One hypothesis is that fat soluble chemicals accumulating in breast milk might induce asthma. If this mechanism is correct, children higher in the birth order would be more likely to have asthma. However, additional adjustment for birth order did not affect the association between asthma and breast feeding (adjusted odds ratio: 1.212; 95 percent confidence interval: 1.057, 1.389; p < 0.01). This indicates that this hypothesis is not supported.