Association between pre-pregnancy BMI and weight gain during pregnancy and fetal growth restriction

Abstract There are many reports on high pre-pregnancy body mass index (BMI), but few reports on low pre-pregnancy BMI. However, while high BMI is a problem, low BMI is also a problem. Therefore, we conducted a study to determine the association between pre-pregnancy BMI, weight gain during pregnancy, and fetal growth restriction (FGR). This is an observational study using data from the Japan Environment and Children’s Study which includes 100,000 mothers. Singleton live births born between 37 and 41 weeks of gestational age were eligible. FGR was defined as a birth weight lower than the 10th percentile of birth weight for infant’s gestational age, sex, and parity. Pre-pregnancy BMI were categorized into four groups; underweight (BMI < 18.5 kg/m2), normal (BMI between 18.5 and 25 kg/m2), overweight (BMI between 25 and 30 kg/m2), and obese (BMI over 30 kg/m2). Using logistic regression analysis, association between FGR and pre-pregnancy BMI and weight gain during pregnancy were calculated. The results showed that low pre-pregnancy BMI increased the odds of FGR by factor of 2.33 (95% confidence interval 1.91–2.85). The interaction between pre-pregnancy BMI and weight gain during pregnancy was significant, indicating that the greatest benefit of weight gain was seen in underweight mothers.