Birth order and ankylosing spondylitis: no increased risk of developing ankylosing spondylitis among first-born children.

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).
Conclusion: 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.