Data from 8193 pregnancies were analyzed for clues to the mechanisms responsible for the fetal growth retardation associated with maternal cigarette smoking. The retardation proved independent of mothers' prepregnancy body weights and pregnancy weight gains; therefore, it was not due to undernutrition. As the retardation was present in intrapair comparisons of siblings whose mothers had smoked in only 1 of their pregnancies, it was not due to genetic factors. Cigarette smoking causes permanent damage to uterine arteries; however, this did not retard fetal growth because retardation was absent when mothers stopped smoking during pregnancy. Results of the study exclude undernutrition, genetic factors, and placental underperfusion as major contributing factors to the fetal growth retardation associated with cigarette smoking.