Association between pre-pregnancy BMI and neonatal weight outcomes in twin pregnancies resulting from assisted reproductive technology: a 10-year cohort study

Background/Objectives: To examine the effect of pre-pregnancy maternal body mass index (BMI) on neonatal weight outcomes of twin infants who were conceived by assisted reproductive technology (ART). Subjects/Methods: A 10-year (2006–2015) Chinese sample of 3431 mothers and their twin infants conceived by ART from a retrospective cohort were included. The effects of pre-pregnancy maternal BMI on gestational age and birth weight were assessed by generalized linear model and generalized estimating equation model. Results: Compared with a normal weight group, pre-pregnancy maternal underweight was associated with lower birth weight and increased risk of small for gestational age (SGA) in twins conceived by ART (birth weight: difference −59.22 g, 95% CI −93.16 to −25.27 g; SGA: RR 1.25, 95% CI 1.09 to 1.43). Pre-pregnancy maternal obesity was associated with higher birth weight and increased risk of preterm birth (birth weight: difference 65.82 g, 95% CI 10.66 to 120.99 g; preterm birth: RR 1.19, 95% CI 1.03–1.37). A nonlinear relationship between pre-pregnancy maternal BMI and SGA was observed. The risk of SGA decreased with the pre-pregnancy maternal BMI up to the turning point (BMI = 21) (RR 0.90, 95% CI 0.86–0.95). Conclusions: Among mothers undergoing ART, pre-pregnancy maternal obesity is associated with higher birth weight and higher risk of preterm birth for twin pregnancy, and pre-pregnancy maternal underweight is associated with lower birth weight and higher risk of SGA. Women preparing for ART should maintain a normal BMI to lower the chances of adverse neonatal outcomes.