The effect of reproductive age (chronologic age at conception minus menarcheal age) and chronologic age on the risks of low birth weight, preterm delivery and intrauterine growth retardation were studied in 1198 primiparous women whose pregnancies ended in singleton live births at Yale-New Haven Hospital, 1980-1982. After adjustment for maternal race and other important confounding variables, neither young reproductive age (less than 8 years) nor young chronologic age (less than 20 years) were strongly related to any of the reproductive outcomes. Older chronologic age (greater than 29 years) was also not strongly related to any of these outcomes, but older reproductive age (greater than 15 years) was moderately associated with low birth weight (OR 1.9; 95% CI 0.9, 3.8) after adjusting for maternal race, religion and smoking status. In addition, extremely young reproductive age (less than 2 years) appeared to be associated with an increased risk of preterm delivery (OR 3.1; 95% CI 0.7, 14.6), but our sample was too small to adjust for confounding factors in this group of women. Reproductive age appears to reflect extremes in menarcheal age, whereas chronologic age does not, and this association may account for any relationship between reproductive age and pregnancy risk in primiparous women.