Background The authors assessed whether socioeconomic inequalities in asthma symptoms were already present in preschool children and to what extent prenatal, perinatal and postnatal risk factors for asthma symptoms mediate the effect of socioeconomic status (SES).

Methods The study included 3136 Dutch children participating in the Generation R Study, a prospective cohort study. Adjusted ORs of asthma symptoms for low and middle SES (household income and maternal education) compared to high SES were calculated after adjustment for potential confounders and also adjusted for prenatal, perinatal and postnatal mediators at preschool age.

Results At age 1 year, low-SES children had a 40% lower risk of asthma symptoms compared to high-SES children (p<0.01). However, the risk of asthma symptoms in 3- and 4-year-old low-SES children was 1.5 times higher compared to their high-SES age mates (p<0.05). The positive associations at age 1 year were particularly modified by postnatal factors (up to 38%). In toddlers, prenatal factors explained up to 58% of the negative associations between SES and asthma symptoms.

Conclusions SES indirectly affects asthma symptoms at preschool age. The inverse association between SES and asthma symptoms emerges at age 3 years. This is particularly due to a high level of adverse prenatal circumstances in low-SES toddlers. Future research should evaluate public health programs (during pregnancy) to reduce socioeconomic inequalities in childhood asthma.