Smoking during pregnancy has been associated with many adverse pregnancy outcomes. However, debate continues about whether smoking causes these outcomes. This review of the criteria of causation found that the relative risk of having a low-birth-weight baby is nearly doubled in women who smoke during pregnancy compared with nonsmoking women and that the relative risks of spontaneous abortion and perinatal and neonatal mortality are increased by about one-third. The magnitude of the long-term effects of maternal smoking on the physical and mental development of the offspring is small but measurable. The epidemiologic evidence associating maternal smoking with major adverse pregnancy outcomes in prospective and case-control studies displays a high degree of consistency. A dose-response gradient for smoking during pregnancy has not been clearly shown in relation to perinatal and neonatal mortality. However, there is strong support for a dose-response gradient in relation to low birth weight and spontaneous abortion and moderate support for long-term developmental effects. Although a biological causal mechanism has not been clearly delineated, considerable evidence indicates that the chemicals in tobacco smoke are capable of producing deleterious changes in the placenta and fetus.