In the NAS-NRC Registry, all major diseases are more common in DZ than in MZ twins. Furthermore, concordance rates for most disorders are lower in the registry than would be expected. In this article we propose a general model which seeks to explain these phenomena. The model explores the impact of traits which increase or decrease the probability of enrollment of individuals given that the registry, like the NAS-NRC, includes only pairs where both members are enrolled. If the trait decreases the probability of selection into the registry, both the prevalence of and concordance for the trait in the registry will be lower than that found in the population. A trait which increases the probability of selection has the opposite effects. However, the magnitude of these effects are a function of the population concordance. If population concordance differs in MZ and DZ twins, the effect of differential enrollment will not be the same for the two zygosity groups. The article examines the impact of differential enrollment on estimates of heritability and common environment and explores ways in which estimates of prevalence and concordance rates can be obtained which are free of the bias introduced by selection.

The number of AgNOR (NOR+) and the amount of AgNOR (NORM+) were analysed by means of two multilevel analyses of variance in a total of 12 twin pairs: 3 female and 4 male MZ and 5 male DZ pairs. In the first analysis, only zygosity was controlled; in the second, chromosome types D and G were controlled as well as the interaction between chromosome type and zygosity. For NOR+ and NORM+, when chromosome types D and G are not distinguished, the within-pair variance is greater, though not significantly, in DZ than in MZ pairs; but it is highly significantly greater when chromosome type (D or G type) is under control. This confirms an important genetic determination of NOR+ and NORM+ when in the ANOVA model the D and G types are controlled. However, nongenetic factors also influence the Ag-NOR patterns, but not enough to conceal the genetically defined rDNA pattern. Indeed, about 50% of the cells transcribe their rDNA in a way not closely dependent on the rDNA background and significant intrapair differences of NOR+ pattern exist in MZ twins.

Genetic and environmental influences on the phenotypic expressions of several biological markers were studied in 18 monozygous (MZ) and 8 dizygous (DZ)twin pairs. Zygosity was determined using ABO, Rh, and HLA. The biomarkers studied included: T & B lymphocytes, suppressor and helper T lymphocytes (Tγ, Tμ), T cell (PHA) mitogen activation (MA), serum immunoglobulins (IgA, IgM, and IgG), plasma carcinoembryonic antigen (CEA), aryl-hydrocarbon hydroxylase (AHH) and sister chromatid exchange (SCE) in lymphocytes. Temporal variation of markers over a 6-month period was not significant. The mean absolute differences between levels from first and second blood draws were less than one standard deviation. Variability associated with age was not significant. Females had higher levels of T lymphocytes than males. A gender related association was observed for the IgM immunoglobulin test: females had a higher mean level of IgM. Smoking was found to influence the levels of SCE, T helper lymphocytes and mitogen activation. The variability of these biomarkers within and between twin pairs was quantified. Immunoglobulin levels, particularly that of IgM, showed statistically greater similarity within MZ twins than within DZ twins. Several other markers suggested heritability.

When the Weinberg estimate of the proportion of monozygotic pairs is quite deviant from that in the source population, it is likely to be wrong because Weinberg's difference is much less stable than the zygosity proportions. A formula is proposed for the probability distribution of possible compositions of a small sample of twins based on sex concordance in the sample and zygosity proportions in the source population.

In 33 out of 51 women studied in late gestation, the uterus was found to phasically compress the pelvic vessels and impede the venous blood flow during quiet standing. This caused a reduction of the cardiac stroke volume with resultant reduction of systemic blood pressure and a compensatory increased heart rate (range of increases 9-51 beats/min). In all cases uterine contractions (mostly subclinical) coincided with the phase of circulatory readjustment. Apparently, the contracting uterus, by changing its position and/or shape, relieves the venous obstruction and prevents decompensation. In the women displaying the uterine compression syndrome (UCS), uterine activity was markedly increased in standing compared to the left recumbent position.

It was also investigated whether the UCS appeared more often and earlier in gestation in women with twins. In all 9 women with twin pregnancies (mean gestational age 28 5/7 weeks) the UCS associated with uterine contractions was apparent in the standing posture. Although at present no definite conclusions can be reached on the effect on the cervix of these contractions, quiet standing especially in twin pregnancies seems to provoke an increased uterine activity and should therefore be avoided.

Acute polyhydramnios in the second trimestr is a typical complication in monozygous twin pregnancies. It is caused by a feto-fetal transfusion with anemia on the donor and polycytemia on the recipient twin. Contrary to the chronic hydramnios, there is no increase in malformations. In view of the high mortality rate (100%, according to most authors), the clinical management has to be reconsidered. During the years 1979 to 1983, 10 cases of acute polyhydramnios have been observed at the University Hospital in Zurich. This corresponds to an incidence of 9% in our twin population. All cases investigated were MZ twin pregnancies. With the exception of one patient, who underwent an abortion, all women were hospitalized, had bed rest and received recurrent removals of amniotic fluid and prophylactic tocolysis. The mean gestational age at the time of diagnosis was 23 4/7 weeks and at delivery 30 3/7 weeks. In two cases – one of which is presented in detail – with an unintentional puncture of a placental vessel, the recurrence of the hydramnios did not appear. Eight of 18 newborns survived. No malformations were found. Bed rest, tocolysis and recurrent amniocenteses seem to have a positive influence on the prolongation and outcome of the gestation in acute polyhydramnios.

Monozygotic twins with typical trisomy 13 are reported. Despite an identical karyotype, the twins were dimorphic for the presence of an omphalocele. Reasons for the rarity of MZ twins with trisomy 13 are presented. It is suggested that the presence of a chromosomal abnormality in MZ twins may predispose to dimorphism.

During the years 1979-1980, a prospective study was carried out to evaluate the value and efficacy of routine hospital bed rest in the prevention of premature birth and pregnancy complications in twin gestation. This was done by comparing hospital bed rest to special antenatal care at outpatient clinic. Since the results did not support the idea of using routine hospital bed rest, this was abandoned. Because the material was rather small, the results of the changed policy of management were controlled in the present follow-up study. The material consists of 102 twin pregnancies diagnosed during the years 1982-1983. The number of deliveries before the 34th gestational week and of < 1500 g weighing infants were still decreasing (9.0% and 3.0%). Perinatal mortality after the 24th gestational week was 2.0%. The follow-up study confirmed the unefficacy of routine hospital bed rest. Early diagnosis is the basis of the management of twin pregnancy. This allows counselling of the pregnant women, regular examinations to detect pregnancy complications, and also planning of the delivery.

An analysis of all perinatal deaths occurring in twin pregnancies in Dundee women from 1956 to 1983 was performed. The uncorrected perinatal mortality rate fell from 116/1000 births in 1956-60 to 16/1000 births in 1981-83, this fall almost entirely taking place after 1975. Causes of death were identified using the Aberdeen Classification and a reduction in deaths in all cause groupings occurred. About half of the deaths were in the Premature, Cause Unknown group and a marked decrease in deaths in this group made the largest contribution to the improved perinatal mortality rate. This fall was partly due to a reduction in the incidence of extreme prematurity and low birth weight. Changes in obstetric management which may have influenced outcome included the introduction of routine early pregnancy ultrasound scanning, the use of tocolytic drugs, intrapartum fetal monitoring, epidural analgesia and an increase in Caesarean section rate from 2% in 1956-60 to 39% in 1981-83.

A specific prenatal care program is proposed to prevent preterm deliveries in twin pregnancies, with ultrasound scanning, early work leave and home visits by midwifes. A four-year study has been conducted on 197 pregnancies (160 early followed and booked and 37 late referred). It can be shown that early preterm births and very low birth weights are less frequent in the followed group than in published data, as well as in the late referred pregnancies. The total perinatal mortality rate is 25.6 per 1000 total births.

We have compared breech twins and breech single births in a population recorded between 1974 and 1978 in 10 Belgian maternity centers. In 190 twin pregnancies, 38 first and 65 second twins were delivered in breech presentation. These twins were compared to 853 singleton breeches, of which 95 delivered by cesarean section were excluded. Data were stratified according to birthweight. No significant difference between twins and singletons was found in terms of perinatal mortality rates. However, Apgar scores below 7 at 5 minutes were significantly less numerous among first twins than among singletons. Therefore, first twins in breech presentation might be at lower risk of fetal distress. This difference should be taken into account in the management of first twins presenting by the breech.

A simple program for management of term twin delivery in the second stage of labor is presented. Provided that given selection criteria are met, twins at term are delivered by the vaginal route

An epidemiological study of all primigravid twin pregnancies delivered in Aberdeen between 1950 and 1969 was performed to determine the pattern of arterial blood pressure changes. There is a greater fall from non-pregnant levels in diastolic blood pressure by mid pregnancy and a greater rise of diastolic pressure by delivery. These changes are independent of age, body size and rate of weight gain during pregnancy. The expected increased incidence of proteinuric pre-eclampsia is also independent of rate of weight gain when defined for twin pregnancies.

No significant difference has been found in either stroke volume or cardiac output between twin pregnancies and singleton pregnancies. Cardiac output is less in preeclamptic twin pregnancies.

In recent years, it has been realized that some infants of frankly alcoholic mothers escape the stigmata of fetal alcohol syndrome (FAS) and others have only a few of the characteristics. These infants are thought to display fetal alcohol effects (FAE). The controversy regarding the amount of alcohol a woman can safely drink during pregnancy and the effects of timing and individual physiology on producing FAS vs FAE in the infant are important questions which can perhaps be partially answered through examining twin pregnancies and offspring. Data are presented regarding the long-term growth and development of a set of dizygotic twins, one with FAS and one with FAE, delivered to a mother who drank moderate amounts of alcohol during pregnancy. The variation in the degree of abnormality found in dizygotic twins exposed to similar amounts of alcohol at the same time during gestation indicates that differences in fetal susceptibility to ethanol dysmorphogenesis are of prime importance to the expression of the fetal alcohol syndrome.

Continuous wave Doppler ultrasound was used to study the twin fetus in 76 multiple pregnancies. The technique is not difficult and allowed the identification of the small for gestional age twin in both intrauterine growth failure and twin to twin transfusion syndrome.