Objective To explore associations between maternal body mass index (BMI) in early pregnancy and childhood infections. Design Birth cohort study linked to primary care records. Setting Bradford, UK. Participants Live singleton births within the Born in Bradford cohort study between 2007 and 2011. Exposures Maternal BMI in early pregnancy. Main outcome measures The total number of infections between birth and ~14 years of age with subgroup analysis by infection type and age. Results A total of 9037 mothers and 9540 children were included in the main analysis. 45% of women were of Pakistani ethnicity and 6417 women (56%) were overweight or obese. There was an overall trend for an increasing infection rate with increasing maternal BMI. In adjusted models, only those with obesity grade 2–3 had offspring with significantly higher rates of infection during the first year of life (RR 1.12 (95% CI 1.05 to 1.20)) compared with women of healthy weight. However, by age 5 to <15 years, children born to overweight women (RR 1.09 (95% CI 1.02 to 1.16)), obese grade 1 women (RR 1.18 (95% CI 1.09 to 1.28)) or obese grade 2 women (RR 1.31 (95% CI 1.16 to 1.48)) all had significantly higher rates of infection compared with those born to healthy weight mothers. Respiratory tract and skin/soft tissue infections made up the majority of excess infections. Conclusions Maternal BMI was positively associated with rates of offspring infection in this study cohort, and suggests that we should be supporting women to achieve a healthy weight for pregnancy. Future research should investigate whether this is replicated in other populations, whether there is a causal association and the potential mechanisms and areas for intervention.