The effect of nicotine content of cigarettes on fetal growth was studied. From April 1985 to April 1987, 86% of all pregnant women in two well-defined geographical areas responded to a questionnaire on social conditions and lifestyle factors in pregnancy. After excluding multiple births and women who gave birth after elective caesarean section, 10,485 pregnant women were available for the study. Results showed that not only smoking, but also nicotine content in cigarettes was related to reduced fetal growth as measured by birthweight, birth length, and head circumference. The timing of smoking during pregnancy played a role. Smoking before pregnancy or smoking early in pregnancy was not related to fetal growth, nor were the partners' smoking habits. The study corroborates the hypothesis that smoking reduces fetal growth and points to nicotine as one of the potential causal factors.