In each species, natural selection has resulted in an optimal litter size, to ensure the largest average number of surviving offspring and the lowest maternal mortality. The terrestrial insectivores from which the primates evolved had large litters. It has been argued that in primates adaptation to an arboreal mode of life led to a reduction of litter size to a single offspring because of the difficulty of producting and caring for a large litter in a tree [6]. A recent critical survey of the literature indicates that twinning frequency in most nonhuman primates is lower than in man [20].

There are clear ethnogeographic differences in the incidence of human twinning. Among peoples of Eastern Asia, multiple maternities are rare; in Japan for instance, the twinning rate is only 3-7 per mill, according both to the official statistics and hospital records. These low rates seem not to be caused by the facts that it is customary in Japan to have children very early in life or that twin births were at one time viewed with displeasure and concealed, or sometimes twins were even killed in some areas [21].

In hospital series among some Negro tribes almost 10 times higher twinning frequencies than among Japanese have been reported e.g. the Yorubas in western Nigeria with values above 60 per mill [24]. However, where national birth statistics of Blacks are available, twinning is lower, e.g. in U.S.A. 13-15 per mill [1].

A healthy 23-year-old woman with amenorrhea was examined at the Mendel Institute. She had been amenorrheic for 4 years, and had not responded to hormone treatment. We therefore decided to study her family tree and karyotype. We describe the results of our study here: the patient was found to have gonadal dysgenesis, caused by translocation of a fragment of the X to a 12 chromosome, resulting from a break at q21, at the end of the q-arm.

This year, the French are commemorating the centenary of the death of their distinguished countryman Louis Pasteur, chemist and bacteriologist. Yet few people in France or elsewhere will be aware of a fact which I recently discovered: Louis Pasteur and the Czech geneticist Gregor Mendel were born in the same year - 1822. That Pasteur and Mendel are exact contemporaries is a most significant coincidence, for their respective careers symbolize the parallel and contiguous development of medical and genetic knowledge in the nineteenth century. This in turn paved the way for man to discuss, investigate, increase and spread understanding of human reproduction, disease and life-span, and to utilize scientific understanding of these subjects to increase the well-being of mankind in the twentieth century. Indeed, the human relevance of these biomedical discoveries caused them to eclipse those of other branches of knowledge in this century.

Large twin samples and recent applications of multiple regression techniques to behavioral genetics methodology makes possible evaluation of genetic and environmental contributions to the articulation proficiency of individual phonemes. Factor analysis of the articulation scores from 256 MZ and DZ twins and 124 of their non-twin siblings (all children ranged from 2; 11 to 9; 8 years) were conducted to reduce a 50-item articulation test to a more manageable set of five articulation factors. The twins' factor scores were then analyzed using multiple regression procedures to determine the extent to which the individual factors resulted from genetic and/or environmental influences. The /r/ and /∫, t∫, dƺ/ factors were found to have strong genetic components, while the /l, j, w/ factor was found to be strongly influenced by environmental sources of variation.

The annual variation of the sex ratio (SR) of 1385 twin births was analyzed and compared to that of 85909 singletons. These births referred not only to live births but also to stillbirths in two southeastern Brazilian maternity hospitals from 1984 to 1993. While the annual variation of the SR of singletons was very small, that of twin births was extremely high, due to the significant heterogeneity of the annual data. It is suggested that the large SR annual variation of the twin birth might be a consequence of the variation of male or female monozygotic twins.The hypothesis that twin births show a lower sex ratio than singletons could not be supported by the present data.

I would like to reply here to a number of criticisms concerning the stance taken by Pope John Paul II on the rights of the unborn child in his book Crossing the Threshold of Hope, made by Dr John Godfrey of the University of Edinburgh in a recent article entitled “The Pope and the Ontogeny of Persons” (Nature, 12 January, 1995).

The accusation levelled against John Paul II is contained in the subheading of John Godfrey's article: “In the recent book… Pope John Paul II airs his views on human reproduction. The pity is that he ignores most of modern genetics and embryology”. Dr Godfrey quotes the following passages written by the Pope in Crossing the Threshold of Hope: “The concept of a ‘person’ is not only a marvellous theory; it is at the centre of the human ethos… in this field more than in any other, collaboration among pastors, biologists and physicians is indispensable”.

The are two common replies to the question of whether carriers of genetic disorder should have children. The first simply ends any argument by claiming that everyone has an inalienable right to have children. In many ways this view is quite attractive. The right is said to be universal as well as inalienable. It avoids odious comparisons and applies to everyone, including those whom society has deemed unfit for the flimsiest reasons.

In the United Kingdom, for example, a High Court judge recently upheld the decision by Sheffield Health Authority to refuse IVF treatment to Julie Seale, who was then 36, on the grounds of her advanced age. This latest salvo in what one writer has called “the fertility war” follows on from the Grand Peninsular Campaign against IVF treatment for post-menopausal women and the recurrent guerilla battle over enforced sterilisation. As this author remarks, “What has happened without our really noticing it is that, with every new skirmish in the fertility war, we are becoming more and more comfortable with the idea that some people deserve to be parents more than others” [1]. And in an age of what is essentially payment by results, there is a great temptation for healthcare providers to think that those who deserve to be parents are those with the best clinical chances.

The objective of this study was to assess the childhood growth of twin children in terms of the effects of intrauterine growth retardation (IUGR) and discordancy at birth on the incidence and severity of stunting and discordancy in current height and weight. The study was part of a cross-sectional field project conducted at the Annual Twins Days Festival in Twinsburg. Ohio, USA, during 1990, 1991, and 1993, and including all twin children between 2 and 12 years of age. Mothers of twins were interviewed regarding their children's birthweights and gestational age; the twin children were measured for their current heights and weights. The study population included 990 twin children, including 555 boys and 435 girls, of which there were 254 boy pairs and 194 girl pairs. Birthweight for gestational age and current weight and height were each converted into Z-scores and characterized as severe (Z-score <−2,0), or moderate IUGR or stunting (Z-score > −2.0 and < −1.2). For the present study discordancy in birthweight, and current height and weight was calculated for like-gender twin pairs. Only twin children with severe IUGR at birth showed an increased risk of stunting in their current height or weight, and this risk was only for moderate, not severe, stunting. Boy twins with severe IUGR at birth were at increased risk of moderate stunting in their current weight (OR 2.67, 95% CI 1.55, 4.58, P = 0.002), while girl twins with severe IUGR at birth were at increased risk of moderate stunting in their current height (OR 4,09, 95% CI 1.49, 10.99, p = 0.003). Among like-gender twin pairs, there were no differences in mean or categories of birthweight or current weight discordancy, but boy twin pairs did show a significantly greater proportion of current weight discordancy compared to girl twin pairs (p = 0.005). Overall, there was a significant tendency for differences in height and weight between like-gender twin pairs to disappear over time, with the effect being greater for boy twin pairs. We conclude from these findings that twin children tend to overcome growth retardation and discordancy present at birth, and although children who had severe IUGR or discordancy at birth were more likely to have some residual moderate stunting or discordancy in height or weight, they still tended to be within normal values for their gender and current age.

All of the recorded twin live births in Washington State birth certificates between 1984 and 1988 were used a retrospective cohort study to determine the risk of zygosity on pregnancy complications and birth outcomes (n = 3458). Relative risks comparing different sex (DS) twins to same sex (SS) twins were corrected to relative risks relating dizygotic (DZ) to monozygotic (MZ) twins, using the Weinberg rule. A higher proportion of DS twin pregnancies (3.5%) than SS pregnancies (1.6%) were complicated by gestational diabetes, resulting in an estimated risk for DZ twin pregnancies relative to MZ pregnancies of 8.6 (95% CI = 3.5-21.0). DZ twin pregnancies were at a lower risk for complications of polyhydraminios (RRDZ∣MZ = 0.2, 95% CI = 0.1-0.4) and of pyelonephritis, (RRDZ∣MZ = 0.3, 95% 0 = 0.1-0.8). MZ twins were more likely to have low birthweight and to have shorter gestations. The proportion of first-born babies of MZ twin pairs who died during their first year was similar to that of first twins of DZ pairs; however, the second-born of MZ twins were more likely to die in infancy than were second-born DZ pairs. First twins of DZ pairs were more likely to die of SIDS (sudden infant death syndrome) than the first of MZ twins (RRDZ∣MZ = 1.5, 95% CI = 0.4-5.1). In contrast, DZ second-born were less likely to die of SIDS than were MZ second-born twins (RRDZ∣MZ = 0.1, 95%CI = 0.1-0.7). DZ twins were less likely to have adverse newborn conditions or malformations. The high risk for gestational diabetes for DZ twin mothers is possibly due to the presence of two placentas which may support the development of greater insulin antagonism than the single placenta in the mother of MZ twins. The reduced risk of DZ relative to MZ twins for selected adverse birth outcomes may result from the increased tendency of MZ twins to be premature.

This is a follow-up study of twins, including 33 twinpairs from the Stockholm area, aiming to study the cognitive development of twins at eight years of age. The twins have been followed at different ages from birth onwards. All children were tested with the Wechsler Intelligence Scale for Children in a Swedish translation (WISC). The WISC test consists of a Verbal and a Performance Scale. There were no significant differences between twin girls and twin boys on these Scales. On the Performance subtests Block Design, Object Assembly, and Coding, however, the twin girls performed significantly better than the twin boys. In comparing cognitive development for twins and singletons, the twin group had somewhat lower average scores than the singletons. Prematurity and low birth weight continued to be related to cognitive development at eight years of age. Also at this age the school teacher completed a questionnaire about the twins social behaviour and some personality traits. There was a relation between one questionnaire factor, a low score of assertiveness, and the mother's negative or ambivalent expectations concerning the twin pregnancy. The twin group with the mother's negative expectations also had significantly lower results on the subtests Comprehension and Coding. Negative mothers had more premature twins than mothers who were positive toward the twin pregnancy.

This study investigated the degree of risk of handicap in twins, triplets, quadruplets and quintuplets and associated factors, and examined the clustering tendency of handicaps. The sample was recruited from the Kinki University Twin and Higher Order Multiple Birth Registry. This panel consisted of 705 pairs of twins (1410 twins), 96 sets of triplets (287 triplets excluding 1 infant death), 7 sets of quadruplets (27 quadruplets excluding 1 infant death), and 2 sets of quintuplets (10 quintuplets), all of whom were born after 1977. The incidence of handicap was 3.7% in twins, 8.7% in triplets, 11.1% in quadruplets, and 10.0% in quintuplets. The risk of prodicing at least 1 handicapped child was approximately 1 in 13 pairs of twins (7.4%), 1 in 4 or 5 sets of triplets (21.6%), and 1 in 2 sets of quadruplet and quintuplets (50%). There was a significantly higher clustering tendency of handicaps in twins and triplets compared with the expected frequency calculated from the incidence rate of handicap. Four significant risk factors for subsequent handicap were found by logistic regression: gestation number, shortening of gestational age, premature rupture of the membrane, and toxemia during pregnancy.

Precision of zygosity determination in twins can be improved by the use of modern methods of DNA analysis. The clinical application of 4 single- (SLS) and 2 multi-locus (MLS), and 6 PCR (polymerase chain reaction) systems for zygosity determination in 12 twin pairs with oral clefts was compared with regard to the quality and quantity of sample material required and the probability of error in monozygosity determination. PCR systems proved to be superior to SLS or MLS, as DNA sampling is much more convenient, while its level of accuracy still fulfils clinical requirements. For this reason, PCR systems should be considered a basic method in modern clinical twin research.

The objective of this study was to evaluate the association between maternal factors, including rates of gestational weight gain before and after 24 weeks' gestation, and adequacy of intrauterine growth for gestational age at birth of triplets, as a mean Z-score of the triplet set. The study design was a retrospective, anonymous, pilot telephone survey of mothers of triplets and an historical cohort analysis of their prenatal weight gain records. The statistical analyses performed included multiple regression analysis to formulate a model for mean triplet Z-score (a measure of birthweight-for-gestational age) and analysis of variance to confirm and simplify the components of this model. Factors significant in the final model and their beta coefficients included weeks' gestation (-0.124, p <0.0001), rate of gain before 24 weeks' gestation (0.606, p = 0.005), and induced conception (−0.404, p = 0.01). Rate of gain > 1.5 lbs/week before 24 weeks was significant in the analysis of variance (p = 0.009). Better intrauterine growth for gestational age is achieved in triplet gestations with maternal weight gains of >1.5 lbs/week before 24 weeks' gestation.

Prior studies have suggested that the risk of testicular cancer among dizygotic twins may exceed that among monozygotic twins or the general population. Cryptorchidism is associated with testicular cancer and twinship, and therefore might potentially explain the findings of the prior studies. In 1993-1994, when they were 66 to 77 years of age, 14,326 twin individuals in the National Academy of Sciencies-National Research Council Twin Registry were interviewed by telephone. A history of testicular cancer was reported by 5 (0.08%) of 5951 monozygotic twins and 11 (0.16%) of 6992 dizygotic twins. Follow-up interviews concerning testicular cancer risk factors and treatment were able to be administered to 4 of the monozygotic and 9 of the dizygotic twins reporting testicular cancer. A history of cryptorchidism was reported in the follow-up interview by only one dizygotic twin. Our data agree with the results of prior studies reporting a more frequent occurrence of testicular cancer among dizygotic than monozygotic twins. Although somewhat limited by small numbers, our study also suggests that the findings of increased testicular cancer in dizygotic twins are not explained simply by increased occurrence of cryptorchidism in twins.

This study investigates the influence of hypertensive disorders on twin pregnancies for an unselected, population-based series. Between 1986 and 1991, out of a total of 56,381, 766 (1.3%) were twin deliveries at our institution, the only tertiary care hospital serving a population of about 400, 000 inhabitants. The incidence of hypertensive disorders was significantly higher in twin gestations than in singleton pregnancies, at 3437/55,615 (6.2%) vs 85/766 (11.1%) (p<0.001, OR=1.8, 95% CI =1.4-2.3). Hypertensive disorders were significantly higher in twin as compared to singleton pregnancies, regardless of parity, and even after adjusting for maternal age. More instrumentai or surgical deliveries were needed when pregnancies were complicated by hypertension, in twin as well as singleton gestations. Despite the association between prematurity and hypertensive disorders, and prematurity and perinatal mortality, no significant difference was found in perinatal mortality between hypertensive and nor-motensive twin pregnancies. The neonatal death-rate in normotensive and hypertensive twin pregnancies (3.7% and 3.5% respectively) was higher than that of stillbirths (respectively 2.3% and 0%).

Numerous reports in the literature suggest that hormones may transfer from one fetus to another, in humans as in animals. In a large sample of over seven thousand Australian adult twins, it was found that opposite-sex females showed a statistically significant tendency to hold more masculine attitudes than did same-sex female twins. This may be due to post-natal social interaction, but could also be caused by the transfer of testosterone from the male to the female fetus in opposite-sex twins.

The retardation of fetal growth in twin as compared to singleton gestations may be due to maternal and/or placental factors. The separate birthweights and related placental weights of 487 dichorial twin pairs were recorded, and the heavier placental weights of firstborns in comparison with second-borns were found to be related to the heavier birthweights of firstborns compared to second-borns. The lighter placental weights of firstborns compared to second-borns were found to be related to the lighter birthweights of firstborns compared to second-borns. Obviously, the placenta itself may may very well be the cause of the retardation of fetal growth in twin gestations.

The term gestational trophoblastic disease is used to indicate a group of both benign and malignant trophoblast, including molar degeneration of villi, hydatid mole, invasive mole and choriocarcinoma.

This study shows a new classification of trophoblastic disease existing with a living fetus or fetuses. Benign hydatid mole is the initial stage of the disease continuum, whereas highly malignant choriocarcinoma is the final stage of this spectrum.

The acute form of twin-to-twin transfusion syndrome is caused by rapid transfer of blood from one of the twins to another via placental anastomoses. Usually, this only occurs during the second stage of labour as a result of a sudden relative rise of blood pressure in one of the fetal circulations. This can result in the sudden intrauterine death of a fetus (or both, as in our case). Currently, there is no reliable means of identifying such an at-risk pregnancy by means of ultrasound antenatally. We would classify this as TTTS Type III.

We evaluated the relative influence of genetic and environmental factors on leukocytes, lymphocytes, neutrophils, monocytes, erythrocytes, reticulocytes, hemoglobin, platelets, T lymphocyte subsets, and NK marker cells in monozygotic (MZ) and dizygotic (DZ) twins. Peripheral blood components were measured in 154 healthy twin pairs (118 MZ pairs and 36 DZ pairs) and lymphocyte subsets in 48 healthy twin pairs (38 MZ pairs and 10 DZ pairs).

Platelets and erythrocytes showed higher intraclass correlations in the MZ pairs than in the DZ pairs in the males. However, in the females, the intraclass correlations for these components were similar between the MZ and DZ pairs. Neutrophils and monocytes showed low intraclass correlations in the MZ pairs. The absolute number of CD4+ cells and the CD4+/CD8+ cell ratio showed high intraclass correlations regardless of age in the MZ pairs in both males and females. The intraclass correlation for the absolute number of CD3+ cells was low in pairs aged ≥ 60 years in the MZ pairs. The absolute numbers of CD16+ cells and CD56+ cells showed high intraclass correlations regardless of age in the MZ pairs.