Certain congenital malformations occur in excess among twins and also among first-degree relatives of twins. In the general population, these anomalies are familially associated with each other, and, like twinning, familially associated with unusual brain function asymmetry. They affect structures built by fusion of bilateral embryonic halves and remodeled under major influence of neural crest mesenchyme. This conjunction of associations suggests that twinning, symmetry development, and this group of malformations might share causai elements at least some of which are heritable. The problem here is the absence of zygosity differences in these relationships, because of which they cannot be explained within the biology of twinning as it has been understood. A potential resolution is offered by way of a mechanism common to MZ and DZ twinning, involving a relationship between oocyte organization and the determination of body symmetries.

A retrospective review of triplets delivered at a Boston perinatal center from 1977 to 1986 was performed. Comparison was made between this group (study group) and previously published data on triplets in our institution (control group). Since 1977 there was a more liberal use of abdominal delivery. Cesarean sections (CS) of all triplets with malpresentation was our protocol. Of the 15 sets of triplet pregnancies in the study group, 11 were delivered by CS and 4 by vaginal delivery, vs only 1 CS in the control group wich consisted also of 15 triplets. The corrected mortality rate in the study group was lower than in the control group (2.6% vs 7.1%) but did not reach statistical significance. Apgar scores at 1 and 5 minutes were significantly higher in the study group (P < 0.002). Apgar scores for the third triplet were also higher in the study group (P < 0.05). In comparing the combined mortality and morbidity between the study group and the control group, no difference was found in the first triplet, but those of the second and third triplets were significantly lower in the study group. Of interest is the finding that the combined mortality and morbidity was not different statistically among the first, second, and third triplets in the study group, while in the control group an increase from the first to the third triplet was noted (21%, 31%, and 43%, respectively). A more liberal approach toward abdominal delivery of pregnancies of higher fetal number is advocated.

The rate of multiple births and the incidence of congenital anomalies in Tokyo Metropolitan Hospitals were studied during the period 1979-1990. The number of twins was 968 pairs (8.23 per 1,000 deliveries) and of triplets 18 sets (15.3 per 100,000 deliveries) among 117,672 deliveries including 1,587 stillbirths after 16 weeks gestation. Multiple birth rates increased yearly. Stillbirth rates in twins and triplets were 5.5% and 16.7% respectively, which were both significantly higher than that in singletons (1.3%). The number of congenital anomalies was 42 in 1,936 twins (2.17%), 2 in 54 triplets (3.7%) and 1721 in 116,686 singletons (1.47%). The most common defects in twins were those of the cardiovascular system (0.72% in twins vs 0.52% in singletons) and of the musculoskeletal system (0.72% in twins vs 0.50% in singletons), followed by upper respiratory tract and/or mouth conditions (0.67% in twins vs 0.35% in singletons), all of which had no significant difference in frequency between twins and singletons. Though some anomalies had a significantly higher frequency in twins than in singletons, the concordance rate in the like-sexed twins was very low.

To search for an association of twin zygosity with tooth size, 56 dental variables measured from 65 pairs of twins (43 MZ, 22 DZ) were studied. Results of the t′ test for equality of the means showed no association of zygosity with any of the variables in males or in females. Results of the F′ test for homogeneity of total variances between zygosities showed evidence for unequal total variances in 15 variables in males and 13 in females. Sex influence was further noted on the association of zygosity with the variance of tooth size. Where total variances were unequal, genetic variance estimates differed when only the within-pair mean squares were used and when combined estimates designed to be unbiased by differences in environmental variances were used.

Among 93 cases of thalidomide embryopathy ascertained in Brazil, one pair of female dizygotic twins was found. One of them presented complete paraxial hemimelia in the right arm while the other showed a triphalangeal thumb at the left hand and had duodenal stenosis. Of the eight dizygotic twin pairs exposed to thalidomide in utero described thus far, only four were concordant for the type of malformation.

Within the Finnish Twin Cohort of like-sexed adult twin pairs, a subgroup of pairs separated at an early age has been identified. In 165 pairs, both cotwins responded to questionnaires in 1975 and 1979. An environmental dissimilarity score was formed which consists of items on whether the twins had lived after separation in the same community, attended the same school, were on the same grade at school, how often the cotwins met, how often they met common friends and relatives and whether they attended the same clubs etc, or not. To validate the zygosity diagnosis obtained by questionnaire in 1975, those pairs whose zygosity was unknown as well as those with the least contact after separation were contacted for blood sampling (11 bloodgroups). Of 15 pairs with no zygosity diagnosis, 10 responded (1 no address,2 abroad,2 refused). Six pairs were classified MZ and 4 DZ. In 12 MZ and 8 DZ pairs undergoing bloodgroup determination, the classification of only one pair changed from DZ to MZ. The following intraclass correlations for height and weight were found:

Twinning rates were studied in Swedes, Åland Islanders, Finns, Germans, and Dutch during years of starvation when death rates were two to three times higher than average. In contrast to the situation among some animals, this study suggests that nutrition above a certain threshold is unimportant for human reproduction, including twinning. The twinning rates for these different populations display marked temporal differences, but low values in the twinning rate are not consistently associated with periods of epidemics, famine, or similar nutritional stress. After years of privation and/or separation of spouses, a rapid “catch-up effect” can often be seen in the twinning rates, as well as marriage and birth rates. Psychoendocrine factors and interparental immunological conditions that may be involved in this phenomenon are discussed.

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.

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 aim of this study on all twin births in Sweden between 1973 and 1985 was to evaluate the effect of maternal age, parity, and sex on twin perinatal mortality (PNM), by using national data held at the Medical Birth Registry, National Board of Health and Welfare, Stockholm. Zygosity in this study was estimated by the Hardy-Weinberg formula. In all, 22,374 twins were born; 972 (4.3%) of these died perinatally. The relative risk for perinatal death (after standardization for maternal age, parity, and like sex) decreased significantly during the study period, and reached its lowest value (0.3) in 1983. Maternal age was not associated with PNM, while women having their first delivery had a significantly higher PNM than the rest of the parturients. The MZ twinning rate increased fom 0.36% during 1973-76, to 0.40% during 1977-81 and 0.44% during 1982-85 (trend analysis: χ2 = 40.1, p < 0.001). After stratification for year of delivery and birthweight, like-sexed twins had a significantly higher PNM than unlike-sexed twins (odds ratio, OR = 1.25, 95% confidence interval, CI = 1.02-1.54). Males had a significantly higher PNM than females in like-sexed pairs (OR = 1.39, 95% CI = 1.16-1.66), and in unlike-sexed pairs (OR = 1.46, 95% CI = 1.01-2.10). MZ twins had a significantly higher estimated PNM than DZ ones (OR = 1.31, 95% CI = 1.02-1.67).

A case of monozygotic male twins discordant for skeletal and cardiac defect is reported. One twin had the hemifacial microsomia type of the oculo-auriculo-vertebral dysplasia. The cotwin had no asymmetry of the face and normal ears, but preaxial polydactyly and ventricular and auricular septal defects. The cotwins were concordant for craniostenosis with a ridge metopic suture. Karyotypes were normal.

Other than a specific case of concordant Vitiligo in MZ twins, the development of which has been followed for more than 11 years up to now, MZ twins suffering from Vitiligo have been described only three times. In all the four cases, three were concordant, one discordant; this last was accompanied by a discordant disorder of basal metabolism, by hyperhydrosis and exophtalmus. The greater degree of concordance found up to the present day, notably contrasts with the relative extreme rarity of familiar Vitiligo.

A survey questionnaire of 200 families with 2-month-old twins assesses the economic, social and psychological impact of the arrival of twins on family life. The study shows the extent of material difficulties mothers of twins are confronted with, and the ways they cope with them — recourse to mother's helpers, assistance from the father and other members of the family (analyzed in terms of parity and socioeconomic/cultural status), twincare organization strategies, and impact on the decision to stop working. The findings provide an overall picture of the real situation, a necessary prerequisite to an understanding of psychological problems.

During the 10-year period, 1975-1984, 105 triplet pregnancies were delivered at Harare Maternity Hospital, Zimbabwe, among 286,338 pregnancies in the Greater Harare Unit, giving an incidence of triplets of 1:2,727. The mean gestational age at delivery was 32.5 wk with 81 women (77.1%) delivering before 37 wk. Primigravidas delivered at a significantly earlier mean gestational age (P < 0.05) and had a higher perinatal mortality (P < 0.001) compared with grand multigravidas. Of the 315 babies, 277 (87.9%) weighed < 2500 g. The overall perinatal mortality rate was 327‰, with a pernital mortality rate of 146‰ for infants weighing ≥ 1000 g. Women hospitalised for bed rest during the antenatal period had fewer perinatal deaths compared with those diagnosed during the antenatal period, but not hospitalised for bed rest (P < 0.02). No difference was found in the mean gestational age at delivery or the mean birth weights between these two groups. Among infants ≥ 28 wk gestation there were fewer perinatal deaths in triplets delivered by cesarean section compared with triplets delivered vaginally (P < 0.0004). This suggests that cesarean section may offer the optimal mode of delivery in triplet pregnancy.

A survey of triplet pregnancies in Linköping, Sweden, 1973-1981, showed a low perinatal mortality (4%) and low morbidity possibly due to prophylactic bed rest and tocolytic treatment. All children were followed at child health clinics. In one child, a persistent growth retardation was recorded at 4 years of age. No psychomotor deviations were found in the group. An interview concerning social and economic factors after delivery was carried out. The answers revealed considerable strain in daily life for all families.

A study of 111 mothers of twins in Nottinghamshire, England, in 1981-82 showed that at least 12 had used ovulation stimulants, compared to 2 of 102 mothers of singletons. Thus, the use of ovulation stimulants increased the twinning rate of this population by about 10 percent; this may explain the recently noted levelling off in the secular decline in twinning rates. In contrast to a recent study in France, no association between twinning and prior use of oral contraceptives was seen.

The NHLBI Twin Study is a longitudinal study of cardiovascular disease risk factors in 514 pairs of white, middle aged, male, veteran twins. The initial examination took place between 1969-1973. Ten years later, 81% of the living cohort returned for a second examination. Data collected up to 30 years prior to recruitment for the initial examination were used to characterize participants and nonparticipants; data from the initial examination were used to characterize returnees and nonreturnees to the second examination. Participants had significantly lower diastolic blood pressure and higher socioeconomic status than nonparticipants as measured thirty years earlier. Between the first and second examinations, the mortality of participants was less than 50% of the mortality of nonparticipants. Returnees to the second examination had a better health profile at the initial examination than nonreturnees, with significantly lower levels of cigarette smoking, glucose intolerance, hypertension, and diabetes and higher levels of pulmonary function. However, returnees were more obese than nonreturnees. Thus, this study of cardiovascular disease risk factors in twins appears to be affected by response bias in a way similar to studies of individuals. Additional analyses of biases that may affect the genetic component of the study indicated that factors related to classical twin analyses were relatively unaffected by selection.

Sixteen pairs of term discordant twins (weight discrepancy of more than 20% when the, lighter twin was compared to the heavier) were evaluated at birth. Weight, length, head circumference, anterior fontanel area, and combined ossification of the knee epiphyses were measured, and ponderal index (weight/length ratio) calculated. The most severely growth retarded infants had markedly decreased ossification and larger anterior fontanels.

Eleven pairs had physical measurements at one year of age. With individual exceptions, the lighter twins at birth remained smaller in all dimensions. Despite these persistent differences between twin pairs, the values for length at one year of age were within normal limits for both the heavier and lighter twins. Infants without ossification at birth had a greater incremental linear growth by one year than those infants with ossification.

37 DZ and 12 MZ twin pairs underwent a clinical and ECG heart examination. As to their zygosity, it is suggested that a concurrent similar clinical finding, very similar ECG and a mean intrapair difference of corresponding unipolar chest lead waves d ≤ 1.5 definitely support monozygosity.

Paired analysis of the ECG curves shows that the difference between MZ and DZ twins is best characterized by Tv1-6 and, to a certain extent, by Rv1-6. The same applies — in partial agreement with Mathers et al. — for PQ and QTc intervals.

Considering the intrapair differences in both MZ and DZ twins, it was found that they tend to increase in the course of one cardiac cycle, thus indicating a gradual prevalence of peristatic factors.

A high frequency of twin births has been observed in Linha São Pedro, a small settlement which belongs to the city of Cândido Godói, located 524 km Northwest from Porto Alegre, Rio Grande do Sul, Brazil, in an ethnically homogeneous population of German descent restricted to a small geographic region. From 1990 to 1994, the proportion of twin births in Linha São Pedro was 10%, significantly higher than the 1.8% rate for the state of Rio Grande do Sul as a whole. Genealogical analysis showed a high recurrence of multiple births within families, as well as a high level of inbreeding in the community. Zygosity data indicated that 9 of the 17 pairs of twins studied (53%) were dizygotic. No external environmental factors were detected that could be influencing the appearance of this characteristic. This preliminary investigation confirmed the presumed existence of a high twinning rate in the community. The high familial recurrence and the high inbreeding rate suggests the presence of genetic twinning factors. Complementary studies of twins that have yet to be evaluated and the search for additional risk factors, as well as linkage studies, should contribute to a further understanding of the biological factors related to twin births in the human species.