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Changes in objectively measured smoking in pregnancy by time and legislative changes in Finland: a retrospective cohort study

28 Nov 2016


To study the changes in prevalence, characteristics and outcomes of pregnant smokers over time and legislative changes.

Design and setting

Retrospective nationwide cohort.


Our study consisted of 9627 randomly selected pregnancies from the Finnish Maternity Cohort (1987–2011), with demographic characteristics and pregnancy and perinatal data obtained from the Medical Birth Registry and early pregnancy serum samples analysed for cotinine levels. Women were categorised based on their self-reported smoking status and measured cotinine levels (with ≥4.73 ng/mL deemed high). Data were stratified to three time periods based on legislative changes in the Tobacco Act.

Primary and secondary outcome measures

Prevalence of pregnant smokers and demographics, and perinatal and pregnancy outcomes of pregnant smokers over time.


Overall, 71.6% of women were non-smokers, 16.2% were active cigarette smokers, 7.7% undisclosed smoking but had high cotinine levels and 4.5% were inactive cigarette smokers. The prevalence of active cigarette smokers decreased from mid-1990s onwards among women aged ≥30 years, probably due to the ban of cigarette smoking in most workplaces. We observed no changes in the prevalence of inactive smokers or women who undisclosed smoking by time or legislative changes.

Women who undisclosed smoking had similar characteristics and perinatal outcomes as inactive and active smokers. Compared with non-smokers, women who undisclosed smoking were more likely to be young, unmarried, have a socioeconomic status lower than white-collar worker and have a preterm birth.


Women who undisclosed smoking were very similar to pregnant cigarette smokers. We observed a reduction in the prevalence of active pregnant cigarette smokers after the ban of indoor smoking in workplaces and restaurants, mostly among women aged ≥30 years.

Click here to view the full article which appeared in BMJ Open