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This study estimates the effects of 23 factors on the prevalence of premature labor and fetal growth retardation across the entire birthweight spectrum. We studied risk factors for premature labor within the domain of babies of appropriate size for their gestational age (N = 9,490). We also studied risk factors for fetal growth retardation among babies born at term (N = 10,889). Preterm labor was associated independently with young maternal age, low prepregnant weight, low weekly weight gain, nulliparity, previous preterm birth, histories of two or more induced abortions, spontaneous abortions, or stillbirths, uterine exposure to diethylstilbestrol (DES), incompetent cervix, uterine anomaly, and pyelonephritis. Odds ratios ranged from 1.7 to 5.9. Fetal growth retardation, as estimated by small-for-gestational-age birth, was associated independently with black race, young maternal age, short maternal height, low prepregnancy weight, low weekly weight gain, and smoking, as well as nulliparity, previous preterm birth, three or more abortions, uterine anomaly, and uterine exposure to DES. Odds ratios ranged from 1.6 to 2.8. Our study shows the importance for etiologic understanding of separating diverse routes to low birthweight and considering the occurrence of adverse pregnancy outcomes along the entire birthweight spectrum. The findings reinforce the need for comprehensive control of confounding in studies of pregnancy outcome.