Studies of pre-eclampsia and childhood asthma are conflicting, and none have performed a formal mediation analysis of preterm birth.We examined the association between pre-eclampsia and asthma at 7 years using national registries, including all births in Norway from 1999 to 2006 (n=406 907), and a subsample of children in the Norwegian Mother and Child Cohort Study (MoBa) (n=45 028) using log-linear regression. We performed a mediation analysis of preterm birth, and a sibling comparison to evaluate unobserved confounding.There was a positive association between pre-eclampsia and asthma in the registry study, with an adjusted relative risk of 1.31 (95% CI 1.22-1.41), but not in MoBa, which had an adjusted relative risk of 1.19 (95% CI 0.99-1.44). The odds ratios for the direct effect not mediated through preterm birth and the indirect effect in the registry linkage were 1.19 (95% CI 1.10-1.29) and 1.12 (95% CI 1.11-1.14), respectively. The sibling comparison indicated no association between pre-eclampsia and asthma (adjusted OR 1.07, 95% CI 0.87-1.33).In this large study, which used different datasets and analytic approaches, there was little evidence for an association between pre-eclampsia and childhood asthma. The association was weak and largely explained by pre-term birth and confounders shared by siblings.
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