Malformations in twins were studied with the use of a Medical Birth Registry covering nearly all births in Sweden, 1973-1981. A total of 15,427 infants marked twins were identified and the malformation diagnoses were analyzed, comparing rates with the overall rates in the same data source. Problems in ascertainment and interpretation are discussed. No excess of neural tube defects was seen, but hydrocephaly was diagnosed more often in twins than in singletons. This can be partly, but not completely, explained by a higher rate of low-weight infants among twins. An excess was noticed in twins in the rates of cardiac defects, spine defects, gut atresias – notably esophageal and anal atresia – and severe kidney malformations. These are components of the so-called VACTERL syndrome – the last component (limb reductions) being more common in twins than in singletons, though statistical significance was not reached. A relation between monozygotic twinning and this syndrome is tentatively suggested.

Procedures are described for the acquisition and analysis of data in a study of the dental and facial characteristics of South Australian twins. Comparisons of the mesiodistal diameters of maxillary incisors in MZ and DZ twins revealed heterogeneity of total variances and evidence of inequality of mean values for some dimensions between MZ and DZ twins. Previous estimates of heritabilities for tooth size, relying on classical assumptions in twin research, may be exaggerated. A preliminary analysis of facial shape was undertaken using a procedure for shape matching based on a least squares fit of homologous coordinates. There was evidence of mirror-imaging in some MZ twin pairs and differences in facial asymmetry between male and female DZ twins. Future extensions of the study using methods for three-dimensional shape analysis are described.

The incidence of preterm delivery among twin maternities in Oxford between 1973 and 1983, was 39.4%. Preterm delivery was more common in MZ than DZ twins, and this was associated with higher incidence of spontaneous rupture of the membranes associated with monozygosity.

The twinning, triplet and quadruplet rates were found to be 23.8, 0.3 and 0.04 per thousand maternities, respectively in a survey of 71,773 maternities carried out in Ibadan and Igbo Ora, Nigeria. This represents a dramatic fall from the rates found in previous studies. The findings are discussed in the light of changing dietary habits.