284 LETTERS William H. James, PhD University College London London, UK 1. James WH. The difficulties in establishing a birth order or maternal age effect in ankylosing spondylitis: comment on the article by Baudoin et al [letter]. Arthritis Rheum 2002;46:563–4. 2. Baudoin P, van der Horst-Bruinsma IE, Dekker-Saeys AJ, Weinreich S, Bezemer PD, Dijkmans BAC. Increased risk of developing ankylosing spondylitis among first-born children. Arthritis Rheum 2000;43:2818–22. 3. Said-Nahal R, Miceli-Richard C, Dougados M, Breban M. Increased risk of ankylosing spondylitis among first-born children: comment on the article by Baudoin et al [letter]. Arthritis Rheum 2001;44:1964–5. 4. Brophy S, Taylor G, Calin A. Birth order and ankylosing spondylitis: no increased risk of developing ankylosing spondylitis among first-born children. J Rheumatol 2002;29:527–9. DOI 10.1002/art.10702 Reply To the Editor: Recently, Dr. James discussed the statistical method used in our study (1). The point of discussion now concentrates on the issue of whether an individual with AS has an equal probability of being first-, second-, or later-born. Dr. James is correct that our method is based on the assumption that, in the absence of a relationship between AS and birth order, the probability of being first-, second-, . . . nth-born would be equal within each family size. He states that this assumption is almost certainly invalid for our study population, because family size distribution is not constant over time. This reasoning would correctly apply to a cross-sectional, random sample (especially of the elderly) but is not applicable to our study population. Our AS patients were followed up at our outpatient clinic shortly after diagnosis (at relatively young ages) during a long period of many years. If Dr. James’ reasoning would still apply to our study population, the arguments for a relationship between AS and birth order would be even stronger, because we observed relatively higher numbers of patients who were first-born (for review, see ref. 1, Tables 2 and 4). Moreover, we already observed this relationship within families with 2 children. We agree that use of a control sample, providing more direct evidence, would be fine, but use of a control sample is not necessary to reach the conclusion presented in our article. The fact that our data on the birth order effect were not confirmed by Brophy et al (2) is, however, disappointing. The lower mean maternal age at first delivery of (healthy) mothers of AS patients compared with mothers of healthy controls that was observed in our study was not taken into account by Brophy et al. This relationship cannot be explained by the observations of other investigators (3), i.e., that mothers with early symptoms of AS are more likely to pass on the disease to their children, because all of the mothers in our study were healthy. Therefore, we still believe that there is an effect of both lower maternal age at first delivery, as was observed in animal models, as well as birth order (being first-born) that increases the risk of AS. Irene E. van der Horst-Bruinsma, MD, PhD P. Dick Bezemer, PhD Ben A. C. Dijkmans, MD, PhD Vrije Universiteit Medical Centre Amsterdam, The Netherlands 1. Baudoin P, van der Horst-Bruinsma IE, Dekker-Saeys AJ, Weinreich S, Bezemer PD, Dijkmans BAD. Increased risk of developing ankylosing spondylitis among first-born children. Arthritis Rheum 2000;43:2818–22. 2. Brophy S, Taylor G, Calin A. Birth order and ankylosing spondylitis: no increased risk of developing ankylosing spondylitis among first-born children. J Rheumatol 2002;29:527–9. 3. Calin A, Brophy S, Blake D. Impact of sex on inheritance of ankylosing spondylitis: a cohort study. Lancet 1999;354:1687–90. DOI 10.1002/art.539 Erratum In the article by Delgado Alves et al published in the October 2002 issue of Arthritis & Rheumatism (pp 2686–2694), the name of the fifth author was spelled incorrectly. The correct spelling is J. Nourooz-Zadeh. We regret the error.