Male and female blastocysts: any difference other than the sex?


      • Comparing embryos, more males than females develop to the blastocyst stage.
      • Male blastocysts exhibit a higher aneuploidy rate.
      • The live birth rate is similar between male and female single blastocyst transfers.


      Research question

      Is there any imbalance in the sex ratio at the blastocyst stage of human embryos? And what is the sex ratio in euploid, transferred, implanted blastocysts and at birth?


      Embryos from 646 women undergoing 921 preimplantation genetic testing for aneuploidy (PGT-A) cycles from September 2017 to February 2020 were included. Data from the chromosomal constitution of 2637 biopsied blastocysts were retrospectively analysed. Trophectoderm samples were analysed by next-generation sequencing. Embryos were categorized as euploid, mosaic or aneuploid. A total of 548 blastocysts diagnosed as euploid were warmed and transferred in a subsequent single-embryo transfer cycle.


      The blastocyst sex ratio was skewed in favour of male sex with 53.1% (1401/2637) of blastocysts diagnosed as male and 46.9% (1236/2637) as female (sex ratio 1.13, 95% confidence interval [CI] 1.05–1.22). Following biopsy and PGT-A, 41.2% (1086/2637) of blastocysts were classified as euploid, 7.7% (202/2637) as mosaic and 51.2% (1349/2637) as aneuploid. More chromosome euploidy was observed among female than male blastocysts (adjusted odds ratio 1.29, 95% CI 1.08–1.55) after adjusting for female age, male age and gonadotrophin dose. Euploid blastocysts were comparable between the sexes (sex ratio 0.99, 95% CI 0.88–1.11). No significant differences were observed between the sexes in implantation (sex ratio 0.86, 95% CI 0.68–1.08), miscarriage (sex ratio 1, 95% CI 0.51–1.97) or live birth rate (sex ratio 0.85, 95% CI 0.66–1.08).


      More male than female embryos develop to the blastocyst stage. Male blastocysts exhibit a higher aneuploidy rate. The capacity to implant and lead to a live birth is similar between the sexes.

      Key words

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      Beatriz Carrasco received her degree in Biology from Córdoba University and her Master's degree in Assisted Human Reproduction also from Córdoba University. She is a Senior Embryologist in the IVF laboratory of the Dexeus University Hospital, Spain, and an active member of the Spanish Association for the Study of Reproductive Biology (ASEBIR).
      Key message
      After in-vitro culture, more male embryos develop to the blastocyst stage compared with female ones. However, male blastocysts exhibit a higher aneuploidy rate. The capacity to implant and to give rise to a live birth is similar between the sexes after euploid blastocyst transfer.