Low pregravid body mass index as a risk factor for preterm birth: variation by ethnic group.

OBJECTIVE: To examine the association between pregravid body mass index (BMI) and preterm delivery among black, white, and Hispanic women.

METHODS: Preterm deliveries among 12,459 women (43.2% black, 39.3% white, and 17.5% Hispanic) enrolled in a large multicenter trial of preterm birth prevention were examined by pregravid BMI category (very low, less than 16.5; low, 16.5-19.7; normal, 19.8-26.0; high, greater than 26) and by pathway (all, early, late, spontaneous preterm labor, and premature rupture of membranes [PROM]).

RESULTS: More than one-fifth of both black (20.1%) and white (28.6%) women had low pregravid BMIs (less than 19.8), whereas only 11.7% of Hispanic women were under-weight. The overall prevalence of preterm delivery (gestational age less than 37 completed weeks) was 8.1% (10.3% in black, 7.3% in white, and 4.8% in Hispanic women). Among black and white women, bivariate analysis revealed an inverse linear association between pregravid BMI and the prevalence of all preterm deliveries (P < or = .001) and between pregravid BMI and the prevalence of late (33-36 weeks' gestation) preterm deliveries (P < .001). No such associations were observed for early (20-32 weeks' gestation) preterm delivery or among Hispanic women. Pregravid BMI was also associated inversely with spontaneous preterm labor among both black (P < or = .01) and white (P < .001) women, but not among Hispanic women. Logistic regression analysis (adjusting for the effects of maternal age, education, smoking, parity, previous preterm delivery, birth interval, and height) revealed that among black and white women, very low and low pregravid BMIs were associated with increased adjusted odds ratios for late (but not early) preterm delivery and for spontaneous preterm labor (but not PROM).

CONCLUSIONS: These observations suggest that low pregravid BMI is associated with an increase in the prevalence of late preterm delivery and of spontaneous preterm labor among black and white, but not Hispanic, women.