Misclassification of maternal smoking status and its effects on an epidemiologic study of pregnancy outcomes

Reliance on self-reported smoking status among pregnant women can result in exposure misclassification. We used data from the Calcium for Preeclampsia Prevention trial, a randomized study of nulliparous women conducted from 1992 to 1995, to characterize tobacco exposure misclassification among women who reported at study enrollment that they had quit smoking. Urinary cotinine concentration was used to validate quit status, and factors associated with exposure misclassification and the effects of misclassification on associations between smoking and pregnancy outcomes were evaluated using logistic regression. Of 4,289 women enrolled, 508 were self-reported smokers and 771 were self-reported quitters. Of 737 self-reported quitters with a valid cotinine measurement, 21.6% had evidence of active smoking and were reclassified as smokers. Women who reported having quit smoking during pregnancy were more likely to be reclassified than women who reported quitting before pregnancy (p