Prematurity and low birth weight as potential mediators of higher stillbirth risk in mixed black/white race couples.

Objective: Although births of multiracial and multiethnic infants are becoming more common in the United States, little is known about birth outcomes and risks for adverse events. We evaluated risk of fetal death for mixed race couples compared with same race couples and examined the role of prematurity and low birth weight as potential mediating risk factors.

Methods: We performed a retrospective cohort analysis using data from the 1998-2002 California Birth Cohort to evaluate the odds of fetal death, low birth weight, and prematurity for couples with a mother and father who were categorized as either being of same or different racial groups. Risk of prematurity (birth prior to 37 weeks gestation) and low birth weight (<2500 g) were also tested to see if the model could explain variations among groups.

Results: The analysis included approximately 1.6 million live births and 1749 stillbirths. In the unadjusted model, compared with two white parents, black/black and black/white couples had a significantly higher risk of fetal death. When all demographic, social, biological, genetic, congenital, and procedural risk factors except gestational age and birth weight were included, the odds ratios (OR) were all still significant. Black/black couples had the highest level of risk (OR 2.11, CI 1.77-2.51), followed by black mother/white father couples (OR 2.01, CI 1.16-3.48), and white mother/black father couples (OR 1.84, CI 1.33-2.54). Virtually all of the higher risk of fetal death was explainable by higher rates of low birth weight and prematurity.

Conclusions: Mixed race black and white couples face higher odds of prematurity and low birth weight, which appear to contribute to the substantially higher demonstrated risk for stillbirth. There are likely additional unmeasured factors that influence birth outcomes for mixed race couples.