BACKGROUND

Studies of the association of prepregnancy body mass index (BMI) and preterm birth have been inconclusive, and no studies have examined the effect of central adiposity on risk. There is also uncertainty about optimal gestational weight gain among Black women.

METHODS

Using self-reported prospective data from the Black Women's Health Study, we investigated the relation of preterm birth to prepregnancy BMI (kg/m), waist circumference, and gestational weight gain among 7840 singletons born to black women, ages 21-44, during 1995-2003. We compared mothers of 1114 infants born 3 or more weeks early (597 spontaneous preterm births and 517 medically-indicated preterm births) with mothers of 6726 term infants. We used generalized estimating equation models to derive multivariable odds ratios (ORs) and 95% confidence intervals (CIs).

RESULTS

Relative to normal weight women (BMI: 18.5-24.9), underweight women (BMI: <18.5) were at increased risk of both preterm birth subtypes; obese women (BMI > or = 30.0) were at increased risk of medically-indicated preterm birth and very early spontaneous preterm birth (<32 weeks' gestation). Waist circumference, a measure of central adiposity, was positively associated with medically-indicated preterm birth. Among obese women, average gestational weight gain in the second and third trimesters of <0.4 or >0.6 lbs/wk was associated with an increased risk of both preterm birth subtypes.

CONCLUSIONS

Our data suggest that underweight increases risk of both preterm birth subtypes, while obesity increases risk of medically-indicated preterm birth and only a subgroup of spontaneous preterm births (<32 weeks' gestation). Central adiposity was an independent risk factor for medically-indicated preterm birth only. Among obese women, gestational weight gain within the range recommended by the 2009 Institute of Medicine report (0.4-0.6 lbs/week in the second and third trimesters) was associated with the lowest risk of preterm birth.