J Am J Med Genet. 1999 Mar 12;83(2):117-24.
Congenital malformations in twins: an international
study.
Mastroiacovo P, Castilla EE, Arpino C, Botting B, Cocchi
G, Goujard J, Marinacci C, Merlob P, M?neki J,
Mutchinick O, Ritvanen A, Rosano A.
Birth Defects Unit, Catholic University, International
Centre for Birth Defects, Rome, Italy. mc8682@mclink. It
Data provided by nine registries based in European and
Latin America countries 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 Diseases (ICD). Malformation rates and
rate ratios (RR) for twins compared to singletons were
calculated for each registry, and the homogeneity of the
RRs was tested using the test of Breslow and Day. If
departure from homogeneity in the different registries
was not significant, registry-adjusted RRs with 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 remaining nine
malformations, heterogeneous estimates of RRs were
obtained. This study confirms the majority of already
known associations and further identifies previously
unreported malformations associated with twins. In
conclusion, there is an excess of malformations in twins
compared with singletons, and all anatomical sites are
involved. The number of specific malformations
associated with twins is higher than that previously
reported in smaller studies.
