Changes in cotinine levels during pregnancy.

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 (> or = 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.