Association between the body mass index of women undergoing IVF and the gender of the offspring

Research question: Is there an association between women's body mass index (BMI) and the gender of offspring generated from fresh or frozen-thawed embryo transfer (FET) cycles? Design: Between 2018 and 2021, 2670 cycles were performed, comprising 786 fresh embryo transfers and 1884 FET cycles. The study analysed the relationship between the BMI of women undergoing IVF and the sex of the newborn and the male-to-female live birth ratio (secondary sex ratio [SSR]). Results: After 2670 single-embryo transfers, 483 (50.31%) male and 477 (49.69%) female infants were born, with an SSR of 101.3. In the fresh embryo transfer group, the SSR ranged from 350 (95% CI 82.67-1481.65) in underweight women (although this was based on very small numbers) to 109.7(95% CI 67.73-176.61) in overweight women, while in the FET group, SSR values varied correspondingly from 130.8 (95% CI 64.47-265.26) to 63.0 (95% CI 46.02-86.24). Results from Bayesian multinomial logistic regression showed that for each unit increase in BMI within the fresh embryo transfer group, the odds of a boy's birth decreased by 6% (odds ratio [OR] 0.94, 95% CI 0.90-0.99) and the odds of a girl's birth by 5% (OR 0.95, 95% CI 0.90-0.99). Within the FET cohort, the odds of a boy's birth decreased by 4% (OR 0.96, 95% CI 0.92-0.99), while the odds of a girl's birth increased by 3% (OR 1.03, 95% CI 1.00-1.06). Conclusions: An increase in BMI is associated with the birth of fewer boys in fresh or frozen embryo transfer cycles, and a significant increase in the birth of girls in FET cycles.