Interactive effect of race and marital status in low birthweight

Although unmarried mothers are at risk of delivering low-birthweight (LBW) infants, the meaning and significance of this variable need to be explored in depth. With data on 216,285 infants born to white and black mothers aged 10 to 49 years in Atlanta, Georgia, between 1980 and 1987, we examined the association of LBW and marital status and the effect of race on the association. Education and age were controlled in the analysis in an attempt to isolate the effect of race on the marital status and birthweight association. The crude LBW rate among infants born to unmarried mothers was about twice the rate among infants born to married mothers (132.8 vs 63.9 per 1000 live births). Adjustment singly for maternal race, age, and education gave risk ratios (unmarried vs married) of 1.50, 2.03, and 1.78, respectively. Simultaneous control for all factors led to a hierarchy of education-adjusted risk: unmarried black adult mothers had the highest risk of delivering an LBW infant (2.49), followed by married black adults (1.93), unmarried black teenagers (1.90), married black teenagers (1.67), unmarried white adults (1.65), unmarried white teenagers (1.35), married white teenagers (1.08), and married white adults (1.0; reference group, with an LBW rate of 51.2/1000 live births). Thus, these data demonstrate both a consistently higher risk for black women and an interactive effect of age on the association of marital status and LBW: unmarried status appears to increase the risk of LBW much more among adult women than among younger women. This finding has implications for research and prevention of LBW.