Heavy maternal drinking during pregnancy has consistently been linked to decreased intrauterine growth, but the effect of smaller amounts of alcohol is less clear. In this study, the relationship between fetal growth and "moderate" drinking by low-risk, nonsmoking prenatal patients is explored. The sample consists of 144 women seen for the first time at the prenatal clinic of University College Obstetrics Hospital, London, England, between July 1979 and May 1980 and meeting the following criteria: white, aged 19-35 years, 8-16 weeks gestation at first prenatal visit, nonsmoker, nonalcoholic, lower middle class or higher, and in general good health. Average daily consumption of 10 g of ethanol (about one drink) in the week prior to recognition of pregnancy is related to a decrease in infant birth weight of 225 g, after adjustment for gestational age, sex of child, and maternal age, weight, height, pregnancy weight gain, social class, gravidity, and parity. In addition, consumption of this amount in the week before first prenatal visit is related to a comparable decrease in birth weight for male but not for female infants. These findings suggest that risk of decreased intrauterine growth begins very early in pregnancy, and that fetal response to later alcohol use may vary with sex of the child.