title: Increased risk of developing ankylosing spondylitis among first-born children.
abstract: OBJECTIVE
  Studies in mice have demonstrated an increased risk of ankylosing enthesopathy in earlier litters compared with later-born offspring. In humans, birth order and maternal age as risk factors for ankylosing spondylitis (AS) have not been investigated previously. This study was undertaken to investigate whether first-born children have a higher risk of AS than later-born children and whether maternal age at delivery is another risk factor.
  
  
  METHODS
  The birth order of 162 AS patients was compared with that of their healthy siblings, both for the total group and with stratification for maternal age at first delivery. Maternal age at the time of delivery of AS patients who were first-born children was compared with the mean maternal age at first delivery in the Dutch population.
  
  
  RESULTS
  The number of first-born children with AS was significantly higher than would be expected in case of an equal risk between first-born and later-born children (26 versus 20 for families with 2 children [P = 0.029] and 63 versus 47.6 for all families [P = 0.004]). Also, the mean maternal age at first delivery was lower in mothers of AS patients (24.8 years) compared with mothers of healthy controls (26.1 years).
  
  
  CONCLUSIONS
  Low birth order is a risk factor for AS in humans.
authors.affiliation: University Hospital, Vrije Universiteit, Amsterdam, The Netherlands.
authors.forename: P
authors.initials: P
authors.lastname: Baudoin
authors.forename: I E
authors.initials: IE
authors.lastname: van der Horst-Bruinsma
authors.forename: A J
authors.initials: AJ
authors.lastname: Dekker-Saeys
authors.forename: S
authors.initials: S
authors.lastname: Weinreich
authors.forename: P D
authors.initials: PD
authors.lastname: Bezemer
authors.forename: B A
authors.initials: BA
authors.lastname: Dijkmans
mesh_terms: D000293:Adolescent; D000328:Adult; D001722:Birth Order; D005260:Female; D006650:Histocompatibility Testing; D006801:Humans; D008297:Male; D008423:Maternal Age; D008875:Middle Aged; D018566:Pregnancy, High-Risk; D012307:Risk Factors; D013167:Spondylitis, Ankylosing / Q000453:epidemiology* / Q000209:etiology*