Changes in prepregnancy body mass index between the first and second pregnancies and risk of large-for-gestational-age birth.

OBJECTIVE: The objective of our study was to determine whether changes in prepregnancy body mass index (BMI) between the first 2 pregnancies is associated with increased risk for large-for-gestational-age (LGA) birth in the second pregnancy. STUDY DESIGN: A population-based, retrospective cohort analysis was performed using the Missouri 1989-1997 longitudinally linked data. Women with the first 2 consecutive singleton live births (n = 146,227) were analyzed. BMI (kilograms per square meter) was categorized as underweight (less than 18.5), normal (18.5 to 24.9), overweight (25 to 29.9), and obese (30 or greater), and LGA was defined as gestational age-specific birthweight greater than the 90th centile. Multivariate logistic regression analysis was used to estimate the odds ratio (OR) with 95% confidence interval (CI). Population attributable fraction for LGA births was calculated. RESULTS: Compared to women with normal BMI in their first and second pregnancies, overweight-overweight (OR 1.7, 95% CI 1.6, 1.8) and obese-obese (OR 2.3, 95% CI 2.2, 2.4) women in their first and second pregnancies were at increased risk of LGA births. Any increase in BMI from normal to obese between pregnancies increased LGA risk (OR 1.6 to 2.0), whereas any decrease in BMI from obese to normal attenuated the risk (OR 1.3 to 1.7). 17.1%, 13.2%, and 7.6% of LGA births are likely preventable had BMI not increased from first pregnancy underweight, normal, and overweight, respectively. CONCLUSION: In comparison with women with normal BMI in both pregnancies, any increase or decrease in prepregnancy BMI between normal and obese is associated with increased risk of LGA birth. A modification in the risk of LGA births by long-term maternal BMI status or maternal genetic factors appears likely