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

Background: Obesity in pregnant women is 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 examined the effect of maternal pre-pregnancy BMI and weight gain during pregnancy on preterm birth. Methods: Data from the Collaborative Perinatal Project (CPP) on 45,824 pregnant women with singleton, live-born infants with no sever congenital anomalies was analyzed. Primary outcome variables included preterm (< 37 weeks of gestation), categorized into spontaneous preterm with and without premature rupture of membrane (PROM) and indicated preterm. Maternal BMI was categorized into underweight (BMI < 18.50), normal weight (BMI =1 8.50 - 24.99), overweight (BMI = 25.00 - 29.99), and obese (BMI >= 30.00). Multinomial regression analysis was conducted and OR and 95% CI were calculated. Results: The rate of spontaneous preterm birth with PROM among overweight women decreased with increasing weight gain but increased among women who had excessive weight gain. Similarly, a U-shaped rate of spontaneous preterm birth with and without PROM was observed in obese women. Gaining less weight was protective of spontaneous preterm with and without PROM among overweight and obese women compared to normal weight women. Among underweight women, gaining < 7 kg or 9.5-12.7 kg was associated with increased odds of indicated preterm birth. Appreciable differences were also observed in the association between pre-pregnancy BMI, gestational weight gain and the subtypes of preterm births among African Americans and Caucasian Americans. Conclusion: Reduced weight gain during pregnancy among overweight and obese women is associated with reduced spontaneous preterm birth with and without PROM. Health care professionals and public health workers should be aware of this risk and adhere to the 2009 IOM guideline that recommended reduced weight gain during pregnancy for obese and overweight women.