title: Seasonal variation in the prevalence of Down syndrome at birth: a review.
abstract: OBJECTIVE
  Many studies on seasonality in Down syndrome (DS) have been performed and have come to different conclusions. It is suggested that seasonal variation in hormone production by the hypothalamus-pituitary-ovarian axis just before ovulation leads to seasonality in conception rates of DS. This study aimed to determine whether there is seasonal variation in the prevalence of DS at birth as a proxy for seasonality in DS at conception.
  
  
  METHODS
  All the English and Dutch articles on this topic were reviewed. Articles published between 1966 and January 1996 were traced by Medline, and by the reference lists.
  
  
  RESULTS
  Twenty articles met the criteria for inclusion. Although seven of these studies reported seasonality in DS prevalence, no consistent seasonal pattern was found in DS at birth in these studies, or in the remaining studies. A seasonal pattern could not have been masked by the effects of maternal age, induced abortions, shortened gestation, or misclassification of DS.
  
  
  CONCLUSIONS
  Seasonality in the prevalence of DS at birth does not exist. Evidence did not support the suggestion that DS occurrence is related to seasonality in hormone production.
authors.affiliation: Department of Medical Informatics, Epidemiology and Statistics, University of Nijmegen, The Netherlands.
authors.forename: A M
authors.initials: AM
authors.lastname: Stolwijk
authors.forename: P H
authors.initials: PH
authors.lastname: Jongbloet
authors.forename: G A
authors.initials: GA
authors.lastname: Zielhuis
authors.forename: F J
authors.initials: FJ
authors.lastname: Gabreëls
mesh_terms: D004314:Down Syndrome / Q000453:epidemiology*; D004739:England / Q000453:epidemiology; D006728:Hormones / Q000096:biosynthesis; D006801:Humans; D007231:Infant, Newborn; D009426:Netherlands / Q000453:epidemiology; D015995:Prevalence; D012621:Seasons
chemical_list: D006728:Hormones