tentially lead to a considerable gap between the self-reported and actual smoking status, as measured by biochemical markers (Fendrich et al. [@CR10]; Pérez-Stable et al. [@CR24]; Secker-Walker et al. [@CR34]; Ford et al. [@CR13]). From the viewpoint of medical practice, the consequence of misreported status may be that a doctor or nurse will not be able to provide proper care if they are not aware of the actual exposure to tobacco smoke (Russell et al. [@CR30]; Connor Gorber et al. [@CR3]). Being aware that there may be a high incidence of unreported smoking levels and aiming at significantly reducing misclassification error confirmed with biochemical tests in earlier surveys, we used a screening question which provided respondents with more answers than a straightforward 'yes' or 'no' (Cummings et al. [@CR5]; Pérez-Stable et al. [@CR25]). Respondents had several options to best describe their behavior concerning smoking in general, and smoking/quitting in pregnancy. Nine questions were addressed only to the women who continued smoking during pregnancy, and four to those who ceased smoking before or during pregnancy.

Biochemical verification of smoking status {#Sec7}
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We decided to use a biochemical marker of cigarette smoking, such as exhaled carbon monoxide to verify self-reported smoking status (Florek et al. [@CR11]). Following the interview, those patients who had given consent were tested with a meter measuring carbon monoxide in exhaled air (MicroCO, MicroMedical, UK). According to the manual provided by manufacturer, a result of \>7 ppm CO in exhaled air indicates current smoking and this cutoff value was used in this study. The CO breath test was carried out in 4145 pregnant women; 98 women refused the test and 269 were not tested because the device was not available or was not working properly in the time of testing. Overall among 3587 women who self-reported non-smoking status at the time of the test, 6.0 % were not confirmed as a non-smoker by a carbon 