¹Centre for Preventive Medicine Barts and the London, Queen Mary's School of Medicine and Dentistry, Wolfson Institute of Preventive Medicine, London, UK.

Correspondence: Dr J Morris, Wolfson Institute of Preventive Medicine, Charterhouse Square, London, EC1M 6BQ, United Kingdom. Tel: 207 882 6274, Fax: 207 882 6270. E-mail: j.k.morris@qmul.ac.uk

We thank the authors for their comments on our paper. They query as to why we do not mention paternal age as a possible explanation for the increase in prevalence of Klinefelters compared to XYY and XXX. We agree that it may be a partial explanation, but we do not believe that there is a substantial body of evidence of an association of paternal age with the birth of a child with Klinefelters. Of the five studies referenced in a recent meta-analysis, only one study found a significant positive association between paternal age and Klinefelters.¹

The authors were correct in the assumption that we did not reference the study by Bojesen et al,² because it did not give the corresponding numbers of XYY and XXX diagnoses and the basis of our paper was to compare the prevalences of the three sex chromosome trisomies. The study by Bojesen et al covers the time period from 1970 to 2000 and they estimate a prenatal prevalence of 2.1 per 1000 (not specifying what proportion of diagnoses are from CVS or from amniocentesis), which compares to the data in our paper of 3.1 per 1000 observed in an amniocentesis series in women over age 35 from 1976–1981. It is difficult to directly compare these two figures as Klinefelters is associated with maternal age and has a fairly high fetal loss rate, so the gestational age at diagnosis is important.