Preconception maternal body mass index and offspring sex ratio in intrauterine insemination cycles.

Abstract

Objective: To investigate the association between maternal preconception body mass index (BMI) and offspring sex ratio among women undergoing intrauterine insemination (IUI) treatment.

Methods: This retrospective cohort study analyzed 17,116 IUI cycles from 9581 women treated at a single large fertility center between 2015 and 2024. Maternal preconception BMI, measured at the start of each IUI cycle, was categorized into four groups: underweight (< 18.5 kg/m²), normal weight (18.5-24.9 kg/m²), overweight (25.0-29.9 kg/m²), and obesity (≥ 30.0 kg/m²). The primary outcome was the proportion of male live births, with secondary sex ratio (SSR; male births per 100 female births) reported descriptively. Associations were estimated using generalized estimating equations to account for repeated cycles per patient, with adjustment for potential confounders using inverse probability weighting based on the generalized propensity score.

Results: Among 3224 live-born infants with documented sex, 1629 (50.5%) were male and 1595 (49.5%) were female, yielding an overall sex ratio of 102.1 males per 100 females. The male-to-female sex ratio declined progressively with increasing maternal preconception BMI: 120.4 for underweight, 106.3 for normal weight, 85.3 for overweight, and 65.3 for obese women. In adjusted analyses, overweight women had a significantly lower likelihood of male offspring (adjusted RR, 0.88; 95% CI, 0.80-0.98) and a correspondingly higher likelihood of female offspring (RR, 1.13; 95% CI, 1.03-1.23) compared with normal-weight women. A directionally similar but statistically non-significant inverse association was observed in the obesity subgroup (adjusted RR, 0.77; 95% CI, 0.58-1.04), which was based on only 81 live births. The inverse trend across BMI categories was significant for male births (P for trend = 0.012), as was the positive trend for female births (P for trend = 0.004).

Conclusion: Higher maternal preconception BMI was associated with a lower proportion of male live births and a reduced secondary sex ratio among women undergoing IUI. These findings support the hypothesis that maternal metabolic status may modulate sex-specific live birth outcomes, possibly through differential embryonic or fetal survival.