Associations between maternal age and socioeconomic status with smoking during the second and third trimesters of pregnancy: a register-based study of 932 671 women in Finland from 2000 to 2015

OBJECTIVES: This study aimed to explore the association between maternal age and smoking during the second and third trimesters of pregnancy across socioeconomic groups and to evaluate the interacting effect of maternal age and socioeconomic status on smoking with a view to informing public health interventions. DESIGN: This is a register-based study. SETTINGS: Data from the Finnish Medical Birth Register were cross-linked with background data from Statistics Finland. PARTICIPANTS: The information of 932 671 pregnant women who gave birth in Finland from 2000 to 2015. MAIN OUTCOME MEASURES: Maternal smoking during the second and third trimesters of pregnancy by occupation and maternal age. RESULTS: The proportion of women who smoked during the second and third trimesters of pregnancy was 10.5%. Using women 30–34 years as the reference group, adjusted ORs (aOR) and 95% CIs for smoking were 6.02 (5.81 to 6.24) in women below 20 years and 2.77 (2.71 to 2.84) in women 20 to 24 years. The prevalence of smoking across socioeconomic groups compared with upper-level employees increased, peaking for women in manual occupations (aOR 3.39, 95% CI 3.25 to 3.52) and unemployed women (aOR 4.49, 95% CI 4.30 to 4.68). Significant interactions on the additive scale with the relative excess risk due to interaction >2 were found for unemployed women aged 25–29 years and for teenage mothers and mothers aged 20–24 years across all socioeconomic groups, but not for self-employed women. CONCLUSIONS: Smoking during the second and third trimesters of pregnancy was most common among teenage mothers across all socioeconomic groups. The association between maternal age and smoking differed by socioeconomic status for young mothers. Interventions should address a wider range of maternal risk factors among young mothers with low socioeconomic status and simultaneously target a broader number of women who smoke during the pregnancy.