Effect modification of birth weight outcomes and gestational weight gain by pre-pregnancy body mass index in a diverse population

Obesity and pregnancy weight gain are changing the face of perinatal outcomes. Here I examine how pre-pregnancy body mass index (BMI) and gestational weight gain influence extreme birth weights in Los Angeles County. The 2007 vital records of live births were reviewed to identify infants born preterm low weight (<37 wks gestation and <2500g), term low weight (>37 wks gestation and <2500g), or macrosomic (>4000 g, also >4500g). New variables collected on the birth certificate and the new Institute of Medicine Guidelines (IOM, 2009) were used. Using logistic regression I examined the influence of BMI and gestational weight gain on these outcomes while controlling for other risk factors. Gaining less or gaining more weight than recommended increased the odds for term low birth weight (TLBW) and macrosomia, respectively. BMI modified these associations in opposing directions: highest effect for TLBW among underweight women, [Adjusted Odds Ratio: 2.5 (95% confidence interval: 1.8, 3.4)], followed by normal weight women, 1.7 (1.5, 1.9); highest effect for macrosomia (>4500g) among normal weight women, 3.0 (2.3, 3.8), followed by obese women, 2.3 (1.7, 3.2). Obesity increased the odds of preterm low weight births, 1.2 (1.1, 1.3). Even though African American women were more often overweight and obese (26.2% and 22.5% respectively) and gained more weight during pregnancy (52.2%), they did not exhibit the highest risk for macrosomia (>4000g: 5.2%). Instead, White and Latino women had the highest risks (9.6% and 7.7% respectively). Weight gain during pregnancy affects BMI subgroups and impacts ethnic groups differently.