Background Previous reports suggest that offspring of mothers who smoke during pregnancy have greater risk of developing depression in adolescence. However, it is unclear whether this association is due to intrauterine effects or to confounding by environmental or genetic factors. Using partner smoking during pregnancy as a negative control exposure for maternal smoking during pregnancy can help to strengthen causal inference. If maternal smoking influences offspring depression via intrauterine effects, this should be more strongly associated with offspring depression than partner smoking.

Methods We compared the associations of maternal and partner smoking during pregnancy with offspring depression in a UK longitudinal birth cohort, the Avon Longitudinal Study of Parents and Children. Maternal smoking during pregnancy was assessed by self-report in questionnaires administered at two times during pregnancy and once post birth. Partner smoking was assessed by both maternal reports and by self-report in questionnaires administered twice during pregnancy. Smoking during pregnancy was defined as any report of smoking at any time during pregnancy. Offspring depression was measured at age 18 years using the Computerised Interview Schedule-Revised (CIS-R), according to ICD-10 criteria. Analyses were conducted using logistic regression, adjusting for offspring age and sex, maternal age, parity, social class, maternal education, maternal depression, housing tenure and crowding and offspring smoking.

Results Of the 15,455 pregnant women enrolled into the study, full data for analysis was available for 3,100 women and their offspring, representing 68% of the 4,564 offspring who completed the CIS-R at age 18. In adjusted analyses, there was weak evidence that maternal smoking during pregnancy was associated with higher odds of offspring depression (OR 1.44, 95% CI: 0.99, 2.07), but no clear evidence that partner smoking was associated with offspring depression (OR 0.87, 95% CI: 0.63, 1.20). There was some statistical evidence that the estimate for maternal smoking differed from that of partner smoking (p = 0.04). Compared to offspring with neither mother or mother’s partner smoking during pregnancy, odds ratios for depression were 0.90 (95% CI: 0.61, 1.32), 1.79 (95% CI: 1.06, 3.01) and 1.70 (95% CI: 1.14, 2.52) for offspring with only their mother’s partners smoking during pregnancy, only their mothers smoking during pregnancy and both mothers and mother’s partners smoking during pregnancy respectively.

Conclusion Maternal smoking during pregnancy was more strongly associated with offspring depression at age 18 years  than partner smoking, although evidence for the association with maternal smoking was not strong after adjustment for confounders. This is consistent with a possible intrauterine effect of smoking on offspring depression.