1 The Generation R Study Group, Erasmus Medical Center, Rotterdam, The Netherlands.
2 Department of Public Health, Erasmus Medical Center, Rotterdam, The Netherlands.
3 PERFORM Centre, Concordia University, Montreal, Quebec, Canada.
4 Centre de recherche, Centre Hospitalier de L'Universite de Montreal, Montreal, Quebec, Canada.
5 École de santé publique, Université de Montréal, Montreal, Quebec, Canada.
6 Department of Program Development and Support, Cree Board of Health and Social Services of James Bay, Chisasibi, Quebec, Canada.
7 Division of Health Sciences, Warwick Medical School, University of Warwick, Coventry, UK.
8 Murdoch Children's Research Institute, Melbourne, Victoria, Australia.
9 Department of Paediatrics, The University of Melbourne, Melbourne, Victoria, Australia.
10 Department of Health, Medicine and Care, General Practice, Linköping University, Linköping, Sweden.
11 Crown Princess Victoria Children's Hospital and Division of Pediatrics, Department of Biomedical and Clinical Sciences, Linköping University, Linköping, Sweden.
12 Division of Respiratory Medicine and Allergology, Department of Pediatrics, Erasmus Medical Center, Rotterdam, The Netherlands.
13 Division of Neonatology, Department of Pediatrics, Erasmus MC, University Medical Center, Rotterdam, The Netherlands.
14 Department of Public Health, Erasmus Medical Center, Rotterdam, The Netherlands h.raat@erasmusmc.nl.

Objective: To examine the associations between maternal education and household income during early childhood with asthma-related outcomes in children aged 9-12 years in the UK, the Netherlands, Sweden, Australia, the USA and Canada.

Methods: Data on 31 210 children were obtained from 7 prospective birth cohort studies across six countries. Asthma-related outcomes included ever asthma, wheezing/asthma attacks and medication control for asthma. Relative social inequalities were estimated using pooled risk ratios (RRs) adjusted for potential confounders (child age, sex, mother ethnic background and maternal age) for maternal education and household income. The Slope Index of Inequality (SII) was calculated for each cohort to evaluate absolute social inequalities.

Results: Ever asthma prevalence ranged from 8.3% (Netherlands) to 29.1% (Australia). Wheezing/asthma attacks prevalence ranged from 3.9% (Quebec) to 16.8% (USA). Pooled RRs for low (vs high) maternal education and low (vs high) household income were: ever asthma (education 1.24, 95% CI 1.13 to 1.37; income 1.28, 95% CI 1.15 to 1.43), wheezing/asthma attacks (education 1.14, 95% CI 0.97 to 1.35; income 1.22, 95% CI 1.03 to 1.44) and asthma with medication control (education 1.16, 95% CI 0.97 to 1.40; income 1.25, 95% CI 1.01 to 1.55). SIIs supported the lower risk for children with more highly educated mothers and those from higher-income households in most cohorts, with few exceptions.

Conclusions: Social inequalities by household income on the risk of ever asthma, wheezing/asthma attacks, and medication control for asthma were evident; the associations were attenuated for maternal education. These findings support the need for prevention policies to address the relatively high risks of respiratory morbidity in children in families with low socioeconomic status.