Congenital malformations in twins: epidemiologic survey.

Among 1195 twins born in the Collaborative Perinatal Project, for whom information was available, 219 (18.33%) were found to have malformations, 179 (14.98%) single and 40 (3.35%) multiple. The frequency of malformations and malformed individuals was significantly higher in twins than in singletons from the same population but the difference was entirely contributed by MZ twins. This held true for both major and minor malformations. There was no significant difference in the frequency of multiple malformations. Negro twins were more frequently malformed than white, and male twins were more frequently malformed than female. Twins had a significant, more than two-fold, increase of cardiovascular and alimentary tract malformations, and smaller increases of central nervous system, musculoskeletal, ear, and respiratory malformations over singletons. Significantly increased among specific malformations were macrocephaly, encephalocele, cleft lip and palate, anomalies of the diaphragm, cardiac septal defects, tracheoesophageal fistula, malformations of the alimentary tract, inguinal and umbilical hernias, and cystic kidney. A significant increase of malformations in twins over singletons occurred only in live births who survived to one year. There was a small but not significant increase of malformations in twin fetal deaths and deaths beyond the neonatal period; but among neonatal deaths, twins were less frequently malformed than singletons. Monoamniotic twins had higher mortality, more complications of delivery, and significantly more congenital malformations than diamniotic twins. One of the 9 pairs of MA twins in the study was a conjoined thoracopagus twin pair with multiple cardiovascular, alimentary, and other malformations. The case is interesting because of the medical history of the mother before and during pregnancy, and its possible bearing on the cause of MZ twinning and of congenital malformations. There was no difference in the frequency of malformations among first- and second-born twins. Concordance rates were significantly higher among MZ than among DZ twins for any malformation as well as for the categories of major, minor, single and multiple malformations. In the majority of cases, the twins were concordant for the same malformation. Concordance rates of MZ twins were consistently higher than those of DZ for all systems, but the difference was significant only for the musculoskeletal system, comprising mostly clubfoot. Because of the small numbers involved, the results of the concordance analysis should be interpreted with caution.