Background It has been suggested that factors in early life including exposure to allergens and microbes may influence the development of asthma.

Objective To identify risk factors for asthma in early childhood.

Methods Eight‐hundred and seventy‐one children of European mothers were enrolled at birth, of whom 385 (44.2%) were born small for gestational age (SGA) and 486 were appropriate for gestational age (AGA). Data were collected at birth, 12 months, 3.5 years of age (y) and 7 y. The outcome of interest (current wheeze) was defined as a positive response to the question: ‘Has your child had wheezing or whistling in the chest in the last 12 months?’

Results Participation rate was 85.4% at 1 y, 63.1% at 3.5 y and 68.0% at 7 y. The prevalence of asthma was 23.8% at 3.5 y and 18.1% at 7 y. Antibiotic use in the first year of life and day care in the first year of life were associated with increased risk of wheeze at 7 y [odds ratio (OR)=4.3 95% confidence interval (CI) (1.8–10.1) and OR=2.8 95% CI (1.2–6.5), respectively], but not at 3.5 y. Exposure to dogs was a risk factor for asthma at both ages [OR=2.1 95% CI (1.1–3.8)] as was sleeping on a used cot mattress in the first year of life [OR=1.8 95% CI (1.0–3.2)].

Conclusions There was a significant association between antibiotic use and day care in the first year of life and wheezing at 7 y but not at 3.5 y. This strengthens the argument that these factors increase the risk of asthma. We have also made the novel observation that sleeping on a used mattress in the first year of life is a risk factor for wheezing at 3.5 and 7 y.

Capsule summary This prospective study of 871 children made the novel observation that sleeping on a used mattress in the first year of life was a risk factor for wheezing at 3.5 and 7 y.