Self-reported nicotine exposure and plasma levels of cotinine in early and late pregnancy

Background. The accuracy of maternal self-reported smoking information, especially exposure to environmental tobacco smoke, has been questioned. This study aimed to validate self-reported smoking, smoking cessation, and environmental tobacco smoke exposure in early and late pregnancy, using the biomarker cotinine as the gold standard. Methods. Prospective cohort study of 953 pregnant Swedish women between 1996 and 1998. In-person interviews and cotinine measurements were performed at 6-12 and 31-34 completed weeks of gestation. All women were asked about nicotine exposures throughout pregnancy, including cigarette smoking, oral snuff, nicotine replacement therapy, and environmental tobacco smoke exposure. Results. The validity of self-reported daily smoking was high in early and late pregnancy. However, among women reporting smoking cessation before the first interview and between the first and second interviews, 13% and 25% misreported active smoking, respectively. According to cotinine measurements, 22% of non-smoking women were exposed to environmental tobacco smoke in early pregnancy, and 8% were exposed in late pregnancy. Self-reported information on environmental tobacco smoke exposure in early and late pregnancy misclassified most exposed women as unexposed. Conclusion. The results of this study indicate that self-reported smoking information among pregnant women can be trusted. However, among women reporting smoking cessation during pregnancy, the misclassification rate increased with recency of quitting. Environmental tobacco smoke exposure was common among non-smokers, and the low validity of self-reported environmental tobacco smoke exposure suggests that future studies on environmental tobacco smoke exposure and risks of pregnancy outcomes may have to rely on biomarker assessments.