IS THERE EVIDENCE OF CLUSTERING IN DOWN-SYNDROME

Background Associations between environmental hazards and the occurren ce of congenital anomalies may be detectable by seeking evidence of no n-random occurrence of cases (clusters). There have been a number of a necdotal reports of occurrences of clusters of Down syndrome (DS). Met hods Data from a national register of cytogenetic diagnoses of Down sy ndrome births and legal terminations occurring between 1989 and 1995 w ere used to examine the possibility of clustering. Space-time clusteri ng at Regional Health Authority (RHA) level was examined by comparing the expected monthly number of DS pregnancies given the maternal age d istribution, with the observed numbers. Knox's method was used to dete rmine if any clustering of RHA of unexpectedly high prevalence had occ urred. Seasonality was also investigated by comparing monthly expected and observed numbers of DS pregnancies. Time clustering was examined by using the scan statistic to determine whether a statistically signi ficant excess of pregnancies in any 3-month period occurred in any ind ividual or adjacent groups of District Health Authority (DHA). Results The numbers of DS pregnancies were no higher than expected (P < 0.05) in the same RHA over consecutive months. There was no evidence of any seasonality of DS pregnancies (P > 0.5). Only two individual DHA and three pairs of adjacent DHA had significantly high scan statistics (P < 0.03), but as over 400 statistical tests had been completed 12 clust ers would be expected to have occurred due to chance alone. Conclusion s There was no evidence of any space-rime clustering in DS at DHA leve l.