title: Reverse causality behind the association between reproductive history and MS.
abstract: BACKGROUND
  Possible associations between childbearing patterns and multiple sclerosis (MS) risk have been studied for a long time, with conflicting results. We aimed to investigate the influence of reproductive history on MS risk.
  
  
  METHODS
  Using a Swedish population-based case-control study involving incident cases of MS (1798 cases, 3907 controls), we calculated odds ratios (OR) for MS comparing parents with childless subjects together with 95% confidence intervals (CI) employing logistic regression.
  
  
  RESULTS
  Overall, there was an association between having children and reduced MS risk among both sexes. Subjects who had become parents within five years prior to the index year had a substantially reduced risk of developing MS (OR 0.6, 95% CI 0.5-0.8 for women, and OR 0.4, 95% CI 0.3-0.6 for men). No association between having children and MS risk was observed when more than 10 years had passed since the birth of the last child. We found no association between increasing offspring number and MS risk.
  
  
  CONCLUSIONS
  The observed association between reproductive history and MS risk is restricted to a limited time period preceding the index year, with similar findings in both sexes, which contradicts biologic impact of pregnancy on MS risk and argues in favor of reverse causality, i.e. that fecundity is affected by yet-undiagnosed MS.
authors.affiliation: Institute of Environmental Medicine, Karolinska Institutet, Sweden.
authors.forename: A K
authors.initials: AK
authors.lastname: Hedström
authors.forename: J
authors.initials: J
authors.lastname: Hillert
authors.forename: T
authors.initials: T
authors.lastname: Olsson
authors.forename: L
authors.initials: L
authors.lastname: Alfredsson
mesh_terms: D016022:Case-Control Studies; D005260:Female; D006801:Humans; D008297:Male; D009103:Multiple Sclerosis / Q000453:epidemiology*; D016017:Odds Ratio; D011247:Pregnancy; D017584:Reproductive History; D012307:Risk Factors
keywords: Multiple sclerosis; reproductive history; reverse causality