The relationship between maternal alcohol drinking during pregnancy and birth weight was examined using prospectively collected data from 31,604 pregnancies. The percentage of newborns below the tenth percentile of weight for gestational age increased sharply with increasing alcohol intake. After adjustment for other risks, a reduction in mean birth weight was seen in drinkers compared with nondrinkers, ranging from 14 g in those drinking less than one drink each day to 165 g in those drinking three to five drinks each day. The adjusted odds ratio for producing a small-for-dates newborn compared with nondrinkers ranged from 1.11 in those drinking less than one drink daily to 1.96 in those drinking three to five drinks daily. Consuming at least one to two drinks daily was associated with a substantially increased risk of producing a growth-retarded infant. Conversely, consuming less than one drink daily had a minimal effect on intrauterine growth and birth weight.