Older maternal age and infant mortality in the United States

We used data from the National Infant Mortality Surveillance project to examine the effect of older maternal age on infant mortality for the 1980 United States birth cohort. The 1,579,854 births and 14,591 deaths of singletons who were black or white and whose mothers were 25-49 years of age were included. Direct standardization was used to calculate birth-weight-adjusted relative risks of neonatal and postneonatal mortality, using the birth weights of infants with maternal age 25-29 as the standard. We found that the risk of infant mortality was nearly equal for infants born to mothers 25-29 and 30-34 years of age; infants born to mothers 35-39 years of age were at a slightly elevated (18% higher) risk, and those born to mothers 40-49 years of age were at a much more elevated (69% higher) risk. Among whites, the higher neonatal mortality associated with a maternal age of 35-39 was mostly due to an increased prevalence of low birth weight; among blacks, it was due to higher birth-weight-specific risks. Neither white nor black postneonatal mortality risks were much elevated until a maternal age of 40-49, and this last elevation was mostly due to higher birth-weight-specific risks. These findings suggest that infertility and fetal mortality aside, and considering only the effect on infant mortality, it is relatively safe for women to postpone childbearing into their middle, and perhaps late, thirties.