The genetic and environmental determinants of a brief assessment of metabolic rate at rest and under psychological stress were studied in 40 pairs of monozygotic and 40 pairs of dizygotic young adult male twins. Height, weight and age were employed as covariates. Univariate analyses showed a high heritability for height and weight and moderate heritability for metabolic rate. Classical twin analyses and multivariate genetic modeling indicated that genetic influences on resting metabolic rate were entirely explained by body weight: there was no independent genetic contribution to resting metabolic rate. Metabolic rate under psychological stress, on the other hand, showed a significant genetic effect. The exponent (3/4) in the power function relating body weight to resting metabolic rate was the same as that found in a wide variety of animal species, a value that has been proposed as defining a body weight set point. We speculate that an adult body weight set point is genetically transmitted. Independent genetic effects on resting metabolic rate would be observed only when the normal equilibrium between body weight and metabolic rate is unbalanced during development, aging or disease. The study illustrates the use of multivariate genetic analyses of twin data which may be readily applied to widely used metabolic rate assessments.

The usual assumption that monochorionic twins are monozygotic has recently been questioned, based on blood group discordance in 3 of 12 monochorionic pairs. Therefore, this study evaluates the validity of zygosity diagnosis based on examination of placental membranes, and at the same time evaluates Weinberg's differential rule in a Danish consecutive twin series. All twin pairs, 265 like-sexed and 87 unlike-sexed pairs, born at Odense University Hospital, Denmark, from 1 January 1980 through 31 August 1988, were ascertained. In 229 like-sexed pairs, zygosity was determined by serological methods, and in 190 of these, the fetal membranes could be reliably classified. Fiftynine monochorionic twin pairs were identified and none of those pairs were discordant for any genetic markers. Therefore, we conclude that monochorionic twins can be considered monozygotic. The distribution of monozygotic and dizygotic pairs was in accordance with Weinberg's rule.

Existing data concerning the effect of gestational diabetes on perinatal outcome in twin pregnancies is scant. We hypothesized that altered carbohydrate metabolism would worsen perinatal outcome in twin gestation in a manner similar to singleton gestation. Thirteen twin pregnancies complicated by gestational diabetes mellitus were matched by gestational age at delivery to 13 twin pregnancies unaffected by gestational diabetes. Comparing infants of diabetic mothers to infants of control mothers, there was a trend of greater likelihood of respiratory distress syndrome, hyperbilirubinemia, and prolonged neonatal intensive care nursery admissions. Our experience suggests that altered carbohydrate metabolism in multiple gestations increases the potential for neonatal morbidity.

Conventionally, the zygosity of twins into monozygotic (MZ) and dizygotic (DZ) is determined by utilizing several blood groups or other genetic markers. Existence of many blood groups or other genetic markers raises the crucial problem of choosing minimum number of blood groups and applying them in a sequence with minimum cost offering maximum effectiveness in classifying the twins by zygosity. In this note, a statistical approach to this problem is made and a solution provided. The efficient sequential search procedure utilizing the gene frequency of four blood groups from the German population of Nordrhein-Westfalen province and applying the results to a twin set of 72 pairs (32 MZ and 40 DZ) of the same population indicates that efficient sequences with 3 and 4 blood groups are AB0→P→Rh and AB0→P→Rh→MNSs, respectively. Further, it is understood that the efficient sequence for zygosity determination will be differing from population to population. As gene frequencies of blood systems differ in the various populations, this will effect the probability that a DZ twin pairs is concordant for a specific marker. Moreover, the relative cost may also turn out to be a decisive factor in the determination of the best sequence. Hence, indiscriminate use of several conventional blood group systems in the determination of zygosity of twins from a particular population should be discontinued. The formulae developed in this note are quite general and the procedure explained can easily be generalized to other situations.

In 1980-82, a mailed questionnaire was completed by 3,810 pairs of adult twins enrolled on the Australian NH&MRC Twin Register. Twins were asked whether they had had their tonsils out and, if so, at what age. The sample was divided into four birth cohorts of approximately equal size, and only childhood tonsillectomy (to the age of 18) was considered. The prevalence of tonsillectomy differed markedly between cohorts, being highest in those born in the 1940s and early 1950s. Within each cohort, the prevalence was very similar in MZ and DZ twins, yet concordance was much higher in MZ twins, indicating the importance of genetic factors in predisposition to tonsillectomy. However, the proportions of variance in liability due to genetic and shared environmental factors differed markedly between cohorts. In the 1950s, when tonsillectomy was fashionable, shared environment accounted for 60% of variance and genetic factors for only 29%. However, by the early 1960s, when tonsillectomy was going out of fashion, heritability was up to 0.82 and shared environment accounted for only 10% of variance. Our results illustrate, once again, that heritability is not a constant, but depends on the precise characteristics of the population and the time at which it is studied.

The small size of most reported triplet series has resulted in conflicting statements about the influence of several clinical variables on triplet birth weight. Therefore, obstetrical and neonatal data were collected on 196 mothers and their 580 infants (8 stillbirths excluded). Gestational age was based on the date of fertilization in 13 IVF triplets and on the date of ovulation in 90 medically induced triplets. Obstetrical and ultrasonic criteria were used to estimate the date of confinement in 93 spontaneous triplets. Birth weight appeared to be higher in males and with higher maternal parity, independent of gestational age. The apparent effect of medical technologies such as ovulation induction or IVF on combined triplet birth weight disappeared when maternal parity and fetal gender were controlled. Preeclampsia, maternal race and zygosity were not significantly associated with birth weight. While birth order did not significantly effect ultimate birth weight, the heaviest triplet did present first more often than would be expected by chance alone. Future evaluation of neonatal outcome data in multifetal gestations should control for gestational age, fetal gender and maternal parity. It appears that triplet birth weight is not affected by etiology, which is important given the significant impact of medical technologies.

This study describes the evolution in fetal and neonatal mortality rates among twin pairs born in 22 hospitals located in the eastern regions of the province of Quebec in 1976-1978 (n = 776 pairs) and 1982-1985 (n = 712 pairs). It also assesses the contribution of maternal factors, obstetrical care and characteristics of twins in the variation of the risk of death over time. The fetal mortality rate did not improve from 1976-1978 (22.6 per 1000) to 1982-1985 (28.1 per 1000). However, the neonatal mortality rate declined from 44.7 to 34.7 per 1000 liveborn first twins and from 56.8 to 36.1 per 1000 liveborn second twins. For first twins as for second twins, birthweight-specific neonatal mortality rates decreased within birth weight categories under 2500 g. In the second period, 96.9% of twin pregnancies were detected before confinement compared to 59.6% in the earlier period. The proportion of twins delivered by obstetricians, the percentage of twin births occurring in ultraspecialized perinatal units and the frequency of caesarean sections increased markedly. The proportion of preterm births increased over time (34.5% vs 43.1%) whereas the percentage of low birthweight twins decreased but not significantly (54.3% 51.6%). In this study, changes in maternal age, parity, educational level, sex of pairs, qualification of the physician, and level of care available at the hospital of birth, did not account for the decrease in neonatal mortality rates among twins. The increase in the frequency of caesarean sections seemed to explain only a small proportion of the decrease in the neonatal mortality rate among second twins. In the second as well as in the first period, the neonatal mortality rate for twins was six times higher than that for singletons.

We examined the neonatal outcome of 644 twins weighing 500 g or more and 656 singletons, born in the years 1984-1986 in the Soroka Medical Center, Beer-Sheva, Israel. There was nearly a four-fold risk of antepartum death in twins vs singletons, which disappeared when birth weight was controlled for. The risks for intrapartum and early neonatal mortality were not raised in this population. A statistically significant relative risk for congenital heart malformations in twins vs singletons remained (RR = 5.0, 95% CI = 1.5-16.3), after controlling for maternal age. Significantly higher rates of hyalin membrane disease, hypoglycemia, hyperbilirubinemia, anemia and septicemia were found in twins. Controlling for the confounding of the association between twinning and mortality or morbidity caused by differences in distributions of mode of delivery or gestational age between twins and singletons, was not as efficient as the controlling for birth weight. Thus, adjustment for birth weight removed all the excess risks detected except in hypoglycemia. Our findings suggest that the-lower birth weight of twins, which is so intimately associated with multiple gestations, is probably the single most important factor associated with neonatal problems found in twin births.