.2<0.01
      BMI group, % (n)0.03
      ≤30 kg/m279.2 (304/384)72.6 (284/391)
      >30 kg/m220.8 (80/384)27.4 (107/391)
      Smoker, % (n)3.1 (12/384)3.3 (13/389)
      The denominator is less than total number of patients in the group because of missing data for that variable. BMI, body mass index; DOR, diminished ovarian reserve; HTN, chronic hypertension; PCOS, polycystic ovary syndrome; PGT, preimplantation genetic testing; RPL, recurrent pregnancy loss.
      0.87
      Parity, % (n)0.43
      Parous32.6 (125/384)29.9 (117/391)
      Nulliparous67.4 (259/384)70.1 (274/391)
      History of pre-existing diabetes, % (n)0.25
      Yes0.0 (0/384)0.8 (3/391)
      No100.0 (384/384)99.2 (388/391)
      History of chronic HTN, % (n)0.22
      Yes0.3 (1/384)1.3 (5/391)
      No99.7 (383/384)98.7 (386/391)
      Diagnosis, % (n)<0.01
      Unexplained26.6 (102/384)12.0 (47/391)
      Anovulation/PCOS3.9 (15/384)37.3 (146/391)
      Male factor31.0 (119/384)21.2 (83/391)
      DOR4.7 (18/384)2.8 (11/391)
      Endometriosis6.5 (25/384)7.2 (28/391)
      Fibroids3.9 (15/384)2.3 (9/391)
      RPL8.9 (34/384)7.2 (28/391)
      Tubal factor8.3 (32/384)6.1 (24/391)
      Other6.3 (24/384)3.8 (15/391)
      Cycle characteristics
      Mean number of embryos transferred (n, mean ± SD)1.2 ± 0.41.3 ± 0.5<0.01
      PGT, % (n)35.2 (135/384)26.3 (103/391)0.01
      a The denominator is less than total number of patients in the group because of missing data for that variable.BMI, body mass index; DOR, diminished ovarian reserve; HTN, chronic hypertension; PCOS, polycystic ovary syndrome; PGT, preimplantation genetic testing; RPL, recurrent pregnancy loss.

       Maternal outcomes

      Maternal delivery characteristics and outcomes are presented in Table 2 and observations specific to HDP are shown in Figure 1. Programmed FET was associated with more overall maternal complications (32.2% [126/391] versus 18.8% [72/384]; P < 0.001) than natural FET, respectively. More specifically, women in the programmed FET group had a higher probability of HDP than the natural FET group (15.3% [60/391] versus 6.3% [24/384]); P = 0.001), respectively. Moreover, women in the programmed FET group more often experienced PPROM and were delivered via caesarean section. Only the rate of overall maternal complications and rate of HDP, however, remained significantly different after the Bonferroni correction was applied. No significant difference was observed between groups in the rate of gestational diabetes mellitus or placenta accreta, although a few cases of placenta accreta were reported.
      Table 2Maternal outcomes for natural and programmed frozen embryo transfer groups
      Maternal complicationsNatural FETProgrammed FETCrude OR (95% CI)Adjusted OR (95% CI)
      Binary logistic regression controlling for covariates, including age, body mass index, smoking status, parity, history of diabetes mellitus or chronic hypertension, primary infertility diagnosis, number of embryos transferred and use of preimplantation genetic testing.
      P- value
      Binary logistic regression controlling for covariates, including age, body mass index, smoking status, parity, history of diabetes mellitus or chronic hypertension, primary infertility diagnosis, number of embryos transferred and use of preimplantation genetic testing.
      Overall, %, n18.8 (72/384)32.2 (126/391)2.06 (95% CI 1.48 to 2.87)2.21 (95% CI 1.51 to 3.22)<0.01
      Gestational diabetes, %, n7.3 (28/384)10.7 (42/391)1.53 (95% CI 0.93 to 2.52)1.69 (95% CI 0.95 to 2.99)0.07
      Hypertensive disorders of pregnancy, %, n6.3 (24/384)15.3 (60/391)2.56 (95% CI 1.56 to 4.25)2.39 (95% CI 1.37 to 4.17)<0.01
      Preterm premature rupture of membranes, %, n0.3 (1/384)2.6 (10/391)10.05 (95% CI 1.28 to 78.91)11.71 (95% CI 1.44 to 95.55)0.02
      Chorioamnionitis, % (n)0.5 (2/384)0.5 (2/391)0.98 (95% CI 0.25 to 8.89)0.76 (95% CI 0.08 to 7.17)0.81
      Placental abruption, % (n)0.5 (2/384)0.8 (3/391)1.48 (95% CI 0.25 to 8.94)1.11 (95% CI 0.12 to 10.15)0.93
      Placenta accrete, % (n)0.3 (1/384)0.5 (2/391)1.97 (95% CI 0.18 to 21.81)2.98 (95% CI 0.25 to 34.95)0.38
      Placenta previa, (%) n2.3 (9/384)2.6 (10/391)1.09 (95% CI 0.44 to 2.72)1.50 (95% CI 0.57 to 3.89)0.42
      Postpartum haemorrhage, % (n)1.0 (4/384)1.3 (5/391)1.23 (95% CI 0.33 to 4.62)2.02 (95% CI 0.49 to 8.30)0.33
      Caesarean delivery, % (n)42.8 (163/381)
      The denominator is less than total number of patients in the group because of missing data for that outcome variable. FET, frozen embryo transfer.
      53.2 (206/387)
      The denominator is less than total number of patients in the group because of missing data for that outcome variable. FET, frozen embryo transfer.
      1.52 (95% CI 1.14 to 2.02)1.44 (1.04 to 2.01)0.03
      a Binary logistic regression controlling for covariates, including age, body mass index, smoking status, parity, history of diabetes mellitus or chronic hypertension, primary infertility diagnosis, number of embryos transferred and use of preimplantation genetic testing.
      b The denominator is less than total number of patients in the group because of missing data for that outcome variable.FET, frozen embryo transfer.
      Figure 1
      Figure 1Rate of hypertensive disorders in pregnancy stratified by (A) age; (B) body mass index; and (C) use of preimplantation genetic testing for aneuploidies. BMI, body mass index; FET, frozen embryo transfer; HDP, hypertensive disorders of pregnancy; PGT, preimplantation genetic testing.
      In view of the differences in baseline characteristics as well as known risk factors for HDP, subgroup analyses were conducted. These subgroup analyses showed that a higher probability of HDP in the programmed compared with natural FET group generally persisted even after groups were stratified by age (15.9% [39/246] versus 6.8% [13/191], P < 0.01; 14.5% [21/145] versus 5.7% [11/193], P < 0.01, P < 35 and P ≥ 35 years, respectively) (Figure 1A), BMI (12.0% [34/284] versus 3.9% [12/304], P < 0.01; 24.3% [26/107] versus 15.0% [12/80], P = 0.12), BMI ≤30 and >30 kg/m2, respectively) (Figure 1B) and the use of PGT (13.9% [40/288] versus 5.2% [13/249], P < 0.01; 19.4% [20/103] versus 8.1% [11/135], P = 0.01, without PGT versus with PGT, respectively) (Figure 1C). Although HDP was significantly higher in programmed compared with natural FET cycles in women with BMI 30 kg/m2 or above, statistical significance was not reached for HDP rates in programmed FET cycles in women with BMI above 30 kg/m2, mainly because of the relatively smaller sample size in the BMI above 30 kg/m2 group. After adjusting for covariates of age, BMI, diagnosis, smoking status, history of diabetes mellitus or HTN, primary infertility diagnosis, number of embryos transferred and use of PGT, the probability of developing HDP was twofold higher in women who used programmed compared with natural FET cycles (adjusted OR 2.39; 95% CI 1.37 to 4.17; P < 0.01) (Table 2).
      In addition, a secondary analysis was conducted in which patients with anovulatory infertility were excluded because they may have a higher likelihood of maternal complications, which could confound results. When patients with anovulatory infertility were excluded from the analysis, the rate of HDP remained significantly higher in the programmed FET group compared with the natural FET group (15.1% [37/245] versus 6.5% [24/369]; P = 0.001), respectively. Similarly, the rate of overall maternal complications also remained significantly higher in the programmed FET group (34.7% [85/245] versus 19.0% [70/369]; P < 0.001) compared with the natural FET group, respectively. In this subgroup analysis, the odds ratio for HDP was 2.56 (95% CI 1.49 to 4.40) and for overall maternal complications was 2.27 (95% CI 1.57 to 3.29).

       Perinatal outcomes

      No significant difference was observed between groups in any of the perinatal outcomes, including the mean birth weight and the probability of having an infant with birth defects (Table 3). Furthermore, no statistically significant difference was found in mean birthweight between groups, including the proportion of macrosomic infants (≥4000 g) in the programmed compared with the natural FET groups (14.0% [54/386] versus (9.4% [36/381]; P = 0.07)). In addition, no minor birth defects were reported in either group and no significant difference was found between groups in reported major birth defects (2.1% [8/384] versus 1.5% [6/391]; P = 0.57).
      Table 3Perinatal outcomes for natural and programmed frozen embryo transfer groups
      Perinatal birth characteristics and complicationsNatural FETProgrammed FETP-value
      Mean gestational age, weeks, mean ± SD38.7 ± 2.338.7 ± 3.20.85
      Gestational age group, % (n)0.59
      ≤28 weeks1.0 (4/384)1.5 (6/391)
      >28 to <34 weeks1.8 (7/384)3.1 (12/391)
      >34 to <37 weeks9.6 (37/384)8.4 (33/391)
      ≥37 weeks87.5 (336/384)87.0 (340/391)
      Weight, g, mean ± SD3318.4 ± 616.23357.9 ± 671.60.40
      Weight group, % (n)0.14
      ≤2500 g7.1 (27/381)
      The denominator is less than total number of patients in the group because of missing data for that outcome variable. FET, frozen embryo transfer; NICU, newborn intensive care unit.
      8.0 (31/386)
      The denominator is less than total number of patients in the group because of missing data for that outcome variable. FET, frozen embryo transfer; NICU, newborn intensive care unit.
      >2500 to <4000 g83.5 (318/381)78.0 (301/386)
      ≥4000 g9.4% (36/381)14.0 (54/386)
      Overall complications, % (n)16.1 (62/384)18.7 (73/391)0.35
      NICU admission, % (n)4.7 (18/394)6.4 (25/391)0.30
      Fetal loss <24 weeks, % (n)0.3 (1/384)0.3 (1/391)1.0
      Fetal loss ≥24 weeks, % (n)0.0 (0/384)0.0 (0/391)1.0
      Major birth defects, % (n)2.1 (8/384)1.5 (6/391)0.57
      Minor birth defects, % (n)0.0 (0/384)0.0 (0/391)1.0
      a The denominator is less than total number of patients in the group because of missing data for that outcome variable.FET, frozen embryo transfer; NICU, newborn intensive care unit.

      Discussion

      Our study evaluated the effect of endometrial preparation on obstetric and perinatal outcomes in a large US cohort, which included only singleton deliveries after transfer of blastocysts that were previously vitrified. We showed that the probability of developing HDP was twofold higher in women who conceived after programmed compared with natural FET cycles (adjusted OR 2.39, 95% CI 1.37 to 4.17; P < 0.01). This result persisted after controlling for important covariates known to be risk factors for developing HDP, including age, BMI, diagnosis, smoking status, history of diabetes mellitus or HTN, primary infertility diagnosis, number of embryos transferred and use of PGT. We did not detect any differences in perinatal outcomes between the two methods of endometrial preparation.
      These results have significant implications for current practice patterns in US fertility centres. Previous studies comparing methods for endometrial preparation in FET protocols demonstrated equivalent pregnancy outcomes, including clinical pregnancy, ongoing pregnancy and live birth rates (
      • Groenewoud E.R.
      • Cantineau A.E.P.
      • Kollen B.J.
      • Macklon N.S.
      • Cohlen B.J.
      What is the optimal means of preparing the endometrium in frozen – thawed embryo transfer cycles?.
      ;
      • Peeraer K.
      • Couck I.
      • Debrock S.
      • De Neubourg D.
      • De Loecker P.
      • Tomassetti C.
      • Laenen A.
      • Welkenhuysen M.
      • Meeuwis L.
      • Pelckmans S.
      • Meuleman C.
      • Hooghe T.D.
      Frozen-thawed embryo transfer in a natural or mildly hormonally stimulated cycle in women with regular ovulatory cycles: a RCT.
      ;
      • Yarali H.
      • Polat M.
      • Mumusoglu S.
      • Yarali I.
      • Bozdag G.
      Preparation of endometrium for frozen embryo replacement cycles : a systematic review and meta-analysis.
      ). Given this information, decisions about which protocol to choose for FET cycles have largely been left to physician preference if a particular patient’s characteristics allowed for both protocol types. In fact, programmed FET is often selected simply because it offers the most flexibility in terms of scheduling owing to the ability to avoid weekend embryo transfers. Our results argue against this approach and provide further justification to strongly recommend natural over programmed FET cycles in all eligible patients.
      Our results are consistent with the findings of previous studies, which most notably demonstrate a significantly higher risk of HDP in pregnancies conceived after programmed FET cycles compared with natural FET cycles (
      • Ernstad E.G.
      • Wennerholm U.
      • Khatibi A.
      • Petzold M.
      • Bergh C.
      Neonatal and maternal outcome after frozen embryo transfer: Increased risks in programmed cycles.
      ;
      • Saito K.
      • Kuwahara A.
      • Ishikawa T.
      • Morisaki N.
      • Miyado M.
      • Miyado K.
      • Fukami M.
      • Miyasaka N.
      • Ishihara O.
      • Irahara M.
      • Saito H.
      Endometrial preparation methods for frozen-thawed embryo transfer are associated with altered risks of hypertensive disorders of pregnancy, placenta accreta, and gestational diabetes mellitus.
      ;
      • Von Versen-höynck F.
      • Schaub A.M.
      • Chi Y.
      • Chiu K.
      • Liu J.
      • Lingis M.
      • Williams R.S.
      • Rhoton-vlasak A.
      • Nichols W.W.
      • Fleischmann R.R.
      • Zhang W.
      • Winn V.D.
      • Segal M.S.
      • Conrad K.P.
      • Baker V.L.
      Preeclampsia Increased Preeclampsia Risk and Reduced Aortic Compliance With In Vitro Fertilization Cycles in the Absence of a Corpus Luteum.
      ). In the present study, this held true even after stratification and controlling for covariates that are potential risk factors for the development of HDP. Our results, more importantly, confirm an increased risk of HDP in current IVF practices, which include blastocyst culture, vitrification cryopreservation protocol and use of modern PGT techniques.
      To the best of our knowledge, this is the largest US cohort of patients that addresses differences in maternal and perinatal outcomes after programmed and natural FET cycles. Two previous studies with large cohorts addressing this question have been conducted internationally, limiting their generalizability to our population (
      • Ernstad E.G.
      • Wennerholm U.
      • Khatibi A.
      • Petzold M.
      • Bergh C.
      Neonatal and maternal outcome after frozen embryo transfer: Increased risks in programmed cycles.
      ;
      • Saito K.
      • Kuwahara A.
      • Ishikawa T.
      • Morisaki N.
      • Miyado M.
      • Miyado K.
      • Fukami M.
      • Miyasaka N.
      • Ishihara O.
      • Irahara M.
      • Saito H.
      Endometrial preparation methods for frozen-thawed embryo transfer are associated with altered risks of hypertensive disorders of pregnancy, placenta accreta, and gestational diabetes mellitus.
      ). Both of these studies, one conducted in a Swedish cohort and the other in a Japanese cohort, demonstrated a higher rate of HDP in programmed FET cycles compared with natural FET cycles. They both, however, included a significant number of cycles with transfer of cleavage-stage embryos (>50% of the embryos in the Swedish study and 20–30% of the embryos in the Japanese study.) Furthermore, in the Swedish study, embryos were transferred that had been cryopreserved using slow freezing. Although the investigators state that they controlled for freezing method, they did not specify what proportion of cycles used slow freezing. Nevertheless, given that their registry included cycles spanning back as far as 2005, one can assume that a significant number of cycles were carried out before the widespread adoption of vitrification. One prospective study has been conducted in a US cohort. The investigators found a significantly higher rate of HDP in programmed FET cycles. Although their analysis covered only procedures carried out in recent years (2011–2017) and mostly involved blastocyst transfers (>95%), their results were limited by sample size, and it was unclear what proportion of embryos were cryopreserved using slow freezing. Furthermore, it was also unclear whether multiple cycles per patient were included, which would affect their results (
      • Von Versen-höynck F.
      • Schaub A.M.
      • Chi Y.
      • Chiu K.
      • Liu J.
      • Lingis M.
      • Williams R.S.
      • Rhoton-vlasak A.
      • Nichols W.W.
      • Fleischmann R.R.
      • Zhang W.
      • Winn V.D.
      • Segal M.S.
      • Conrad K.P.
      • Baker V.L.
      Preeclampsia Increased Preeclampsia Risk and Reduced Aortic Compliance With In Vitro Fertilization Cycles in the Absence of a Corpus Luteum.
      ).
      It has been proposed that this heightened risk of HDP is because programmed FET protocols do not mimic physiologic conditions, resulting in abnormal placentation (
      • Conrad K.P.
      • Belen M.
      • Post E.D.
      Emerging role for dysregulated decidualization in the genesis of preeclampsia.
      ). It is accepted that HDP disorders develop from uteroplacental insufficiency (
      • Mol B.W.
      • Ph D.
      • Barnhart K.T.
      Scratching the Endometrium in In Vitro Fertilization — Time to Stop.
      ); however, more recently it has been proposed that imbalances in angiogenic factors play a role in the pathogenesis of HDP (
      • Vidaeff A.
      • Pettker C.M.
      • Simhan H.
      Gestational Hypertension and Preeclampsia ACOG PRACTICE BULLETIN.
      ).
      • Von Versen-höynck F.
      • Narasimhan P.
      • Seda E.
      • Tierney S.
      • Martinez N.
      • Conrad K.P.
      • Baker V.L.
      • Winn V.D.
      Absent or Excessive Corpus Luteum Number Is Associated With Altered Maternal Vascular Health in Early Pregnancy.
      ;
      • Von Versen-höynck F.
      • Schaub A.M.
      • Chi Y.
      • Chiu K.
      • Liu J.
      • Lingis M.
      • Williams R.S.
      • Rhoton-vlasak A.
      • Nichols W.W.
      • Fleischmann R.R.
      • Zhang W.
      • Winn V.D.
      • Segal M.S.
      • Conrad K.P.
      • Baker V.L.
      Preeclampsia Increased Preeclampsia Risk and Reduced Aortic Compliance With In Vitro Fertilization Cycles in the Absence of a Corpus Luteum.
      ) were the first to show that the absence of corpus luteum may affect vascular health during early pregnancy by altering maternal circulation. In an initial study, the investigators analysed a small cohort of patients undergoing IVF based on number of corpus luteum present, and found that, in the absence of a corpus luteum, mean arterial pressure did not decline as expected in pregnancy and also that the measures of endothelial cell function and arterial stiffness were significantly altered. In addition, circulating angiogenic progenitor cells were lower and the concentration of relaxin, a potent vasodilator secreted by corpus luteum was lower; this contributes to maternal vascular adaptations in pregnancy as shown in animal models (
      • Marshall S.A.
      • Leo C.H.
      • Senadheera S.N.
      • Girling J.E.
      • Tare M.
      • Parry L.J.
      Relaxin deficiency attenuates pregnancy-induced adaptation of the mesenteric artery to angiotensin II in mice.
      ). In a subsequent study, they further showed that vascular compliance was blunted in the absence of a corpus luteum (
      • Arthur I.D.
      • Anthony F.W.
      • Thomas E.J.
      Serum relaxin and the major endometrial secretory proteins in in-vitro fertilization and down-regulated hormone-supported and natural cycle frozen embryo transfer.
      ;
      • Strauch N.K.
      • Von Versen-ho F.
      • Liu J.
      • Chi Y.
      • Keller-woods M.
      • Conrad K.P.
      • Baker V.L.
      Effect of Mode of Conception on Maternal Serum Relaxin, Creatinine, and Sodium Concentrations in an Infertile Population.
      ;
      • Von Versen-höynck F.
      • Schaub A.M.
      • Chi Y.
      • Chiu K.
      • Liu J.
      • Lingis M.
      • Williams R.S.
      • Rhoton-vlasak A.
      • Nichols W.W.
      • Fleischmann R.R.
      • Zhang W.
      • Winn V.D.
      • Segal M.S.
      • Conrad K.P.
      • Baker V.L.
      Preeclampsia Increased Preeclampsia Risk and Reduced Aortic Compliance With In Vitro Fertilization Cycles in the Absence of a Corpus Luteum.
      ;
      • Von Versen-höynck F.
      • Narasimhan P.
      • Seda E.
      • Tierney S.
      • Martinez N.
      • Conrad K.P.
      • Baker V.L.
      • Winn V.D.
      Absent or Excessive Corpus Luteum Number Is Associated With Altered Maternal Vascular Health in Early Pregnancy.
      ). On the basis of these findings, one might argue that natural or modified natural FET protocols, in which one or more corpora lutea are present, should be recommended as the FET protocols of choice for eligible patients to reduce their risk of HDP.
      We observed a higher rate of PPROM in the programmed compared with the natural FET group. When the Bonferroni correction was applied, however, this result was no longer significant. Furthermore, there does not seem to be a physiological explanation for the increased rate of PPROM. Moreover, we did not find an increased risk of preterm delivery, so it is unlikely that this finding is clinically significant. Additionally, unlike previous reports (
      • Ishihara O.
      • Ph D.
      • Araki R.
      • Ph D.
      • Kuwahara A.
      • Ph D.
      Impact of frozen-thawed single-blastocyst transfer on maternal and neonatal outcome : an transfer cycles from 2008 to 2010 in Japan.
      ;
      • Saito K.
      • Kuwahara A.
      • Ishikawa T.
      • Morisaki N.
      • Miyado M.
      • Miyado K.
      • Fukami M.
      • Miyasaka N.
      • Ishihara O.
      • Irahara M.
      • Saito H.
      Endometrial preparation methods for frozen-thawed embryo transfer are associated with altered risks of hypertensive disorders of pregnancy, placenta accreta, and gestational diabetes mellitus.
      ), we did not find a higher rate of placenta accreta in programmed FET pregnancies. It is reasonable to assume that, if placental development is compromised in programmed FET cycles, there should be a higher risk of disorders of abnormal placentation, including placenta accreta and placenta previa. Placenta accreta, however, is a rare event, ranging from 1 in 272 to 1 in 533 pregnancies (
      • American College of Obstetricians and Gynecologists and Society for the Maternal-Fetal Medicine
      Obstetrics Care Consensus: Placenta Accreta Spectrum.
      ). It is, therefore, notable that, in the study by
      • Saito K.
      • Kuwahara A.
      • Ishikawa T.
      • Morisaki N.
      • Miyado M.
      • Miyado K.
      • Fukami M.
      • Miyasaka N.
      • Ishihara O.
      • Irahara M.
      • Saito H.
      Endometrial preparation methods for frozen-thawed embryo transfer are associated with altered risks of hypertensive disorders of pregnancy, placenta accreta, and gestational diabetes mellitus.
      , the prevalence of placenta accreta was less than 1% in both the programmed and natural cycles groups (0.1% versus 0.9%; P < 0.01), which is consistent with the probability noted in our study (0.3% versus 0.5%, P = 1.0). They found a statistically significant difference, however, because of their large sample size and the clinical significance of this finding is debatable. Similarly, we did not detect a significant difference in gestational diabetes mellitus as described by
      • Saito K.
      • Kuwahara A.
      • Ishikawa T.
      • Morisaki N.
      • Miyado M.
      • Miyado K.
      • Fukami M.
      • Miyasaka N.
      • Ishihara O.
      • Irahara M.
      • Saito H.
      Endometrial preparation methods for frozen-thawed embryo transfer are associated with altered risks of hypertensive disorders of pregnancy, placenta accreta, and gestational diabetes mellitus.
      .
      Although multiple studies have demonstrated a higher incidence of macrosomic infants born after programmed FET (
      • Ishii R.
      • Shoda A.
      • Kubo M.
      • Okazaki S.
      • Suzuki M.
      • Okawa R.
      • Enomoto M.
      • Shitanaka M.
      • Fujita Y.
      • Nakao K.
      • Shimada N.
      • Horikawa M.
      • Negishi H.
      • Taketani Y.
      Identifying a possible factor for the increased newborn size in singleton pregnancies after assisted reproductive technology using cryopreserved embryos, in comparison with fresh embryos.
      ;
      • Ernstad E.G.
      • Wennerholm U.
      • Khatibi A.
      • Petzold M.
      • Bergh C.
      Neonatal and maternal outcome after frozen embryo transfer: Increased risks in programmed cycles.
      ;
      • Saito K.
      • Kuwahara A.
      • Ishikawa T.
      • Morisaki N.
      • Miyado M.
      • Miyado K.
      • Fukami M.
      • Miyasaka N.
      • Ishihara O.
      • Irahara M.
      • Saito H.
      Endometrial preparation methods for frozen-thawed embryo transfer are associated with altered risks of hypertensive disorders of pregnancy, placenta accreta, and gestational diabetes mellitus.
      ), we did not detect a significant difference in mean birth weight according to FET protocol nor when birth weight was stratified into groups. In general, it is difficult to reconcile how programmed FET cycles result in both high rates of HDP, which is often associated with fetal growth restriction, and fetal macrosomia. Potential explanations may include distinctive parental traits, such as more gestational diabetes mellitus in women who give birth to macrosomic infants. We also did not detect a significant difference in major or minor birth defects, but as birth defects are so rare, a much larger data set may be required to discern such a difference.
      Our study is unique because we included only singleton deliveries that resulted from transfer of vitrified–warmed autologous blastocysts. One may argue that the findings of this study are limited by the fact that all cycles were performed at a single fertility centre, but it is also a strength as each group was treated uniformly in the medications that were used and how they were monitored. In contrast, in the Swedish cohort study of 10,000 singleton births discussed above (
      • Ernstad E.G.
      • Wennerholm U.
      • Khatibi A.
      • Petzold M.
      • Bergh C.
      Neonatal and maternal outcome after frozen embryo transfer: Increased risks in programmed cycles.
      ), the type of FET cycle carried out and medications used were assumed based on what drugs had been prescribed for each patient. Therefore, it was possible that centres across the country were using vastly different protocols and that the medications prescribed were not the ones necessarily used by the patient. Moreover, our study was conducted retrospectively, so there is a risk of confounding and bias. Logistic regression was conducted to control for potential significant confounders, including age, BMI, smoking status, parity, history of pre-existing diabetes or HTN, primary infertility diagnosis, number of embryos transferred and use of PGT, and still found a significantly higher risk of HDP in the programmed FET group. Furthermore, as significantly more women had anovulatory infertility in the programmed FET group, we carried out a secondary analysis and excluded these patients as they may have a higher probability of maternal complications. We still found that rate of HDP and overall maternal complications was significantly higher in the programmed FET group.
      Nevertheless, we recognize that other possible confounders may exist that that we could not account for, such as history of other concurrent medical conditions that may increase the risk of HDP (renal disease, autoimmune disease) or prior history of HDP. In addition, we did not have information on vanishing twins or fetal demise of a twin. Research suggests that pregnancies complicated by vanishing twin syndrome may be associated with increased perinatal complications, including low birthweight and preterm delivery (
      • Almog B.
      • Levin I.
      • Wagman I.
      • Kapustiansky R.
      • Lessing J.B.
      • Amit A.
      • Azem F.
      Adverse obstetric outcome for the vanishing twin syndrome.
      ;
      • Evron E.
      • Sheiner E.
      • Friger M.
      • Sergienko R.
      • Harlev A.
      Vanishing twin syndrome: Is it associated with adverse perinatal outcome?.
      ;
      • Timur H.
      • Aksoy R.T.
      • Tokmak A.
      • Timur B.
      • Coskun B.
      • Uygur D.
      • Danisman N.
      Maternal and perinatal outcomes of dichorionic diamniotic twin pregnancies diagnosed with vanishing twin syndrome: A retrospective analysis from a single clinical center.
      ). To our knowledge, however, vanishing twin syndrome has not been associated with increased risk of HDP. Nevertheless, it may represent a potential confounder for outcomes. As such, we hope that our results prompt further investigation. A prospective randomized controlled trial would be the optimal method to validate our findings.
      In conclusion, blastocyst transfer in a programmed FET cycles is associated with a higher risk of overall maternal complications, particularly HDP. It is important to highlight that HDP is one of the leading causes of maternal and perinatal death around the world (
      • Vidaeff A.
      • Pettker C.M.
      • Simhan H.
      Gestational Hypertension and Preeclampsia ACOG PRACTICE BULLETIN.
      ). Currently, no accurate screening methods are available for the early detection of women at risk of developing HDP. Low-dose aspirin may modestly reduce the risk of developing HDP, but no interventions can completely eliminate it (
      • American College of Obstetricians and Gynecologists
      ACOG Committee Opinion No.
      ). Therefore, the most effective strategy to reduce a patient’s risk for HDP may be to evaluate for modifiable risk factors, one of which is selecting the type of FET protocol type used by women during IVF. As such, natural FET protocols should be recommended as first-line for all eligible patients undergoing IVF to reduce the risk of HDP.

      Acknowledgements

      We would like to thank the physicians and nursing staff at CARS for their support.

      References

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      Biography

      Dr Reeva Makhijani obtained her medical degree from NYU School of Medicine in 2014. She completed her Obstetrics and Gynecology residency in 2018 at Women and Infants Hospital in Rhode Island. She is currently a fellow in Reproductive Endocrinology and Infertility at the University of Connecticut.
      Key message
      Programmed frozen embryo transfer cycles are associated with more obstetric complications, particularly hypertensive disorders of pregnancy, compared with natural cycles. Therefore, natural frozen embryo transfer protocols should be recommended for endometrial preparation in eligible patients.