LONGITUDINAL CHANGES IN THE RELATIONSHIP BETWEEN BODY-MASS INDEX AND PERCENT BODY-FAT IN PREGNANCY

Objective: To determine the longitudinal relationship between body mas s index (BMI) and percent body fat in women before and during pregnanc y. Methods: 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. Results: The correlation between BMI and pe rcent body fat before conception was r = 0.693 (P < .005); in early ge station 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 conf idence interval (CI) for percent body fat was 18.2, 26.5%. For the mea n BMI of 22.26 in early gestation, the predictive 95% CI for percent b ody fat was 20.0, 29.0%. In late gestation, the mean BMI was 26.04 wit h a predictive 95% CI for percent body fat 22.5, 30.8%. Conclusion: In nonobese women the correlation between BMI and percent body fat remai ns significant during pregnancy, although the 95% CI for predicting pe rcent body fat from the mean BMI ranges widely. Copyright (C) 1997 by The American College of Obstetricians and Gynecologists.