Conclusions
We provide evidence that is consistent with the hypothesis that maternal exposure to greenness during pregnancy increases the risk of developing asthma in childhood. Our results indicate that this is strongest when the exposure occurs in the spring season and in the third trimester of pregnancy. The mechanisms underlying these adverse effects are not fully understood, but they are likely to be related to high allergen concentrations, especially pollen from high vegetation biomass. Moreover, it is important to note that the timing of exposure during pregnancy may play a critical role in the effects of greenness on risk of developing asthma in childhood and later in life.