title: [Sibships characteristics, influenza and risk of schizophrenia. A population-based cohort study].
abstract: BACKGROUND
  To address the hypothesis that infections, perhaps prenatal exposure to influenza virus, might increase the risk of schizophrenia we studied the possible association between schizophrenia risk and birth order, sibship size, interval between siblings, and influenza prevalence.
  
  
  METHODS
  We established a population-based cohort of 1,746,366 persons born to Danish woman who themselves were born since 1935 using data from the Civil Registration System. Schizophrenia in cohort members (n = 2669) and their parents was identified by linkage with the Danish Psychiatric Case Register. Influenza notifications per month in Denmark were obtained from the National Board of Health and Statens Serum Institut.
  
  
  RESULTS
  We found no association between birth order and schizophrenia risk or between schizophrenia risk and influenza prevalence during any month of prenatal life. Coming from a large sibship and having a short interval to the nearest older or younger sibling was associated with an increased risk of schizophrenia.
  
  
  CONCLUSIONS
  Our findings do not add support to the hypothesis that schizophrenia is associated with prenatal exposure to influenza virus or other common infections, but are compatible with the hypothesis that environmental exposure, perhaps to common infections in childhood, may be a risk factor.
authors.affiliation: Statens Serum Institut, afdeling for epidemiologisk forskning. twe@ssi.dk
authors.forename: T
authors.initials: T
authors.lastname: Westergaard
authors.forename: P B
authors.initials: PB
authors.lastname: Mortensen
authors.forename: C B
authors.initials: CB
authors.lastname: Pedersen
authors.forename: J
authors.initials: J
authors.lastname: Wohlfahrt
authors.forename: M
authors.initials: M
authors.lastname: Melbye
mesh_terms: D000293:Adolescent; D000328:Adult; D001722:Birth Order; D002648:Child; D015331:Cohort Studies; D003718:Denmark; D004781:Environmental Exposure / Q000009:adverse effects; D005191:Family Characteristics; D005260:Female; D006801:Humans; D007251:Influenza, Human / Q000150:complications*; D008297:Male; D011247:Pregnancy; D011251:Pregnancy Complications, Infectious / Q000821:virology; D011297:Prenatal Exposure Delayed Effects; D012559:Schizophrenia / Q000209:etiology* / Q000235:genetics / Q000821:virology