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].

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.

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.

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.

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.

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.

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.

To study whether apparently more fecund women having delivered twins at first birth have traits of higher twin-proneness, we performed a retrospective cohort study on population-based historical vital records of the 17-18th century French Canadian immigrants and their descendants under natural fertility conditions. Among 24896 mothers who had at least one child, 248 had twin maternities at their first birth (twinning rate = 1.0%). Among 21508 mothers with a valid marriage-first birth interval, twinning rate was 0.97% among prompt conceptions (7.0-11.0 months), with a particularly high rate at the interval of 7.0-8.0 months (2.2%). Marriages in August-October resulted in a higher twinning rate particularly for the slow conceptions than those in the other seasons. Promptly-conceived mothers of twins at the first delivery may seem to have higher fecundity, but subsequent births from these mothers (n = 88) show a lower twinning rate (1.7%) particularly at younger maternal age than from the other mothers who had slowly conceived twins at their first birth (n = 112). The latter show a 4.5% twinning rate as a whole among their second or later births. So-called twin-proneness of a mother, whether genetic or acquired, was not connected to higher conception rate of twin's mothers immediately after marriage. Reduced fecundity, which may have been imposed by some environmental factors, could raise the chance of twinning.

Fucosidosis is an autosomal recessive disorder caused by a deficiency of alpha-L-fucosidase. Up to now 79 cases have been described and several others identified but not yet published. The higher incidence of the disease is in Italy, where nearly 20 patients have been identified. Fourteen disease-causing mutations have been detected and four of them, Q422X, G60D, E375X, P141fs are present in more than 70% of the forty patients studied. In Italian patients, only seven mutations have been described and P141fs and G60D mutations are present in more than 50% of the cases. The P141fs mutation is absent in other ethnic groups. It has been impossible to establish genotype-phenotype correlation so far and the clinical variability of the disease cannot be explained only by genetic heterogeneity.

Whereas modern assisted conception with such techniques as in vitro fertilisation now helps many subfertile couples to fulfil their ambition to have a child, it has not been without a price. The increased incidence of multiple pregnancies, with their attendant maternal and perinatal sequelae following assisted conception is well known, but perinatologists may be less familiar with the Ovarian Hyperstimulation Syndrome (OHSS) which is the other major complication when controlled ovarian hyperstimulation is used during assisted conception treatment. Mild forms of OHSS are common and require no more than symptomatic treatment. Severe forms of OHSS are uncommon occurring in 0.6% to 14% of IVF cycles [15, 5], but are nonetheless very important to identify as they may lead to thrombo-embolic disease, cardiorespiratory dysfunction, renal failure and even death [6]. This review considers whether OHSS may be related to multiple pregnancy by reviewing the available literature and local experience.