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

ABSTRACT

Research Question: Is there any imbalance in the sex ratio (SR) at the blastocyst stage of human embryos? And what is the SR in euploid, transferred, implanted blastocysts and the SR at birth? Design: Embryos from 646 patients 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 was retrospectively analysed. Trophectoderm samples were analysed by NGS. Embryos were categorized as euploid, mosaic or aneuploid. A total of 548 blastocysts diagnosed as euploid were warmed and replaced in a subsequent single embryo transfer cycle. Results: Blastocyst SR was skewed in favor of male sex with 53.1% (1401/2637) diagnosed as male and 46.9% (1236/2637) as female (SR 1.13 [95%CI: 1.05-1.22]). Following biopsy and PGT-A, 41.2% (1086/2637) of the 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 blastocysts when compared to male ones (aOR 1.29 [95%CI: 1.08-1.55]) after adjusting for female age, male age and gonadotropin dose. The SR of euploid blastocysts was comparable between sexes (SR 0.99 [95%CI:0.88-1.11]). No significant differences were observed between sexes in implantation rate (SR 0.86 [95%CI: 0.68-1.08]), miscarriage rate (SR 1 [95%CI: 0.51-1.97]) and live birth rate (SR 0.85 [95%CI: 0.66-1.08]). Conclusion(s): More male embryos develop to the blastocyst stage compared to female ones. Male blastocysts exhibit a higher aneuploidy rate. The capacity to implant and to give rise to a live birth is similar among sexes.