Two hundred two poor, mainly black women were studied to assess the effects of selected social, behavioral, and biologic factors on birth weight. A path analysis was used to model hypotheses about the interrelationships among these variables. Six sociocultural factors had direct paths to quality of prenatal care: amount of insurance, delay in telling others about the pregnancy, attitudes toward health professionals, month of gestation in which the pregnancy was suspected, perception of the importance of prenatal care, and initial attitude toward the pregnancy. Together, these factors accounted for 64% of the variance in the quality of prenatal care received. Four variables had direct paths to birth weight: month in which the pregnancy was suspected, quality of prenatal care, hypertension, and substance abuse, which together accounted for 13% of the variance in birth weight. The key findings were the relative impact that the quality of prenatal care, especially the source of care, had on birth weight, and the potential for improving birth outcomes by addressing the negative effects of underlying social factors.