Maternal Smoking During Pregnancy and the Risk of Psychiatric Morbidity in Singleton Sibling Pairs

INTRODUCTION: Maternal smoking during pregnancy has been associated with an increased risk for psychiatric morbidity. We further studied this with Finnish siblings to control for genetic/familial factors. METHODS: From the Finnish Medical Birth Register, sibling pairs were selected as the first two children born 1987–1995 to the same mother (n = 150 168 pairs), along with information on maternal smoking (no smoking/smoking). Information on the children’s psychiatric diagnoses related to outpatient care visits (1998–2013) and inpatient care (1987–2013), and the mothers’ psychiatric morbidity (1969–2013) was derived from the Finnish Hospital Discharge Register. The first pair analysis compared siblings of mothers who only smoked in the first pregnancy (Quitters, 4.7%) and mothers who smoked in both pregnancies (Smokers, 9.6%); the second analysis included mothers who smoked only in the second pregnancy (Starters, 3.3%) and mothers who did not smoke in either pregnancy (Nonsmokers, 77.5%). Smoking information was missing for 5.0% of pairs. Psychiatric morbidity of the siblings and mother was included in the statistical analyses. RESULTS: The risk of psychiatric diagnoses was significantly lower for the second child of quitters (adjusted OR 0.77, 95% CI 0.72–0.83) compared to the risk among smokers. A higher risk for psychiatric diagnoses was found for the second child of starters (1.39, 1.30–1.49) compared to the risk among nonsmokers. The effect of smoking was more robust for externalizing diagnoses. CONCLUSIONS: Maternal smoking was independently associated with a higher risk for psychiatric morbidity in children, even when controlling thoroughly for genetic and familial factors. IMPLICATIONS: Maternal smoking during pregnancy has an independent effect on the risk of psychiatric morbidity in children, even after controlling for non-measurable genetic/familial factors by using a sibling pair design. The effect of maternal smoking was robust for externalizing diagnoses. Maternal smoking during pregnancy had an effect on diagnoses both in outpatient and inpatient care.