Pre-pregnancy BMI and weight gain: Where is the tipping point for risk of preterm birth?

Background: Obesity in pregnant women is becoming a major problem affecting both the mother and her offspring. Literature on the effect of obesity on preterm birth is inconsistent and few studies have investigated the influence of weight gain during pregnancy. This study examines the effect of maternal pre-pregnancy BMI and weight gain during pregnancy on preterm birth.

Methods: Data from the Collaborative Perinatal Project (CPP) on 52,574 pregnant women with singleton, live-born infants was analyzed. The main outcome variable, gestational age, was dichotomized as preterm (< 37 weeks) or full-term (>= 37 weeks) pregnancy. Maternal BMI was categorized into underweight (BMI < 18.50), normal weight (18.50 to 24.99), overweight (25.00 to 29.99), and obese (BMI >= 30.00). Multiple logistic regression was conducted and OR and 95%CI were calculated.

Results: This study reported a statistically significant interaction between pre-pregnancy BMI and weight gain during pregnancy. Among overweight and obese women, the odds of preterm birth decreases with decreasing weight gain during pregnancy. Among obese women who gained 36lbs or more during pregnancy, the odds of preterm birth was 1.64 times higher than normal weight women [OR=1.64, 95%CI=1.04-2.60].

Conclusion: Excessive weight gain during pregnancy among overweight and obese women is associated with preterm birth. Health care professionals and public health workers should be aware of this risk and educate pregnant women on the appropriate amount of weight gain for their BMI.