Congenital malformations in twins: An international study

Data provided by nine registries based in European and Latin America countr ies were analyzed to assess whether there is an excess of malformations in twins compared to singletons. Specific congenital malformations were coded according to the ninth revision of the International Classification of Dise ases (ICD), Malformation rates and rate ratios (RR) for twins compared to s ingletons were calculated for each registry, and the homogeneity of the RRs was tested using the test of Breslow and Day. If departure from homogeneit y in the different registries was not significant, registry-adjusted RRs wi th 95% confidence intervals were calculated. Overall, among 260,865 twins, 5,572 malformations were reported. A total of 101 different types of malformations or groups of defects was identified, and a homogeneous estimate of the RRs among registries was found for 91.1% of the malformations. Thirty-nine of the 92 malformations with homogeneous estimates of RRs were more common in twins than in singletons. For the rema ining nine malformations, heterogeneous estimates of RRs were obtained. Thi s study confirms the majority of already known associations and further ide ntifies previously unreported malformations associated with twins. In concl usion, there is an excess of malformations in twins compared with singleton s, and all anatomical sites are involved. The number of specific malformati ons associated with twins is higher than that previously reported in smalle r studies. Am. J, Med, Genet, 83:117-124, 1999, (C) 1999 Wiley-Liss, Inc.