Combined Effects of Prepregnancy Body Mass Index and Weight Gain during Pregnancy on the Risk of Preterm Delivery

Background: The association between excessive gestational weight gain and preterm delivery is unclear, as is the association between low gestational weight gain and preterm delivery among overweight and obese women. Methods: Using data from the Pregnancy Risk Assessment Monitoring System in 21 states, we estimated the risk of very (20-31 weeks) and moderately (32-36 weeks) preterm delivery associated with a combination of prepregnancy body mass index (BMI) and gestational weight gain among 113,019 women who delivered a singleton infant during 1996-2001. We categorized average weight gain (kilograms per week) as very low (0.79). We categorized prepregnancy BMI (kg/m²) as underweight (