The present study aimed to determine a reliable tool to estimate the interval time to delivery in assisted conception twin pregnancies. Mid-gestation cervical length was prospectively measured using transvaginal sonography (TVS) during routine antenatal care. Fifty-seven of 101 suitable women were longitudinally followed and two TVS measurements of their cervical length were obtained, first at approximately 24 weeks gestation and then at approximately 27 weeks gestation. The mean cervical length decreased from 37 ± 12mm at first measurement to 34 ± 11mm at the second one. A linear regression model was found between the time interval of the first (R = 0.656, p < 0.001) and the second (R = 0.435, p < 0.001) assessments and the week of delivery. The current data confirm that the length of the preserved segment of the cervix is an important indicator of its competence. A simple equation using the cervical length (mm) divided by 3 can predict mid gestation scan-to-delivery interval in twin gestation.

Although, in general, twins have higher perinatal mortality rates than singletons, preterm twins have lower perinatal mortality rates than singletons of the same birth weight or gestational age. This study investigated the hypotheses that this paradoxical twin advantage: 1) is due to gestational age distribution differences between the singleton and twin populations, and 2) is due to increased likelihood of birth having occurred in a tertiary perinatal center. A pre-existing, time-limited data set of all births in the province of Ontario in odd years between 1979 and 1985 was chosen for this study because of the large sample size (n = 618,579). Multivariable logistic regression of the relationship between perinatal mortality and twin status was controlled for mother’s age, hospital level and gestational age. Findings confirm the lower mortality of preterm twins. After controlling for level of hospital of birth this difference remained, suggesting that level of hospital of birth was not a major factor responsible for the twin advantage. Analyses in which gestational age was standardized indicate that, for those whose gestational age was less than 2 SD below the mean for their particular group (twin or singleton), twins were actually at higher risk than singletons. These results support hypothesis 1 and do not strongly support hypothesis 2. The results also support earlier authors’ suggestions that the definition of term birth should be different for twins and singletons

Aretrospective study involving 623 twin and 1246 singleton births was conducted to compare the two groups with regard to selected maternal, fetal and labor and delivery characteristics and outcomes. Maternal age and parity were significantly higher for twins. The risks of preterm delivery, arrival in the labor ward in second stage of labor, cesarean births and postpartum haemorrhage were significantly higher in twin than in singleton births. In vaginal deliveries twin mothers were significantly less likely to have had episiotomies or perineal lacerations. There was no difference in the duration of the third stage of labor or in the incidence of retained placentae. Antepartum haemorrhage was a less likely indication for cesarean delivery among twins, while there was no significant difference in the likelihood of severe pre-eclampsia/eclampsia being an indication. Singleton babies were significantly heavier than twins. The incidences of malpresentation, low birth weight, stillbirths and of admission of live births to the neonatal intensive care unit were significantly higher in twins. There was no difference in the rate of instrumental vaginal delivery, or in the route of delivery of fetuses presenting by the breech. There is the need for detailed study of the incidences of antepartum haemorrhage and hypertensive diseases in twin and singleton pregnancies and of the factors determining the mode of delivery when such complications arise. Labor and delivery should also be examined to determine any differences between the two groups, especially in the first and second stages.

Dizygotic compared with monozygotic conceptions are at decreased risk of fetal and infant death and serious morbidity in surviving infants. Different sex twin maternities must be dizygotic but miscoding and incorrect registration of sex and number of fetuses may lead to an incorrect assignment of zygosity. The aim of the study was to validate the coding and registration of number and sex of births in multiple pregnancies. Fetal and infant death registrations from all multiple maternities in England and Wales 1993–1998 were examined. There were 51,792 twin, 1627 triplet and 51 higher order multiple maternities that were registered. Among these there were 1926 fetal deaths, 58 of which were registered as being of indeterminate sex but were coded as male in 56 and female in 2 cases. A fetus papyraceous was registered as male in 19 and as female in 19 cases. Other fetal deaths weighing ≤100g, with no mention of papyraceous on the death certificate, nevertheless, likely to be of indeterminate sex, were registered as male in 26 and as female in 23 cases. In 13 maternities, the number of infants registered at birth was less than the number mentioned on the registration certificate. It cannot be assumed that multiple births of different registered sex are dizygotic. As surviving infants from a monozygotic multiple birth are at much greater risk of infant death and serious morbidity than dizygotic multiple births, incorrect assignment of sex has important implications for parental counselling and may have medicolegal relevance when attributing negligence as the cause of morbidity in a survivor from a multiple pregnancy.

The aim of this study was to compare the educational achievement of female twins (n = 577) and their matched singleton controls (n = 447), who were selected from participants of a national test of educational achievement in the years 1993 to 1998. To assure the representativeness of the selected groups we also compared the achievement scores of the twins and the controls to those of the total Dutch female population tested in the same period. We analyzed the results of the following educational achievement scales: Language, Mathematics and Information Processing. The results indicated that the singleton classmates performed significantly better than the twins on all three scales. However, the twins performed equally well as compared to the Dutch female population. We believe that our singleton control group was not as properly selected as we intended, a selection bias operative at the level of the schools may have confounded the comparison. We therefore conclude on the basis of a comparison with the performance of the total Dutch female population that there are no differences in educational achievement between female twins and singletons.

To explore age-related mechanisms in the expression of recurrent headache, we evaluated whether genetic and environmental influences are a function of the reporting age using questionnaire information that was gathered in 1973 for 15- to 47-year-old Swedish twins (n = 12,606 twin pairs). Liability to mixed headache (mild migraine and tension-type headache) was explained by non-additive genetic influences (49%) in men aged from 15 to 30 years and additive genetic plus shared environmental influences (28%) in men aged from 31 to 47 years. In women, the explained proportion of variance, which was mainly due to additive genetic effects, ranged from 61% in adolescent twins to 12% in twins aged from 41 to 47 years, whereas individual specific environmental variance was significantly lower in twins aged from 15 to 20 years than in twins aged from 21 to 30 years. Liability to migrainous headache (more severe migraine) was explained by non-addi-tive genetic influences in men, 32% in young men and 45% in old men, while total phenotypic variance was significantly lower in young men than in old men. In women, the explained proportion of variance ranged from 91% in the youngest age group to 37% in the oldest age group, with major contributions from non-additive effects in young and old women (15–20 years and 41–47 years, respectively) and additive genetic effects in intermediate age groups (21–40 years). While total variance showed a positive age trend, genetic variance tended to be stable across age groups, whereas individual specific environmental variance was significantly lower in adolescent women as compared to older women.

It has been suggested that dizygotic twin pairs share two HLA haplotypes more often than ordinary siblings and thus might be genetically more alike. We tested this hypothesis in dizygotic twin pairs from the Danish Twin Registry. A total of 114 (60 female and 54 male) same-sexed healthy twin pairs aged 18–45 years participated. Dizygosity was established by means of DNA sequencing of nine polymorphic markers. HLA-A, B and Cw specificities were typed with serology, and if data were inconclusive, with DNA typing. If twin partners had the same HLA-types, they were assumed to share two haplotypes. If they had 1 HLA A, B and C antigen group in common they were assumed to share one haplotype and if they had no HLA types in common they were assumed to share zero haplotypes. Since HLA-types from parents were unavailable we could not test for identity-by-descent and thus had a risk of overestimating the number of twins sharing two haplotypes. A Chi-square test was used to compare observed numbers in each haplotype sharing group with the expected numbers. Twenty-nine (expected 28.5) twin pairs had two HLA-types in common, 52 (expected 57) had one HLA-type in common and 33 (expected 28.5) had zero HLA-types in common, p = 0.56. Our data show that DZ twins are not more similar than sibs from different pregnancies in general.

In this work we present a new method for genetic analysis of twin data which is based on generalized estimating equations and allows for analysis of various response types (e.g., continuous, binary, counts) combined with estimation of residual correlations. The new approach allows for control of covariates of any kind (e.g., continuous, counts) by modeling the dependence of mean and variance on background variables. The proposed method was applied to identify the covariates that have a significant influence on elderly people’s functional abilities, and find the estimates for the correlation coefficients of residuals for MZ and DZ twins in a sample of 2401 Danish twin 75 years of age or older. The bootstrap method was used to obtain standard errors for correlation coefficients. It was shown, that the chosen covariates have similar effects on MZ and DZ twins, and that the residual correlation in MZ twins is significantly higher than in DZ twins, which indicates that genetic factors play an etiological role in the determination of physical status of elderly people, controlled for 10 background variables.

Paternal influence on twinning was investigated through a study of all the state and church records of Scotland for the period 1800–2000 (nine generations) in relation to one Scottish patronymic — in total 50,000 births. All recorded twins born with the chosen patronymic were identified and their whole paternal ascent and descent on the male line were charted for twins. There were established three pedigrees A, B and C manifesting clear paternal twinning hereditary transmission. Detailed familial reproduction patterns were traced for pedigree A, including phenotypic identification of twin zygosity in relation to seven same sexed pairs of twins in the pedigree and one same sexed pair out of dizygotic triplets. It is the most comprehensive description to date of such a kind of twin familial trait. The data presented show (i) the unique feature of clear direct paternal influence on twinning in three families; (ii) paternal factor(s) determination in both DZ and MZ twinning; and (iii) a definite association of twinning tendency with a partial male infertility, which corresponds to the prediction of the Infertility/Twinning Paternally Dependent syndrome hypothesis. The hypothesis of a founder effect explaining the similarity of A, B, C families and the possible localisation of the paternally dependent twinning factor on the Y-chromosome are currently under molecular investigation.

Many twin researchers occasionally encounter twin pairs displaying unusual physical, medical or behavioral traits. Unfortunately, these interesting cases are often unreported because they do not fit in easily with ongoing projects. However, these “odd cases” may suggest new hypotheses, new designs or entirely new projects. In this paper, I argue in favor of publishing case studies of exceptional twins, and refer to several large scale studies inspired by such reports. Several informative case studies are then summarized: DZ twins discordant for hyperinsulinism, DZ twins born after ICSI, “simultaneous madness in twins, and sixteenth century conjoined twins. The article concludes with a review of parenting twin pandas.