Mechanisms for maternal age differences in birth weight.

The authors studied three hypothesized explanations for reduced birth weights of infants born to US adolescent mothers--social disadvantage, biologic immaturity, and unhealthy behaviors during pregnancy. A hierarchical regression analysis was pursued to evaluate these explanations using data from the National Longitudinal Study of Youth on 1,754 first births between 1979 and 1983 to women aged 14-25 years at the time of birth. The birth weights of infants of mothers aged 14-17, 18-19, and 20-23 years were 133, 54, and 88 g less than for infants of mothers aged 23-25. The regression results indicate that the reduced birth weights of infants born to young mothers, particularly women aged 14-17, were related to their disadvantaged social environment. When adjustment was made for poverty and minority status, there were no maternal age differences in birth weight. The reduced birth weights were not related to the young woman's health behaviors during pregnancy or her biologic characteristics. Ethnicity, poverty status, age at menarche, maternal height, net maternal weight gain, and smoking during pregnancy had an independent effect on birth weight in this sample of young women.