The association of embryo developmental rate and morphological grading with neonatal sex ratio

Embryo developmental rate and morphological grading are strongly associated with implantation potential. However, it remains unclear whether these parameters influence neonatal sex, which may be affected by embryo selection criteria. The aim of this study was to investigate the influence of embryo developmental rate and morphological grading on neonatal sex. This single-center retrospective study included 4278 singleton live births resulting from single embryos transfers, conducted between 2011 and 2024. Binary logistic regression was performed to evaluate the association between embryo developmental rate, morphological grading, and neonate sex. A significantly higher proportion of male live births was observed in blastocyst-stage transfers (55.3%) compared to cleavage-stage transfers (50.9%; P = 0.005). Multivariable logistic regression analysis revealed that day 5 blastocyst transfers were significantly associated with a higher likelihood of male neonates compared to day 3 embryo transfers (Adjusted odds ratio (aOR), 1.21; 95% confidence intervals (CI), 1.02-1.43; P = 0.027). Furthermore, day 5 blastocysts with expansion stages 4 and 5 showed 2.30-fold and 2.71-fold increased probabilities of male neonates, respectively, compared to expansion stage 3 blastocysts (aOR, 2.30; 95% CI, 1.44-3.68; aOR, 2.71; 95% CI, 1.54-4.77; P = 0.001). Additionally, TE grades B and C blastocysts demonstrated 39% and 57% reduced probabilities of male neonates, respectively, compared to TE grade A blastocysts in day 5 transfers (aOR, 0.61; 95% CI, 0.48-0.79; aOR, 0.43; 95% CI, 0.28-0.65; P < 0.001). In conclusion, the blastocyst transfer may influence neonatal sex ratios, particularly in day 5 blastocysts with expansion stages 4-5 and high TE scores.