Longitudinal changes in the relationship between body mass index and percent body fat in pregnancy.

Abstract

To determine the longitudinal relationship between body mass index (BMI) and percent body fat in women before and during pregnancy. Twenty-seven healthy, nonobese women were evaluated before conception, in early gestation (12-17 weeks), and in late gestation (33-36 weeks). Height and weight were measured and BMI was calculated. Percent body fat was estimated using hydrodensitometry with correction for residual lung volume. The correlation between BMI and percent body fat before conception was r = 0.693 (P < .005); in early gestation it was r = 0.723 (P < .005) and in late gestation r = 0.633 (P < .005). The mean pregravid BMI was 21.54 and the 95% predictive confidence interval (CI) for percent body fat was 18.2, 26.5%. For the mean BMI of 22.26 in early gestation, the predictive 95% CI for percent body fat was 20.0, 29.0%. In late gestation, the mean BMI was 26.04 with a predictive 95% CI for percent body fat 22.5, 30.8%. In nonobese women the correlation between BMI and percent body fat remains significant during pregnancy, although the 95% CI for predicting percent body fat from the mean BMI ranges widely.