Effect of pre-pregnancy body mass index on neonatal outcomes in women undergoing autologous frozen thawed embryo transfer

Objective: To investigate the associations between pre-pregnancy body mass index (BMI) and neonatal outcomes in women undergoing autologous frozen-thawed embryo transfer (FET). Design: Retrospective cohort study. Setting(s): University-affiliated reproductive medical center. Patient(s): A total of 16,240 women with singleton deliveries achieved by autologous FET. Intervention(s): None. Main Outcome Measure(s): Neonatal outcomes included preterm birth (PTB), low birth weight (LBW), small-for-gestational age (SGA), large-for-gestational age (LGA), fetal macrosomia, and birth defects. Result(s): After adjusting for confounding factors, our study showed that in autologous FET cycles, the overweight women (23 kg/ m(2) <= BMI <27.5 kg/m(2)) were associated with increased rates of PTB (adjusted odds ratio [aOR], 1.226; 95% confidence interval [CI], 1.060-1.418), macrosomia (aOR, 1.692; 95% CI, 1.491-1.921), and LGA (aOR, 1.980; 95% CI, 1.715-2.286); and the obese women (BMI >= 27.5 kg/m(2)) were significantly associated with increased PTB (aOR, 1.503; 95% CI, 1.167-1.936), early PTB (aOR, 2.829; 95% CI, 1.679-4.765), very LBW (aOR, 3.087; 95% CI, 1.720-5.542), macrosomia (aOR, 2.325; 95% CI, 1.862-2.904), and LGA (aOR, 3.235; 95% CI, 2.561-4.085). The rate of SGA infants was higher in the underweight women (BMI <18.5 kg/m(2)) (aOR, 1.687; 95% CI, 1.375-2.071) than that in the normal-weight women (18.5 kg/m(2) <= BMI <= 23 kg/m(2)). No significant difference was observed in the risk of birth defects between normal-weight cases and other BMI categories. Conclusion(s): Among women undergoing FET, pre-pregnancy BMI affected neonatal outcomes of singletons. BMI in Asian categories for overweight and obese showed significant increases in PTB, macrosomia, and LGA; early PTB and very LBW only increased in obese cases. In addition, underweight status was associated with increased risk of SGA. In contrast, there was no association between prepregnancy BMI and birth defects in FET cycles. (C) 2021 by American Society for Reproductive Medicine.