Accurate, unbiased malformation rates in twins must be obtained unselectively from population-based studies that include livebirths and stillbirths after a thorough ascertainment of cases. This type of study was conducted in Los Angeles County, California, where 28 twins with a neural tube defect (NTD) were identified. The prevalence in twins (1.6/1,000) was significantly higher than in singletons (1.1/1,000). The study then was expanded to include population-based data from the Medical Birth Registry of Norway which has a comparable overall NTD prevalence (1.0/1,000) and twinning rate (2%). The combined material shows a higher prevalence of anencephaly and encephalocele but not of spina bifida in twins compared to singletons. The male/female ratios in total twin and singleton cases were comparable (0.8), but varied by specific defect. Like-sex twin females appeared at highest risk for NTD as well as for fetal death.

This study supports theories which associate NTDs with monozygotic twins, either through developmental disruptions that cause susceptibility to environmental agents or through a common etiology. Furthermore, it suggests that twins and singletons differ in their response to etiologic factors for the development of NTDs and that the development of each type of NTD may be related to different factors.

Twinning rates in all of Japan for 1951-1968 and 1974-1990 were analyzed using data from vital statistics of Japan. The twinning rate per 1,000 births was 6.43 in 1951 and remained nearly constant until 1968, then decreased to 5.79 in 1974 and gradually increased to 7.00 in 1990. The MZ twinning rate increased slightly up to 1966, but decreased thereafter, whereas the DZ rate declined over the entire period. As for maternal age, the DZ rate increased up to the age group 35-39 and decreased thereafter. The same tendency is seen in the MZ rate, but the maternal age effect is less marked than in DZ twins. The same tendency is also seen in the overall rate for 1975-1985. As for geographical variations, the MZ and DZ rates were computed in each prefecture for 1955-1959 and in 1974. The DZ rate increased from a low level in the southwest of Japan to a high level in the northeast for both periods, whereas the MZ rate was rather constant in 1974 throughout Japan. As for seasonal variation, the highest rates per 1,000 births for MZ(4.56) and DZ(2.20) twins were seen in April, with the lowest rate in September (3.69) for MZ twins and in July (1.71) for DZ twins.

In order to more adequately characterize patterns of intrauterine growth retardation in twins, the mean birthweights of all nonanomalous white or black twins born between 24 and 41 weeks of gestation and surviving until discharge over an 11-year period (547 infants) and all similar singletons (19,072 infants) were compared by completed weeks of gestation. Between 24 and 35 weeks of gestation, the mean birthweights for the 547 twins and the 19,072 singletons were comparable and did not consistently differ statistically. From 36 to 41 weeks gestation, however, the difference became large, consistent, and statistically significant for each week at P < 0.0001. This difference was present among all subgroups of twins, ie, in all males, females, blacks, and whites; it was still evident when the sample was further stratified by both race and sex (black males, white males, black females, white females). These data suggest a pattern of growth retardation in twins compared to singletons which is large, consistent, and statistically significant beginning at 36 weeks gestation. Clinically, these data also suggest the need for ultrasonic examination early in gestation (24-32 weeks) to document normal growth and to provide baseline data, and show the importance of such monitoring later in gestation, specifically after 36 weeks.

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.

The birthweights and placental weights of 3000 singletons, 1500 twin pairs and 67 triplets are studied in relation to the gestational age. From 30-32 weeks onwards, newborns of twin- and triplet gestation show lower birthweight as compared to singleton newborns. Placental indices (placental weights related to birthweights) are about alike in singletons, twins and triplets. From about 24 weeks onwards placental weights of twin and triplet newborns are smaller as compared to those of singletons. Obviously, children in multiple gestations become growth retarded, preceded by and very likely due to poor early placental development (placental crowding of the uterus).

It is estimated that at least one dizygotic (DZ) twin maternity in twelve is preceded by superfecundation (the fertilization of two ova by sperm from different coitions). Presumably this parameter varies from population to population eg. with coital rates and rates of double ovulation. Sometimes superfecundation occurs by two different men. The frequency with which this occurs must depend on rates of infidelity (promiscuity). It is suggested that among DZ twins born to married white women in the U.S., about one pair in 400 is bipaternal. The incidence may be substantially higher in small selected groups of dizygotic twin maternities, eg. those of women engaged in prostitution.

The dizygotic twinning rate in Western Nigeria is the highest on record (45—50 per mil maternities). It is suggested that, rather than to peculiarities in the population structure or to genetic factors, such a high incidence might be due to the presence in the diet of estrogen-like substances.

Data on alcohol use and smoking habits was available from the 1975 questionnaire of the entire cohort. Prior to pairwise analyses, the data of individuals was compared to that of age-sex matched groups of pairs reared together. The early separated twins had a higer alcohol consumption, while for smoking only slight differences were observed compared to twins reared together. Probandwise concordance rates were computed from smoking status (ever smoker/never smoker), alcohol use (user/nonuser) and “heavy” drinking (half-bottle of spirits on one occasion at least once a month). The following results were obtained in those pairs with the environmental dissimilarity score > 15:

Male, monozygotic twins (six pairs) were repeatedly tested before and after d-amphetamine, l-amphetamine, or placebo administration. Drug effects on cognitive, psychomotor, personality, mood, and pain variables were assessed. Members of a twin pair tended to respond similarly on several tests under placebo conditions, indicating genetic determination of the behavioral variables. In addition, cotwins tended to show similar responses to amphetamine as measured by one test of cognitive function, by several mood and personality variables (hostility, autonomic arousal, friendliness, feelings of tension and loss of control), and tended to have similar plasma levels of both amphetamine isomers. Although shared environmental effects cannot be ruled out, the results are consistent with genetic mediation of a variety of behavioral effects of amphetamines.

Reference intervals for human placental lactogen (hPL) and dU-estrogens (dU-E) in uncomplicated twin pregnancies are found to be higher than singleton pregnancies and considerably wider. Significantly more monozygotic than dizygotic pregnancies, and significantly more monoplacental than diplacental pregnancies showed hPL values below the median on the normal range curve. The same was not found for dU-E. Significantly more growth-retarded fetuses were found in monoplacental than diplacental pregnancies. The benefit of measuring the two parameters in order to identify the intrauterine growth retarded fetuses were evaluated in terms of sensitivity and specificity. Both parameters were found less suitable for the purpose.

The risk for specific defects among twins compared to singletons was studied using data collected by the Spanish Collaborative Study of Congenital Malformations (ECEMC). A total of 136 twins had a major and/or minor congenital defect. The overall rate of congenital defects in twins (2.37%) did not deviate significantly from the rate in singletons (2.21%). Like-sex (LS) and male-male (MM) twin pairs had a slightly higher rate of birth defects than unlike-sex (US) and female-female (FF) pairs, respectively. Defects of the central nervous system, cardiovascular system and genitourinary system were significantly more frequent in LS twins than in singletons, with relative risks of 2.8, 2.5 and 1.6, respectively. No significantly increased risk was found among US twins. Among defects of the central nervous system, the rates of anencephaly, encephalocele and hydrocephaly were significantly higher in total and LS twins; however, no significantly increased risk for spina bifida was observed when compared to singletons. MM twins were also 1.9 times more likely to have hypospadias, but the risk among males of male-female (FM) pairs was decreased.

Twenty–three cases of symmetrical conjoined twins were registered by the Latin-American Collaborative Study of Congenital Malformations (ECLAMC) in 1,714,952 births, which were observed during the 1967-1986 period in 95 maternity hospitals distributed in eleven Latin-American countries. This results in a birth prevalence rate of about 1/75,000 births. The secular and geographic distribution of this material do not depart from random in spite of one hospital with three cases, and two hospitals with two cases each, within a short time period. These 23 cases include one diprosopus, 3 dicephalus, one ischiopagus, 5 pygopagus, none dipygus, 3 syncephalus, none craniopagus, 9 thoracopagus, one omphalopagus, and none rachipagus. Sex distribution is even, with 12 male and 11 female cases.

The authors describe a new case of degenerative nanism « typus amstelodamensis », which shows all fundamental characteristics of the syndrome. From the hormonic research, which was carried through for the first time, resulted normal values (for the age of patient); this fact proves the fundamental normality of the endocrine system, which was already found by De Lange in the anatomical-pathological field.

Moreover, the genealogical examination led for the first time to the assumption of the existence of a genetic factor causing this particular type of a multiple degeneration.

A reanalysis of the data presented in a recent large twin study suggests that opposite-sexed (OS) pairs may be not as exhaustively tested as same-sexed (SS) pairs on genetic markers. This is contrary to an assumption I made in estimating that there are about 8 SS dizygotic pairs to every 7 OS pairs (thus impugning Weinberg's differential rule). If this assumption is false also in regard to the samples I discussed, then that estimate is unsound and Weinberg's rule is unscathed by empirical data. However, regardless of such considerations, there are strong theoretical reasons for questioning the status of Weinberg's rule. It is based on two assumptions, namely that p (the probability that a dizygotic twin zygote is male) is equal and independent for all dizygotic twin zygotes. Data are adduced here to suggest that both assumptions are false. The upshot is that, at present, without testing, we cannot know, of any given population, whether the rule holds or not. Otherwise, though Weinberg's rule may be useful (like Hellin's law) as a rule-of-thumb, it cannot be assumed as a basis for serious scientific argument.

By dosing the serum-cholesterol in basal conditions in 5 0 MZ (25 ♂ ♂ and 25 ♀ ♀) and 50 DZ (16 ♂ ♂ 18 ♀ ♀ and 16 ♂ ♀ ) twin-pairs, the Authors found an intra-pair dissimilarity significantly different among the pairs of the two zygosity groups. On this basis, the genotypic origin of cholesterol biogenesis is pointed out. The twins, aged from 6 to 19, were all living with their respective families: nutrition and environment were thus the same for each pair.

The number of twins in 942 families where the propositus had a congenital malformation of the heart has been studied. The 38 pairs were not significantly more than could be expected by chance: nor were the 21 pairs where one member had a malformation of the heart. In 16 pairs, one was affected and the other normal; and in 5 pairs, one was affected and the other unknown. In 2 pairs, both were unknown because of early deaths; and in 15, both members of the pair were normal. In no pair were both members known to be affected.

There is clear evidence from this series and from the series of Uchida and Rowe (1957) and of Lamy et al. (1957) that where one member of a pair of monozygotic twins has a congenital malformation of the heart, the other is generally normal. It is the exception rather than the rule for both members to be affected, though several examples of such pairs have been reported.

There is some evidence from these three series that monozygotic twins are more liable to have a malformation of the heart than dizygotic twins and their sibs.

It follows from these two findings, that the immediate cause of the malformation of the heart must be environmental, though probably there is some genetic predisposition. A disturbance of the foetal circulation to the affected twin because of a single placenta and chorion seems to be the most likely environmental cause.

I should like to thank Professor Gedda of Rome and Professor Polani for their helpful suggestions and Dr. I. E. Evans for letting me see his unpublished paper on the evidence for monozygosity in some of the pairs of twins included in this paper.

Serum gonadotropin, estradiol, prolactin and alpha-1-antitrypsin levels were measured in the first four days of the menstrual cycle in 14 women who were cycling normally. FSH, and to a lesser extent LH levels, were significantly higher in a group of 8 women who had at least one set of DZ twins (6 of whom had 2 sets) than in a control group of 6 women with no DZ twins. Estradiol levels also tended to be higher in mothers of twins but there were no significant differences in prolactin or alpha-1-antitrypsin concentrations. There were no differences between the means of the two groups in age, height, weight or number of pregnancies.

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.

A kindred is described with a probably new “cervico-dermo-reno-genital syndrome”, comprising a congenital muscular torticollis, multiple spontaneous keloids, cryptorchidism, pyelonephritis and other anomalies.

We believe that in this observation a single gene is responsible for the multiple mesodermal and ectodermal defects. Pleiotrophic effects of a single X-linked gene is the most likely possibility for this inborn error of connective tissue.

From an extensive review of the literature we may conclude that the nosological association of congenital muscular torticollis, multiple spontaneous keloids and cryptorchidism has thus far never been published.

Sex linked or sex limited inheritance for congenital muscular torticollis or multiple keloidosis has not yet been published with certainty.

Our observation and the frequent keloidosis in gonadal dysgenesis, may suggest the presence of gene material on the X-chromosome responsible for connective tissue hyperplasia.

This study is based on data from 165 adult twin pairs separated at 10 years or less. Information on personality factors: extraversion (E) and neuroticism (N) (EPI scale short from), life satisfaction (LS) (Allardt) and stress of daily activities (SDA) was obtained as part of the questionnaire study carried out in the entire Finnish Twin Cohort in 1975. Later in 1979 a questionnaire sent to the twins reared apart yielded a scale (range 7-30 points) measuring the environmental dissimilarities after separation (reliability 0.83). The effect of separation on personality factors by analysis of variance of individual data was studied. Sex, zygosity and age-at-separation were included in the models. The overall expalanatory rates were low (2.1-4.4%). The definitive study group was formed by selecting those pairs with a dissimilarity score greater than 15. The following intraclass correlations were obtained: