Summary: We measured maternal cotinine levels on residual sera of antenatal blood samples to biochemically document changes in smoking between early and late pregnancy. It was a random sample of 404 mothers who had both an early and late sample. Cotinine levels were used to categorize maternal smoking into nonsmoker (<15 ng/mL) and smoker (≥15 ng/mL) groups. Designated smokers were further partitioned into lighter (15–100 ng/mL) and heavier (>100 ng/mL) semiquantitative groupings. There was a positive cotinine result in 113 (28%) mothers in early pregnancy; of these smoking women, 35 (31%) had quit smoking by the time of their late pregnancy blood test and 28 (25%) had reduced their cotinine level by at least 25%. Many more lighter smokers had quit (59%) compared to heavier smokers (17%) (X2= 20.9, df = 1, p<0.001). By late pregnancy, 86 (21%) mothers were still defined as smokers. Almost 30% of pregnant women in this sample were smoking during early pregnancy declining to 21% in late pregnancy.