To investigate the association between breastfeeding duration and age of complementary food introduction with the development of childhood asthma. Previous studies with similar outcome measures have been mixed without a clear consensus, but large prospective cohort studies are lacking.

The study included patients in the Norwegian Mother and Child Cohort Study, a prospective population-based pregnancy cohort directed by the Norwegian Institute of Public Health.

Between 1999 and 2008, 41 020 mother-child pairs were enrolled for the primary outcome of asthma at the age of 7 years. Two baseline questionnaires were conducted at ∼18 weeks’ gestation followed by 4 additional questionnaires through the child’s first 7 years of life. Asthma diagnosis was defined as ≥2 asthma medications dispensed with at least 1 within 12 months of the child turning 7 years of age. Prescription information was obtained from the Norwegian Prescription Database.

In the primary analysis, 4.8% of the children had asthma at the age of 7 years on the basis of asthma medications dispensed. After confounders were accounted for, there was no difference in adjusted relative risk between asthma and breastfeeding duration (0 to <6 months, 6 to 12 months, or ≥12 months). In addition, no significant association was found between the introduction of complementary food and asthma at 7 years of age. In a secondary analysis, a significantly increased risk of early transient asthma was noted at 3 years of age in those who were breastfed for <6 months compared with ≥12 months (adjusted relative risk: 1.46 [95% confidence interval: 1.15–1.84]). In a subgroup analysis, children with parents without atopic disease were at an increased risk of asthma at 7 years old if they were breastfed for <6 months (P = .043).

In this large prospective study, no association was found between breastfeeding duration or the timing of complementary food introduction with asthma at the age of 7 years. There appears to be a higher risk of transient early asthma in those who were breastfed for <6 months, which is consistent with results in previous studies.

In this study, it is suggested that there is an association of longer breastfeeding duration with a lower risk of early wheezing that does not seem to extend into later childhood. Although previous cross-sectional and case-control studies have been highly variable, the conclusions drawn from this investigation are consistent with those of similar large cohort studies. In addition, children without a family history of atopic disease may be at an increased risk of persistent asthma with shorter breastfeeding duration. Additional studies are needed to further clarify this association. This study was limited by selection bias because the participants were of higher socioeconomic status and had lower rates of smoking than the general population.