A large number of smoking women cannot quit their habit when they become pregnant. Preliminary evidence suggests an enhanced nicotine clearance rate in late pregnancy. To evaluate the change in nicotine metabolism in late pregnancy, we undertook a prospective cohort study of a large, diverse group of pregnant women recruited from four Montreal maternal hospitals. Smoking histories were obtained by structured questionnaires administered at 24 to 26 weeks of gestation and postpartum. Hair concentrations of nicotine and cotinine were measured by immunoassays for each trimester based on sectioning the hair and assuming average hair growth of 1 cm per month. A strong correlation was observed between average number of cigarettes smoked per day and hair nicotine and cotinine in all three trimesters. A significant decrease in hair nicotine was observed among steady smokers from the first to third trimester paralleled by a significant increase in hair cotinine. The ratio of hair nicotine:cotinine decreased significantly from the first to the third trimester. Increased nicotine metabolism in late pregnancy results in lower systemic exposure to nicotine. This phenomenon may explain why many pregnant women feel the urge to continue smoking and why standard-dose nicotine replacement therapy has not been effective in reducing smoking during pregnancy in several clinical trials.