Background: There is evidence of higher prevalence of asthma among children of parents with lower socioeconomic status. A potential explanation for this association is exposure to risk factors associated with deprivation, including smoking and obesity.

Aim: to quantify the extent to which maternal smoking and body mass index (BMI) can explain the association between maternal education and offspring asthma.

Methods: In a population-based study linking Swedish health and sociodemographic registers with the Register of Education, we identified 393,061 children with at least one younger sibling born within 5 years, both included in the Medical Birth Register (MBR). Exposure was low maternal education when the child was born. Asthma/wheeze at ages 2 and 5 years were defined based on a diagnosis of asthma combined with dispenses of asthma medication. Mediators were maternal smoking during pregnancy (SDP), BMI in early pregnancy and smoking when expecting the younger sibling (maternal smoking in early childhood) all from the MBR. All analyses were adjusted for maternal immigrant status.

Results: The association between maternal education and asthma/wheeze at age 2 years was OR=1.17 (95% CI: 1.15-1.20) and OR=1.06 (95% CI: 1.03-1.08) at 5 years. SDP could explain 17% of the association with asthma at age 2 years, maternal smoking in early childhood explained 17% and 20% was explained by maternal BMI. The corresponding estimates for asthma at 5 years were 6%, 0% and 54%, respectively.

Conclusion: Maternal BMI and smoking may be important factors to target in order to reduce asthma/wheeze and education-related health disparities in young children.