Diverging associations of maternal age with low birthweight for black and white mothers.

BACKGROUND In the US, the risk of low birthweight appears to increase more quickly with maternal age for black women than it does for white women. Our aim was to investigate correlates and causes of the racial/ethnic divergence in low birthweight by maternal age. METHODS We analysed birth certificate data from 96 887 singleton births to black and white mothers in Chicago from 1994 to 1996 to determine if the association between maternal age and low birthweight differed by maternal ethnicity, marital status, and socioeconomic disadvantage. The association of maternal age with risk of low birthweight was examined, considering especially the interactions of maternal age with maternal race/ethnicity, marital status, education, and neighbourhood poverty. RESULTS In unadjusted data, the risk of low birthweight rose steeply with maternal age for black, but not white, mothers. Adjustment for the main effects of maternal education, marital status, adequacy of prenatal care, cigarette smoking, and neighbourhood poverty accounted for some of the age-related excess risk of black mothers. Further adjustment for interactions of maternal age with these risk factors eliminated the ethnic divergence in age slopes. The fully adjusted analysis indicated that the risk of low birthweight rises more quickly with maternal age for disadvantaged women, regardless of race/ethnicity. CONCLUSION This analysis suggests that hardships act cumulatively to threaten reproductive health. The particularly steep increase in risk of low birthweight with increasing maternal age for black women is explained by the high prevalence of disadvantage in this population.