Aim To determine the magnitude of the epidemiological association of childhood obesity with childhood asthma and pregnancy smoking.

Methods Retrospective cross-sectional surveys were conducted in 1998 and 2006 using a standardised respiratory health survey instrument to determine the association of obesity risk with asthma prevalence and maternal smoking during pregnancy. A total of 3040 schoolchildren (n  =  1964 in 1998 and n  =  1076 in 2006) aged 5–11 years in Merseyside were enrolled using a parent-completed questionnaire and child height and weight were measured.

Results Mean birth weight was 3455 + 695 g and mean body mass index 17.1 ± 1.8 kg/m². The prevalence of overweight was 22.3% and obesity was 11.4%. These prevalence estimates were 31.7% and 15.3%, respectively, among asthmatic children (p = 0.08 and p = 0.06, respectively, compared with non-asthmatic children) and 29.2% and 15.3% in children whose mothers smoked during pregnancy (p = 0.07 and p = 0.01, compared with children born to non-smoking mothers). Regression analysis adjusting for household socioeconomic status, preterm birth, low birth weight and child age showed significant associations of combined childhood asthma and maternal smoking during pregnancy on obesity risk (adjusted odds ratio 1.9, 95% CI 1.4 to 2.4, p = 0.001).

Conclusion Childhood asthma and maternal pregnancy smoking were associated with the development of obesity in childhood. These characteristics should be more widely recognised as markers associated with childhood obesity. The biological mechanisms underlying the association with prenatal smoking require further study.