title: Birth order and ankylosing spondylitis: no increased risk of developing ankylosing spondylitis among first-born children.
abstract: OBJECTIVE
  In the HLA-B27 transgenic mouse model the first litters have been shown to have a higher percentage of diseased offspring than later litters. First-born children (n = 162) have also been shown to have a higher risk of ankylosing spondylitis (AS) than later-born children. We examined this effect of birth order using similar methods but larger numbers.
  
  
  METHODS
  Patients from the Bath AS database (n = 4517; M:F = 2.5:1) were examined according to position of birth within the family. Chi-squared analysis was used to examine if AS was more prevalent among first-born than later-born children.
  
  
  RESULTS
  The first-born child was not significantly more likely to have AS than later-born children (p = 0.295). [Observed compared to expected: 1607 (36%) compared to 1641.13 (36%) for first-born children and 2910 (64%) compared to 2876.3 (64%) for later-born children, respectively.] There was no biological gradient (i.e., inverse correlation between birth order and disease risk).
  
  
  CONCLUSIONS
  There was no statistically significant effect of birth order based on our data. Findings suggesting a birth order effect may be skewed, as it is possible that those parents who do have AS will be less likely to have a large family and yet it is their offspring who will be at greatest risk of developing disease. This will affect the data, as those children born into a large family (i.e., high birth order children) will be at a lower risk of AS than any child born into a small but family-history-positive unit.
authors.affiliation: Epidemiology Department, Royal National Hospital for Rheumatic Diseases, University of Bath, UK.
authors.forename: Sinead
authors.initials: S
authors.lastname: Brophy
authors.forename: Gordon
authors.initials: G
authors.lastname: Taylor
authors.forename: Andrei
authors.initials: A
authors.lastname: Calin
mesh_terms: D000328:Adult; D001722:Birth Order; D002648:Child; D016208:Databases, Factual; D005191:Family Characteristics; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D015995:Prevalence; D012307:Risk Factors; D013167:Spondylitis, Ankylosing / Q000453:epidemiology*