It is a frequent observation that twinning runs in the families [5] and we have recently reported that such is also the case with Gregor Mendel's pedigree [4]. The problem we are concerned with, now, is the identification of the “gemellogenetic” factor postulated by Gedda already in 1951 [2].

We thought we should start by going back to experimental embryology. Clearly, as shown by the experimentally induced MZ twinning in Triton or the spontaneous multiple MZ twinning in Armadillo, there must exist a temporal threshold below which individual parts of a single conceptus can still show totipotence.

Interestingly, recent studies of molecular histology have stressed the role of cell membranes and of membrane receptors in the transmission of specific messages related to the position and function of the cell. This has led Edelman [1], in particular, to identify a specific kind of molecule that appears early in-embryonic development and is responsible for cell adhesion, thus contributing to organogenesis and the shaping of the embryo by regulating cell movements and the topologic development of tissues.

These so-called cell-adhesion molecules (CAM) are proteins that can differentiate both in function and in expression. In terms of function, there are molecules responsible for the adhesion of nerve cells (N-CAM), of liver cells (L-CAM), etc. In terms of expression, there are embryonic forms (E-CAM) and adult forms (A-CAM), the former being characterized by larger quantities of sialic acid. This chemical difference accounts for a different aggregation effect on the cells, that is, for a different cell adhesion. Moreover, the E-CAM and A-CAM can be interconverted, at different times and with variable speed.

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.

Although familial aggregation of blood pressure is well documented, few studies have considered the changing contribution of genetic and environmental influences during adulthood. Applying maximum likelihood model fitting to blood pressure covariation in balanced pedigrees including both parents and their young adult twin offsprings (25 MZ, 32 DZ, aged between 16 and 24 years), it is shown that the increased variation in parents is explained by such developmental changes. For DBP, an apparent reduction in heritability from 68% to 38% from young adulthood to middle age results from the increasing impact of individual environmental experience (E1), with little or no influence from shared family environmental (E2). For SBP, shared environmental effects may play a part. Given the relatively small size of the present sample, the conclusions are to be seen as tentative. An augmented family study, incorporating middle aged twins and their young adult offspring, will clarify the causation of these developmental changes.

The distribution of plasma creatine kinase (CK; E.C.2.7.3.2) is known to be skewed, and this has made it difficult to analyse the sources of variation. We have studied plasma CK in 206 pairs of twins and have analysed the results after separating them into what appear to be two Gaussian frequency distributions. The results in the main distribution (CK < 300 iu/1) are apparently affected by genetic factors common to both men and women, and by environmental factors which are of much greater effect in men. The tendency for some men to have very high CK values may also have a genetic basis.

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.

A statistical model was developed to determine the likelihood of a twin kinship, that is, the offspring of a pair of monozygotic or dizygotic twins under three types of inheritance: sporadic, single locus fetal genetic, and single locus maternal genetic. Samples of 8,000 kinships were simulated for a discrete trait under various hypotheses, and the likelihood determined for each type of etiology. The results indicated that the pedigree analysis procedures formulated here could efficiently detect sporadic or single locus effects with a power approaching 100%, although the parameter estimates obtained were slightly biased. Further analyses revealed that the type of pedigree analysis formulated in this study was found to have equivalent power for equal or unequal frequencies of kinships by the sex and zygosity of the twin parent. It was suggested that further studies be carried out that included the twins and spouses in the likelihood equations, as well as tests of more sophisticated models.

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.

Heritability estimation is possible from concordant twin pairs alone, based on the proportion of like-sexed pairs among all concordant affected pairs. The method is limited to conditions found in both sexes in the prevalence range 0.1% to 10%, and a relatively large population size is required to give an adequate sample of twin pairs. However, the method has the considerable advantage that zygosity determination is not required and that any bias due to incomplete diagnosis/ascertainment is likely to be small. The method is particularly suited to diseases where registration is obligatory and computerised so that the register can be scanned for pairs of individuals with the same date of birth, place of birth and birth surname.

The incidence of twinning in Australia from 1853 to 1982 is described. The overall trend was determined by changes in the dizygotic rate and, for more than a century, it has been similar to that found in Finland and the South of Italy. It is not explained by demographic changes and there is no clear relationship with industrialisation or psychosocial factors. Monozygotic twinning has increased, most markedly in the 1970s. No adequate explanation has been given for this increase, so continuing surveillance in many different populations is necessary.

A holoacardius from a monozygotic, monochorionic twin pregnancy of 36 weeks is described. Trisomy-18 was diagnosed in the viable female cotwin. Computerized tomographic, radiologic, and ultrasonographic procedures were applied to the acardiac. Its three-vessel umbilical cord was velamentously attached to the single placenta. Grossly malformed and poorly developed craniofacial structures, absence of neck and upper extremities, sirenomelic lower extremity, absence of heart, and development of only a few skeletal elements were prominent features. A review of selected world literature emphasizes the limits of the estimates dealing with the total number of reported cases and the incidence of acardii.

Serial ultrasound examinations have demonstrated that one of two gestational sacs in a twin pregnancy may often disappear. When it disappears at an early stage of gestation, the pregnancy may advance without any disturbance and the cotwin can be delivered well developed and lively. When the intrauterine death occurs in the second trimester, the dead fetus usually results in a fetus papyraceus and the cotwin continues to be alive near term. However, when death occurs in the last trimester, the viable twin may be spontaneously delivered soon and be premature. In some cases of late fetal death, the dead fetus may induce intravascular thromboses in many organs of the surviving cotwin, so that the living infant may develop cerebral palsy later after birth.

The course and outcome of 23 monoamniotic (MA) twin pregnancies, delivered in Tampere University Central Hospital during the years 1964-1984, were studied retrospectively and compared to 1056 diamniotic (DA) twin pregnancies. The frequency of MA twins was 2.1% of twin pregnancies. Polyhydramnion complicated the pregnancy in 26% of MA vs 6% of DA pregnancies. Two cases were defined as acute polyhydramnion. Preterm labour was stated in 70% of MA pregnancies and deliveries before the 34th week were 4 times more common in MA than DA pregnancies. The cesarean section rate was more than double in MA pregnancies (39%). Entanglement of the umbilical cords was noted four times, and prolapse of the cord in three vaginally delivered cases. Perinatal mortality was 28% in MA vs 5% in DA twins. The most common causes of death were respiratory distress syndrome, congenital malformation and feto-fetal transfusion.

Twin to twin transfusion, complicated by acute polyhydramnios in a monozygous twin pregnancy, is a difficult clinical problem. A precipitous course usually results in termination of the pregnancy within a few days and often is associated with a high perinatal mortality rate. Two cases are presented that were treated with repeated amniocenteses for the relief of extreme abdominal discomfort and to prevent imminent premature labor. The amount of amniotic fluid removed each time varied from 300 cc to 1200 cc, which was enough to relieve symptoms but not enough to induce uncontrolled uterine activity. A total of 3500 cc and 4750 cc of amniotic fluid were removed from the first and the second patient, respectively. The procedure was found to be safe and resulted in prolonging the pregnancies by 14 and 11 days, respectively. This management, with the addition of tocolysis and close fetal surveillance can offer some hope in an otherwise hopeless situation.