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

Abstract

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?

Design: 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.

Results: 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).

Conclusions: 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.

Keywords: Embryo development; Euploid blastocyst; Female blastocyst; Male blastocyst; Sex ratio.