Fetal weight gain at term: linear with minimal dependence on maternal obesity.

OBJECTIVE: Our purpose was to determine the shape of the function relating birth weight to gestational age between 37.0 and 42.0 weeks of pregnancy and to determine the effect of maternal obesity on term fetal weight gain in normal pregnancies.

STUDY DESIGN: Birth weights in 326 singleton, term gestations were studied in nondiabetic, nonsmoking white women who had well-dated, uncomplicated pregnancies.

RESULTS: Birth weight was a linear function of gestational age between 37.0 and 42.0 weeks. Mean (+/- SD) fetal weight gain was 12.7 +/- 1.4 gm per day (95% confidence interval 9.9 to 15.5 gm/day). Fetal weight gain was only 0.6 gm per day greater for obese pregnant women than for those of normal weight for height. Male fetuses gained weight 0.5 gm per day faster than females.

CONCLUSION: Fetal weight gain is a linear function of gestational age between 37.0 and 42.0 weeks of pregnancy for normal gestations. The effect of maternal obesity on fetal weight gain at term is small and not clinically significant.