Data with regard to the proportion of congenital malformations found at birth are presented from a survey of 657 pairs of twins of known zygosity and placentation delivered in Aberdeen and Northeast Scotland between 1968 and 1979. There was an excess of malformed individuals from monozygotic (MZ) pairs, but this did not reach statistical significance. The possible effect of monochorionic placentation in the causation of malformations in MZ twins in general is discussed. It seems that this type of placentation may be of less causative importance than has been previously suggested.

Data are presented from a large-scale population-based study in Northern Ireland, in which multiple sources of ascertainment were used. As found in other studies, the overall prevalence at birth of congenital anomalies amongst twins (285.4/10,000) was somewhat higher than the rate amongst singletons (241.8/10,000). Unlike in other studies, however, the rate amongst twins of like sex (287.8/10,000) was not markedly higher than that amongst twins of unlike sex (252.3/10,000). Problems of comparison between series are discussed.

In a large population-based study in Northern Ireland during the period 1974-1979, the rate of anencephalus in twins (9.1/10,000) was found to be less than that in singletons (24.3/10,000). This finding is in contrast with most other studies and the possibility of underascertainment of twin cases is considered, but it is concluded that chance is the likeliest explanation. The rate of spina bifida in twins (36.4/10,000) was similar to that in singletons (31.9/10,000). All of the twins with anencephalus were female and from pairs of like sex. Rates of spina bifida in twins from pairs of the two sex types were similar but, unusually, there was a male preponderance. As in previous studies, the great majority of twins with NTDs had unaffected cotwins.

Problems of ascertainment bedevil the investigation of the etiology of congenital anomalies in singletons and in multiple births by epidemiological methods. It is shown that the definition of the population of affected births is ambiguous and that the problem of tracing etiology is complicated by the systematic way in which anomalies may be missed at birth. The available methods of dealing with problems of ascertainment are reviewed. Methods of adjusting for possible bias of ascertainment of affected births, by fitting statistical models to data from several sources, have been employed in some previous studies. In these methods, it is assumed that there are no errors of diagnosis or of recording. However, it is shown that there may be discrepancies in recorded diagnoses between sources, rendering this assumption untenable. In these methods, it is also assumed that the model which is the best fit to the data on the ascertained cases in also the best model for the cases which were not ascertained. This assumption is tested indirectly in a retrospective analysis of data from Aberdeen and Belfast collected concurrently through routine recording systems. It is demonstrated that there was a social process in ascertainment which renders the methods of adjusting for bias of ascertainment at best very complicated and at worst inapplicable.