Income incongruity, relative household income, and preterm birth in the Black Women's Health Study

Relative income may be a better predictor of health outcomes than absolute income. We examined two measures of relative income—income incongruity and relative household income—in relation to preterm birth in a study of U.S. Black women. Income incongruity is a measure that compares the median household income of an individual's residential area with that of others who have the same level of marital status and education, but who may live in different areas. Relative household income is a measure that compares an individual's household income with the median household income of her residential area. We used data collected biennially (1997–2003) from participants in the Black Women's Health Study: 6257 singleton births were included in the income incongruity analyses and 5182 in the relative household income analyses; 15% of the births were preterm. After adjusting for confounders, we found no overall association of income incongruity or relative household income with preterm birth. For relative household income, but not for income incongruity, there was suggestive evidence that neighborhood composition modified the association with preterm birth: higher relative household income was associated with higher risk of preterm birth in neighborhoods with a high percentage of Black residents, and higher relative household income was associated with lower risk in neighborhoods with a low percentage of Black residents.