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AbstractTwins are not a homogeneous group. According to zygosity and chorionicity essential differences exist. The lower sex proportion at birth is due to the monozygotic twins, especially the monoamnionic variety. X-inactivation patterns in monozygotic twin girls are totally symmetrical in monoamnionic pairs, almost symmetrical in monochorionic diamnionic pairs and can be very asymmetrical in the dichorionic variety.

AbstractThis article discusses findings of two recent studies conducted in collaboration with the East Flanders Prospective Twin Survey in the field of cognitive ability. The first study examined the effect of chorion type on heritability estimates of intelligence in children. The second study investigated the causes of association between child psychopathology and lower cognitive ability. Findings of these studies are discussed in the light of the current view on cognitive ability (or 'g') and recommendations for future research are made.

AbstractBoth zygosity and chorionicity provide important information in twin research. The East Flanders Prospective Twin Survey (EFPTS) determines zygosity and chorionicity at birth and therefore provides a gold standard for the testing of diagnostic parameters that can be used to determine chorionicity. The aim of the present study was to investigate whether birthweight discordancy can be used as an indicator of chorionicity. The study sample consisted of 4,060 live-born twin pairs from the EFPTS. We studied MZ twins, using univariate and multivariate logistic regression analyses to calculate odds ratios (OR) and 95% confidence intervals (CI) of being MC in relation to discordancy level. Diagnostic parameters, including sensitivity and specificity, were calculated. A two-fold cross-validation was carried out and a bootstrap distribution with 10,000 samples was created to estimate the standard deviations. For discordancy levels of below 10%, 10–15%, 15–20%, 20–25% and above 25%, the ORs (95% CI) were 1.16 (0.91–1.47), 1.38 (1.05–1.80), 2.13 (1.51–3.01), 2.73 (1.73–4.29) and 2.81 (2.81–4.35) respectively. There were no gender differences. Sensitivity was 42.2% (SD 5.6%), specificity was 72.8% (SD 6.3%), positive predictive value was 72.8% (1.5%) and the negative predictive value was 39.2% (0.7%). In conclusion, although a higher discordancy level resulted in higher ORs of being an MC twin, birthweight discordancy level can only be used to some weak extent as a proxy for chorionicity, highlighting the need to assess and record chorionicity data in obstetrical units.

AbstractThe focus of research in depression is on negative affect. However, positive affect is under-investigated and plays an important role in resilience against depression by neutralizing the effects of genetic vulnerability to depression.

AbstractAn adverse intra-uterine environment has been associated with abdominal fat distribution in singletons. Twins often have a low birth weight and a short gestation. Therefore, they may have an increased risk to develop abdominal obesity. Furthermore, monozygotic monochorionic twins (MZ MC) have a larger intra-pair birth weight difference compared to monozygotic dichorionic twins (MZ DC). If adult anthropometry is programmed in utero, this may affect the intra-pair correlations in adulthood and, consequently, also the results from the classic twin method to estimate genetic and environmental influences. In the present study, we compared the absolute values, the intra-pair differences, and the intra-pair correlations of body mass, height, BMI, and abdominal fat distribution of 424 MZ MC, MZ DC and dizygotic (DZ) twin pairs (aged 18–34 yrs). DZ, MZ DC and MZ MC twins did not differ for most anthropometric characteristics. Only MZ women tended (p = 0.03) to accumulate more abdominal fat compared to DZ twins. Overall, the contribution of zygosity and chorion type to adult anthropometry was rather low (≤ 1.7%). Although the intra-pair birth weight difference of MZ MC pairs (10.5% in men, 12.3% in women) was significantly larger compared to that of MZ DC pairs (6.9% and 9.2% resp.), the intra-pair differences in adult anthropometry were similar for both MZ twin types. Also the intra-pair correlations of MZ MC and MZ DC pairs were strikingly alike, suggesting no significant influence of the prenatal environment on adult concordance. In conclusion, the substantial difference in the prenatal environment of MZ MC and MZ DC twins did not result in a difference in intra-pair concordance of adult anthropometry and fat distribution. Therefore, we suggest that the chorion type of MZ twins does not bias the twin design and the estimation of the genetic contribution to adult anthropometry.

AbstractUnlike-sex twins provide a unique natural experiment to investigate the influence of sex on gestation. Our data showed that length of gestation of unlike-sex pairs is similar to that of female same-sex pairs, and significantly (0.4 wks,p= .02) longer than that of male same-sex pairs. Birthweight of female unlike-sex twins was similar to female same-sex twins, but male unlike-sex twins weighed 78 g more than male same-sex twins (p= .001). These data show that in unlikesex pairs it is the girl that prolongs gestation for her brother, resulting in a higher birthweight than that of same-sex boys.

The East Flanders Prospective Twin Survey (EFPTS) is a prospective, population-based registry of multiple births in the province of East-Flanders, Belgium. EFPTS has several unique features: it is population-based and prospective, with the possibility of long-term follow-up; the twins (and higher order multiple births) are recruited at birth; basic perinatal data recorded; chorion type and zygosity established; and since 1969 placental biopsies have been taken and frozen at −20 °C for later determination of genetic markers. The EFPTS is the only large register that includes placental data and allows differentiation of three subtypes of monozygotic (MZ) twins based on the time of the initial zygotic division: the dichorionic–diamnionic pairs (early, with splitting before the fourth day after fertilization), the monochorionic–diamnionic pairs (intermediate, splitting between the fourth and the seventh day post-fertilization), and the monochorionic–monoamnionic pairs (late, splitting after the eighth day post-fertilization). Studies can be initiated taking into account primary biases, those originating 'in utero'. Such studies could throw new light on the controversy over the validity of the classic twin method, the consequences of early embryological events, and the gene–environment interactions as far as periconceptional and intrauterine environment are concerned.

AbstractThe East Flanders Prospective Twin Survey (EFPTS) is a registry of multiple births in the province of East Flanders, Belgium. Since its start in 1964, over 10,000 twin-pairs have been registered. EFPTS has several unique features: it is population-based and prospective, with the possibility of long-term follow-up; the twins (and higher order multiple births) are recruited at birth; basic perinatal data are recorded; chorion type and zygosity are established; since 1969, placental biopsies have been taken and frozen at –20°C for future research. Since its origin, the EFPTS has included placental data and allows differentiation of three subtypes of monozygotic twins based on the time of the initial zygotic division: the dichorionic–diamniotic pairs (early, with splitting before the fourth day after fertilization), the monochorionic–diamniotic pairs (intermediate, splitting between the fourth- and the seventh-day postfertilization) and the monochorionic–monoamniotic pairs (late, splitting after the eighth day postfertilization). Studies can be initiated taking into account primary biases, those originating 'in utero'. Such studies could throw new light on the consequences of early embryological events and the gene–environment interactions as far as periconceptional and intrauterine environment are concerned.

AbstractThe East Flanders Prospective Twin Survey (EFPTS) is a prospective, population-based registry of multiple births in the province of East Flanders, Belgium. EFPTS has several unique features: it is population based and prospective, with the possibility of long-term follow-up; the twins (and higher order multiple births) are ascertained at birth; basic perinatal data recorded; chorion type and zygosity established; and since 1969 placental biopsies have been taken and frozen at −20 °C for later determination of genetic markers. The EFPTS is the only large register that includes placental data and allows differentiation of 3 subtypes of monozygotic (MZ) twins based on the time of the initial zygotic division: the dichorionic–diamnionic pairs (early, before the 4th day after fertilization), the monochorionic–diamnionic pairs (intermediate, between the 4th and the 7th day post fertilization), and the monochorionic–monoamnionic pairs (late, after the 8-day post fertilization). This added a new dimension to didymology (the science of twins; didymos is the Greek word for twin): the timing of MZ twinning. Studies can be initiated taking into account primary biases, those originating in utero. Such studies could throw new light on the controversy over the validity of the classic twin method, the consequences of early embryological events (before and just after implantation of the embryo), the origin of congenital malformations, the sex proportion of multiples, the gene–environment interactions as far as intrauterine environment is concerned, to name but a few.

AbstractThe purpose of this study is to present curves of estimated placental growth in twins and to evaluate the relative contribution of gestational age, zygosity, chorionicity, fusion of the placentas, sex of the individual and of the twin pair, site of the umbilical cord insertion, birth order, maternal age, and parity. Perinatal data and placental data were obtained from 6315 live-born twin pairs from the East Flanders Prospective Twin Survey. Of 4318 twin pairs, with no missing values, the placental weights of different gestational ages were analyzed using a nonlinear multivariate Gaussian regression. Two groups were distinguished: (1) twins with two separate placentas, and (2) twins with only one placental mass (one placenta in case of monochorionic twins or two fused placentas in case of dichorionic placentas). Overall, placental weight was influenced by gestational age, fusion of the placentas, and parity. In the case of one placental mass, monozygotic dichorionic twins had the lowest weights. If two separate placentas were present, birth order played a role in favor of the first-born twin. For parity and zygosity, the differences were most pronounced between 27 and 29 weeks, whereas the difference for birth order was most pronounced between 33 and 37 weeks. In conclusion, basic physiological characteristics, routinely examined at birth, influence placental weight. Taking these covariates into account allows a better evaluation of the placental weight given a gestational age, as an indicator of growth.