Maternal prepregnancy underweight and risk of early and late stillbirth in black and white gravidas.

Objective: The association between underweight and stillbirth remains poorly defined, especially across racial/ethnic sub-populations. We investigate the association of pre-pregnancy underweight on the risk for early and late stillbirth among black and white mothers.

Methods: We conducted analysis on the Missouri maternally linked data files covering the period 1989-1997 inclusive. Using body mass index (BMI), we categorized mothers as underweight (BMI <18.5) and normal weight (BMI = 18.5-24.9). By applying logistic regression modeling with adjustment for intracluster correlation, we estimated the risk for total, early (-28 weeks of gestation), and late stillbirth (>28 weeks of gestation) among black and white mothers.

Results: A total of 1808 cases of stillbirth were registered. The rate of stillbirth among white mothers was 3.7 per 1000, while the rate among blacks was 7.1 per 1000. Underweight black mothers had comparable risk for total (OR, 0.9; 95% CI, 0.7-1.2), early (OR, 1.1; 95% CI, 0.8-1.5), and late stillbirth (OR, 0.8; 95% CI, 0.5-1.2) as compared to their normal-weight counterparts. By contrast, underweight white gravidas had a 30% reduced likelihood (OR, 0.7; 95% CI, 0.6-0.9) for late stillbirth as compared to normal-weight white mothers. However, the risks for total and early stillbirth among underweight white mothers were similar to those of normal-weight white mothers.

Conclusion: Low prepregnancy BMI has similar effects on fetal survival in both blacks and whites except for late stillbirth. The underweight white survival advantage over blacks in late pregnancy could probably be due to greater access for identified white at-risk groups to effective obstetrical interventions as previously reported.