Assisted reproductive techniques have led to an increase in the proportion of maternities that are multiple. Though predominantly dizygotic, they are at greater risk of monozygotic division than those spontaneously conceived. England and Wales data 1974–99 on stillbirths and livebirths were analysed for 4 periods: 1974–80 (pre-assisted reproduction; 1982–8; 1989–91 (pre-redefinition of stillbirth); 1993–9 (post-redefinition of stillbirth). For twin data, Weinberg's rule was applied to estimate the proportions that were mono- (MZ) and dizygotic (DZ). Compared with the period before assisted reproduction, the most recent period shows an increase in twin maternities of 3.81 per 1000 comprised of 3.22 (95% CI 3.10 to 3.33; p < 0.0001) DZ and 0.60 (95% CI 0.51 to 0.68; p < 0.0001) MZ twins. It is estimated that 15.7% of assisted reproduction twins are MZ. Higher order multiple births showed an increase of 3.06 (95% CI 2.85 to 3.29; p < 0.0001) per 10,000 maternities. Stillbirth rates in MZ twins are of the same order of magnitude as those in higher order multiple births but higher than those in DZ twins. The improvement in stillbirth rates over the 26 year study period is of the same order magnitude in singletons, DZ and MZ twins and higher order multiples. Assisted reproduction has led to a significant increase in the proportion of MZ twins. These are at high risk of fetal death and this needs to be considered when local stillbirth and perinatal mortality rates are used in auditing obstetric services.

The objective of this study was to compare perinatal results in multifetal pregnancies where the reduction to twins was performed with non-reduced twin pregnancies. Perinatal results in 99 sets of twins after transabdominal multifetal pregnancy reduction of triple and higher-order multiple pregnancies performed in a single center were compared with a control group consisting of 151 twin pregnancies conceived in the same time period after infertility treatment, which were not a result of reduction. The main outcome measures were length of pregnancies, weight of the newborns, percentage of miscarriages and the mode of the delivery were analyzed. No significant difference could be found at a 5% level of significance regarding the average duration of pregnancy or average weight of the twins. Fisher test on 5% significance level did not ascertain any significant difference in the probability of miscarriage between the group with reduction (5.26%) and the group without reduction (12.84%). At a 5% level of statistical significance, no significant difference in probability of perinatal death of the fetus or delivery of a stillborn fetus was found. The percentage of cesarean sections did not differ significantly in both groups. The analysis of both groups demonstrated that reduction of multifetal pregnancies to twins may not influence perinatal results in comparison to twin pregnancies where reduction was not performed.

While the true figures are not well established, outcomes of twin pregnancies are directly dependent on a small number of preterm births between 22 and 27 weeks. Observation of perinatal outcomes in twin pregnancies yields two contradictory results. Firstly, it shows an improvement in perinatal mortality figures. Secondly, it reveals an increase in the rates of preterm deliveries. These findings result from the observation of 783 twin pregnancies followed and delivered in a level 3 perinatal centre in Paris between 1993 and 1998. Women followed since the beginning of pregnancy through the outpatient clinic of the institution are included in this number, as are women who were referred or transferred to the centre at a later date due to complications. This analysis reflects the influence of two contrasting policies. The first, and less recent policy is devoted to the prevention of preterm births, and is reflected by the low number of extremely preterm deliveries at 22–32 weeks. The second is the effect of our new approach to the prevention of foetal deaths in relation to foetal growth retardation in twins which has resulted in increased medical intervention such as the induction of labour or scheduled Caesarean birth. This has resulted in an increase in twin preterm births from 33 to 36 weeks, with the expected result of fewer foetal deaths.

With improved technology in assisted reproductive medicine, there has been an absolute increase in the numbers of twin pregnancies with an associated increase in perinatal mortality and morbidity. This increase in perinatal mortality and morbidity is largely due to a higher incidence of delivering preterm as compared to singletons. Twin pregnancies have their unique complications that include abnormal placental communication and discordant growth which are associated with perinatal mortality and morbidity. The objectives of this study were two-fold: i) to determine if the morbidity/mortality outcome at 18–24 months corrected age seen in a cohort of twins born between 24–30 weeks gestation was significantly different as compared to singleton preterm infants of the same gestation; and ii) to determine and evaluate any differences between monochorionic (MC) and dichorionic (DC) twins. Twins 24–30 weeks gestation at birth born between 01/01/97–30/06/99 were identified and prospectively followed to 18–24 months corrected age (c.a.). They were matched with a singleton infant of the same gender and within 1 week of the same gestation. Obstetrical, neonatal and neurodevelopmental data were gathered and analyzed. The primary outcome was death or the presence of a severe neurodevelopmental deficit at 18–24 months corrected age. Of the 56 sets of twins identified, 52 sets were followed prospectively with 101 infants available for matching. In this cohort, twin pregnancies had a lower incidence of pregnancy-induced hypertension and premature rupture of membranes than singletons (p < 0.05). The two groups were comparable in neonatal characteristics. The incidence of death or severe disability was 29.7% in twins vs. 22.8% in singletons (p = 0.337, Fisher's exact test). The major area of deficit was in the cognitive category for both groups, 9.9% vs. 7.9% respectively. MC twins made up 35.6%; DC twins 64.4%. Twin to twin transfusion syndrome (TTTS) occurred in 6.9%. Discordant growth occurred more frequently in MC pregnancies (p = 0.016). MC twins tended to be more premature, lower in birth weight, and experience neonatal morbidity in the form of patent ductus arteriosus and sepsis (p < 0.05) as compared to DC twins. However, the primary outcome of death or severe neurodevelopmental deficit at 18–24 months c.a. was not significantly different between the two groups, 38.9% (MC) vs. 24.6% (DC), (p = 0.173, Fisher's exact test). Neurodevelopmental morbidity or mortality in twins with TTTS was 42%. Mortality and severe neurodevelopmental morbidity were not significantly higher in twins as compared to singletons in this cohort. However, the trend is slightly higher in twins, which may have clinical significance. Though not statistically significant, the incidence of 38.9% in adverse outcome with MC twins may be clinically significant. With the number of twins steadily increasing, further monitoring is required to determine future directions in intervention and research. Early recognition of monochorionicity remains essential to optimize care and neurodevelopment for these infants.

The purpose of the present study is to clarify the mechanism of Japanese self-esteem (SE) in genetic and environmental influences using twin methodology. Eighty-one pairs of adolescent twins, including 50 pairs of monozygotic (MZ) twins and 31 pairs of dizygotic (DZ) twins, participated in this study. Self-esteem was assessed using the Rosenberg Self-Esteem Scale (RSES), translated into Japanese. As a result of using univariate twin analyses, model comparisons using the Akaike Information Criterion (AIC) indicated that the AE model was the best fit (AIC = −5.35). In the best-fitting AE model, the heritability (a2) of SE was revealed to be moderate, accounting for 49% of the variance; environmental influences (individual-specific environmental factors) explained 51% of the variance. These results are consistent with the findings of some behavioral genetics studies of SE in the West and show that there is no difference between Western and Japanese populations in the mechanism of SE considering genetic and environmental influences. The results also suggest the importance of considering both genetic and environmental factors in studies of Japanese SE.

While it is widely assumed that twinship involves high levels of intimacy to the detriment of other relationships, an examination of twinship using measures of intimacy has not yet been conducted. Participants were 214 individual twins (128 MZ, 62 DZ same-sex, and 24 DZ mixed-sex) between 16–73 years of age. Twins completed measures of intimacy in reference to their co-twin, their closest same-sex friend, and their closest other-sex friend. As expected, the highest level of intimacy was reported for the co-twin. Contrary to expectation, MZ twins did not report significantly higher levels of intimacy with their co-twin, or significantly lower levels of intimacy with their closest friends when compared to DZ same-sex or DZ mixed-sex twins. Furthermore, twins who reported high levels of intimacy with their co-twin did not report significantly lower levels of intimacy with their closest friends. Implications and directions for future research are discussed.

Problem behavior in children shows a high degree of cooccurrence, both within the domains of internalizing and externalizing behaviors and across domains. Maternal ratings of 207 same- and opposite-sex twin pairs on the Child Behavior Checklist (CBCL/4–18; Achenbach, 1991) were used to determine the etiology of these associations. In the current sample of 4- to 11-year-old children (mean age: 7.6 years) phenotypic correlations were .68 between Internalizing and Externalizing, between .41 and .66 within the internalizing scales, and between .58 and .71 within the externalizing scales. Correlations across domains on the scale level were of similar magnitude. Genetic and shared environmental correlations were generally high, with the exception of the somatic complaints scale which showed low genetic correlations with other scales. These findings of uniformly high shared etiology was seen within and across domains. For example, Attention Problems showed genetic correlations of .65 with Delinquent Behavior and .76 with Aggressive Behavior; the genetic correlations of Attention Problems with internalizing scales were .71 for Anxious/Depressed and .79 for Withdrawn; a low genetic correlation was only shown for the Somatic Complaints scale (rg = .16). Although the magnitude of shared environmental contributions was lower, the environmental correlations were close to unity, indicating that the same family environmental factors were involved in aspects of problem behavior when assessed across domains. Results for the Attention Problems scale were similar to other disruptive behavior, justifying its inclusion with the externalizing scales.

Identifying twins for a population-based register can be achieved through birth records or community surveys. We studied the feasibility and effectiveness of different methods of identifying and recruiting twins to establish a population based register. To trace twins a population survey was carried out using an interviewer administered questionnaire. We also inspected the birth registration certificates at a divisional secretariat reported from a specified hospital between the years of 1985–1997 and compared it to the birth register of this same hospital. To recruit twins a random sample of 75 twin pairs (150 twins) identified at the Divisional Secretariat were contacted through the post and 25 twin pairs (50 twins) were personally visited. The prevalence of twins was 6.5 twins per 1000 people in the area surveyed. The twinning rate at the hospital was 18.92 twins per 1000 births. A discrepancy of 38 multiples births between the hospital labour room records and those registered at the DS was noted. The response from the postal invitation for recruitment was 59% and the response from the personal invitation was 68%. (Difference 9.4% 95% CI; 7.06–11.73). Community survey and systematic inspection of birth records either at the hospital or the birth registration office was an effective method to trace twins. Once traced, personal contact was more effective than the postal invitation for recruitment of younger twins. A cost-effective approach would be to use a postal coverage followed by personal contact for non-responders. The alternative method, community coverage, would have financial implications.

The classic twin study is sometimes described as “the perfect natural experiment” for the investigation of the aetiology of complex disease, but assumptions of the twin design need to be empirically tested if their results are to be considered unbiased and representative of singleton populations. In this study comparisons of disease and prevalence of lifestyle characteristics have been made between twin participants in the St Thomas' Hospital UK adult twin registry, the largest twin volunteer register in the UK for the study of diseases of ageing, and a parallel population-based study of singleton women. The only differences found were for weight, where monozygotic (MZ) twins were lighter and had a smaller variance than dizygotic (DZ) twins and singletons. For the other variables studied, volunteer twins were not found to differ from age-matched singleton women in distribution or prevalence of: bone mineral density, osteoarthritis, blood pressure, hypertensive drug use, height, history of hysterectomy and ovariectomy, menopausal status and current alcohol and overall tobacco consumption. We conclude that the results of twin studies can be generalised to singleton populations for these measures and disease outcomes.