Inequality in smoking and related risk factors for smoking in expectant mothers - a nationwide Danish register-based study

Aims: Foetuses exposed to smoking during pregnancy are disadvantaged due to numerous adverse obstetric outcomes. This study aimed to examine 1) inequality in maternal smoking between subgroups of pregnant women and 2) significant risk factors of maternal smoking. Methods: Data were collected from Danish registries. Trends in maternal smoking within each study period, T1 (2000-2002) and T2 (2014-2016), were investigated by Poisson regression calculating prevalence proportion ratios, and trends between study periods were studied by adding an interaction term. The significance of risk factors for maternal smoking (low age, low education, living alone and having a moderate/severe mental health condition) were studied by interaction analysis on the additive scale. Results: The prevalence of maternal smoking decreased from 21% in 2000 to 7% in 2016. Decreases were found in all subgroups of maternal age, cohabitation status, educational level and mental health condition. However, large differences in smoking prevalence between subgroups were found, and inequality in maternal smoking increased from 2000 to 2016. The probability of maternal smoking increased with the addition of risk factors, and positive additive interactions were found for almost all combinations of multiple risk factors. Conclusions: Our results provide knowledge on risk factors and increasing levels of inequality in maternal smoking which points to a need for targeted interventions in relation to maternal smoking for subgroups of pregnant women in future smoking cessation programmes and in antenatal care.