Maternal smoking during pregnancy is causally related to fetal growth retardation. Smoking more than doubles the risk for the birth of a small-for-gestational-age infant. About 30% of the Swedish pregnant population smoke in early pregnancy. Despite that practically all pregnant women in Sweden today are aware of the hazards of smoking, about 70% of the smokers continue to smoke during pregnancy. Women who continue to smoke during pregnancy differ in terms of socio-demographic characteristics from those who stop smoking during pregnancy.