[Association between maternal pre-pregnancy body mass index and clinical pregnancy outcomes and perinatal outcomes of single live birth in frozen-thawed embryo transfer cycles].

Objective: To investigate the relationship between maternal pre-pregnancy body mass index (BMI) and clinical pregnancy outcomes and singleton live birth outcomes in frozen-thawed embryo transfer (FET) cycles. Methods: A total of 45 856 patients who underwent FET in Henan Provincial People's Hospital and the Second Affiliated Hospital of Zhengzhou University from January 2013 to December 2023 were retrospectively analyzed, including 15 960 singleton live births. Based on the female's BMI, they were divided into the underweight group (BMI<18.5 kg/m2), the normal weight group (BMI 18.5-<25.0 kg/m2), the overweight group (BMI 25.0-<30.0 kg/m2), and the obese group (BMI≥30.0 kg/m2). Multivariate linear models and logistic regression models were used to evaluate the effects of maternal pre-pregnancy BMI on clinical pregnancy outcomes and singleton live birth outcomes. Generalized additive model was used to fit the curve between maternal pre-pregnancy BMI and perinatal outcomes and to determine the threshold. Results: (1) Among 45 856 FET patients, there were 2 390 cases (5.21%) in the underweight group, 30 177 cases (65.81%) in the normal weight group, 10 835 cases (23.63%) in the overweight group, and 2 454 cases (5.35%) in the obesity group. The early abortion rate of overweight and obesity group, the clinical pregnancy rate and live birth rate of underweight group, the clinical pregnancy rate of obesity group were higher than those of normal weight group, and the live birth rate of overweight group was lower than that of normal weight group, and the differences were statistically significant (all P<0.01). (2) Among the 15 960 singleton live birth FET patients, 926 (5.80%) were in underweight group, 10 620 (66.54%) were in normal weight group, 3 561 (22.31%) were in overweight group, and 853 (5.34%) were in obesity group. The incidences of preterm birth, extremely preterm birth, macrosomia and large for gestational age (LGA) in the overweight and obese groups were significantly higher than those in the normal weight group (all P<0.05). The incidence of small for gestational age (SGA) infants in the underweight group was significantly higher than that in the normal weight group, while the incidences of preterm birth, macrosomia and LGA infants were all significantly lower than those in the normal weight group (all P<0.05). (3) Maternal pre-pregnancy overweight and obesity were associated with early miscarriage (aOR=1.20, 1.37), preterm birth (aOR=1.49, 1.70), very preterm birth (aOR=1.64, 2.09), macrosomia (aOR=1.87, 2.57), and LGA (aOR=1.55, 1.95), and the risks of live birth (aOR=0.87, 0.80) and low birth weight infant (aOR=0.65, 0.51) were significantly decreased (all P<0.01). Maternal pre-pregnancy underweight had no significant effect on early miscarriage, clinical pregnancy and live birth (all P>0.05), but the risks of preterm birth (aOR=0.69), macrosomia (aOR=0.34) and large for gestational age (aOR=0.54) were significantly reduced; the risk of SGA (aOR=1.52) was significantly increased (all P<0.05). (4) The results of curve fitting and threshold effect analysis showed that the risk of very preterm birth (aOR=1.09, 95%CI: 1.05-1.13; P<0.001) increased with the increase of maternal pre-pregnancy BMI, and the risk of low birth weight infants (aOR=0.93, 95%CI: 0.90-0.96; P<0.001) decreased with the increase of maternal pre-pregnancy BMI, and the curve fitting was linear. The relationship between preterm birth, macrosomia, SGA, LGA and maternal pre-pregnancy BMI were nonlinear, and the thresholds were all 25.0 kg/m2. Conclusions: Maternal overweight and obesity before pregnancy could increase the early abortion rate, decrease the live birth rate, and increase the risk of preterm birth, very preterm birth, macrosomia and LGA of single live birth infants in FET. Women with pre-pregnancy BMI≤25.0 kg/m2 could obtain better perinatal outcomes.