Dichorionic Monozygotic Twins Research Articles

Cellular mechanisms of monozygotic twinning: clues from assisted reproduction.

Monozygotic (MZ) twins are believed to arise from the fission of a single fertilized embryo at different stages. Monochorionic MZ twins, who share one chorion, originate from the splitting of the inner cell mass (ICM) within a single blastocyst. In the classic model for dichorionic MZ twins, the embryo splits before compaction, developing into two blastocysts. However, there are a growing number of ART cases where a single blastocyst transfer results in dichorionic MZ twins, indicating that embryo splitting may occur even after blastocyst formation. For monochorionic MZ twins, we conducted a comprehensive analysis of the cellular mechanisms involved in ICM splitting, drawing from both ART cases and animal experiments. In addition, we critically re-examine the classic early splitting model for dichorionic MZ twins. We explore cellular mechanisms leading to two separated blastocysts in ART, potentially causing dichorionic MZ twins. Relevant studies including research articles, reviews, and conference papers were searched in the PubMed database. Cases of MZ twins from IVF clinics were found by using combinations of terms including 'monozygotic twins' with 'IVF case report', 'ART', 'single embryo transfer', or 'dichorionic'. The papers retrieved were categorized based on the implicated mechanisms or as those with unexplained mechanisms. Animal experiments relating to MZ twins were found using 'mouse embryo monozygotic twins', 'mouse 8-shaped hatching', 'zebrafish janus mutant', and 'nine-banded armadillo embryo', along with literature collected through day-to-day reading. The search was limited to articles in English, with no restrictions on publication date or species. For monochorionic MZ twins, ART cases and mouse experiments demonstrate evidence that a looser ICM in blastocysts has an increased chance of ICM separation. Physical forces facilitated by blastocoel formation or 8-shaped hatching are exerted on the ICM, resulting in monochorionic MZ twins. For dichorionic MZ twins, the classic model resembles artificial cloning of mouse embryos in vitro, requiring strictly controlled splitting forces, re-joining prevention, and proper aggregation, which allows the formation of two separate human blastocysts under physiological circumstances. In contrast, ART procedures involving the transfer of a single blastocysts after atypical hatching or vitrified-warmed cycles might lead to blastocyst separation. Differences in morphology, molecular mechanisms, and timing across various animal model systems for MZ twinning can impede this research field. As discussed in future directions, recent developments of innovative in vitro models of human embryos may offer promising avenues for providing fundamental novel insights into the cellular mechanisms of MZ twinning during human embryogenesis. Twin pregnancies pose high risks to both the fetuses and the mother. While single embryo transfer is commonly employed to prevent dizygotic twin pregnancies in ART, it cannot prevent the occurrence of MZ twins. Drawing from our understanding of the cellular mechanisms underlying monochorionic and dichorionic MZ twinning, along with insights into the genetic mechanisms, could enable improved prediction, prevention, and even intervention strategies during ART procedures. N/A.

Birth of monozygotic dichorionic twins after a single blastocyst embryo transfer: a case report of genetic determination of zygosity

To report a case of monozygotic dichorionic (DC) twins after a single cryopreserved blastocyst embryo transfer followed by genetic determination of zygosity postpartum. Case report. University hospital. A 26-year-old woman with polycystic ovary syndrome and her 36-year-old male partner with severe oligozoospermia, resulting in a 1.5-year history of primary infertility. Controlled ovarian stimulation and intracytoplasmic sperm injection treatment with single cryopreserved embryo transfer at blastocyst stage. Ultrasound images of the fetuses and short tandem repeat genotyping postpartum. A DC twin pregnancy following a single cryopreserved blastocyst embryo transfer was confirmed at the first trimester screening. Confirmatory testing performed postpartum included short tandem repeat analysis determining monozygosity and pathology examination reporting DC placental configuration. Dichorionic monozygotic twins are thought to arise from the splitting of an embryo before the blastocyst stage. This case suggests that placental configuration of monozygotic twins may not strictly depend on timing of embryo division. Genetic analysis is the only tool to confirm the zygosity.

Early-life residential green spaces and traffic exposure in association with young adult body composition: a longitudinal birth cohort study of twins

BackgroundGlobally, the rapid increase of obesity is reaching alarming proportions. A new approach to reduce obesity and its comorbidities involves tackling the built environment. Environmental influences seem to play an important role, but the environmental influences in early life on adult body composition have not been thoroughly investigated. This study seeks to fill the research gap by examining early-life exposure to residential green spaces and traffic exposure in association with body composition among a population of young adult twins.MethodsAs part of the East Flanders Prospective Twin Survey (EFPTS) cohort, this study included 332 twins. Residential addresses of the mothers at time of birth of the twins were geocoded to determine residential green spaces and traffic exposure. To capture body composition, body mass index, waist-to-hip ratio (WHR), waist circumference, skinfold thickness, leptin levels, and fat percentage were measured at adult age. Linear mixed modelling analyses were conducted to investigate early-life environmental exposures in association with body composition, while accounting for potential confounders. In addition, moderator effects of zygosity/chorionicity, sex and socio-economic status were tested.ResultsEach interquartile range (IQR) increase in distance to highway was found associated with an increase of 1.2% in WHR (95%CI 0.2–2.2%). For landcover of green spaces, each IQR increase was associated with 0.8% increase in WHR (95%CI 0.4–1.3%), 1.4% increase in waist circumference (95%CI 0.5–2.2%), and 2.3% increase in body fat (95%CI 0.2–4.4%). Stratified analyses by zygosity/chorionicity type indicated that in monozygotic monochorionic twins, each IQR increase in land cover of green spaces was associated with 1.3% increase in WHR (95%CI 0.5–2.1%). In monozygotic dichorionic twins, each IQR increase in land cover of green spaces was associated with 1.4% increase in waist-circumference (95%CI 0.6–2.2%).ConclusionsThe built environment in which mothers reside during pregnancy might play a role on body composition among young adult twins. Our study revealed that based on zygosity/chorionicity type differential effects of prenatal exposure to green spaces on body composition at adult age might exist.

P-139 The incidence of monozygotic twins after the usage of calcium ionophore or after trophectoderm biopsy in ART treatment

Abstract Study question To investigate whether the usage of calcium ionophore or trophectoderm biopsy affect the occurrence of monozygotic twins (MZT) in assisted reproductive treatment (ART) treatment. Summary answer Despite the increase was not statistically significant, higher percentage of MZT were observed in both group with calcium ionophore usage and trophectoderm biopsy. What is known already The incidence of monozygotic twins (MZT) in natural conception is approximately 0.4% worldwide but it is estimated that there is at least a 2-fold rise in the incidence of MZT after assisted reproductive technology compared with natural conception, although the reason is still under debate. The effectiveness of calcium ionophore in assisting oocytes activation and fertilisation has been widely studied. Most studies reported no impact upon embryo viability and reproductive potential after trophectoderm biopsy. However, the impact of calcium ionophore or trophectoderm biopsy on subsequent in vivo development of the blatocyst remains unclear. Study design, size, duration This is a retrospective observational cohort study of frozen embryos transfers (FET) that have achieved clinical pregnancy between January 2019 and October 2021. In this study, 709 FET cycles were included and analysed based on i) with or without calcium ionophore treatment after ICSI/IVF on oocytes retrieval day; ii) with or without trophectoderm biopsy on day 5 or 6. The results were analysed to identify possible risk factors affecting the prevalence of monozygotic twins. Participants/materials, setting, methods The cases with positive beta-hCG test were followed-up and analysed for the occurrence of MZT. The diagnosis of MZT was established by transvaginal ultrasound performed at 4-6 weeks after the transfer. The visualization of uteri containing more than one fetus, either in one sac (monochorionic monoamniotic) or in two sacs separated by a septum (monochorionic diamniotic) is suggestive of MZT. Data were analysed by Fisher’s exact test and Odd Ratio. Main results and the role of chance Data comprising a total of 709 FET cycles in a single clinical setting were analysed. Among 449 cases of clinical pregnancies, 8 cases of MZT occurred. A higher percentage of MZT (1.78%) after blastocyst transfer in ART treatment was observed as compared to natural conception (0.04%). There was a higher incidence of MZT in trophectoderm biopsy group as compared with non-biopsy group (2.3% vs 1.45% respectively, odds ratio [OR]= 1.59, 95% CI 0.39–6.46). However, there was no statistically significant (P-value=0.556, CI = 95%). The incidence of MZT following the calcium ionophore usage was higher compared with the group without calcium ionophore usage (2.33% versus 1.72%, OR = 1.36, 95% CI 0.16–11.3). There was no significantly higher risk of MZT with calcium ionophore versus non-calcium ionophore cycles (P-value= 0.493, CI = 95%). Limitations, reasons for caution The sample examined in this study is limited in size thus more studies are needed to achieve satisfactory statistical power. MZT diagnosed with ultrasound is difficult to differentiate from dichorionic monozygotic twins. The definitive diagnosis of monozygosity is only possible by DNA analysis of all the twins born. Wider implications of the findings These findings can draw attention to study the effect of calcium ionophore on embryo development and the mechanism behind MZT. Based on the findings, the patients undergoing ART treatment should be informed with the risk associated with higher incidence of MZT. Trial registration number NA

Foeto-foetal transfusion syndrome in dichorionic diamniotic twins: a clinical case

Background. Management of multiple gestation complicated by the foeto-foetal transfusion syndrome is among most intricate modern obstetric issues. The syndrome develops in 10–20% of monochorionic diamniotic twins leading to 80–100% mortality in one or both twins if left uncorrected, especially in early syndrome cases. Although foeto-foetal transfusion usually develops with monochorionic placentae, there are notable exceptions of vascular placental anastomoses reported with dichorionic monozygotic twins. The disease supposedly entails from an imbalanced blood interflow between dichorionic twins due to placental vascular anastomoses.Clinical Case Description. Patient S., 32 yo, visited perinatal diagnostics at the Territorial Perinatal Centre of the Children’s Territorial Clinical Hospital with a preliminary diagnosis: 22 weeks and 5 days’ pregnancy. Dichorionic diamniotic twins. Threat of extremely preterm birth. Two caesarean uterine scars. Gestational diabetes mellitus.Medical files: patient history, pregnancy calendar. Pregnancy was regularly monitored with main ultrasound foetometry, foeto-foetal transfusion dynamics control and Doppler velocimetry.Ultrasonographic signs of abnormal haemodynamics underlying the foeto-foetal transfusion syndrome were detected at the first scan at 11–13 weeks 6 days’ term as collar space thickening in one foetus. Hydramnios in one foetus and oligohydramnios in the other were diagnosed at 28 weeks’ gestation conclusively indicating the foeto-foetal transfusion syndrome. This evidence suggested the formation of placental anastomoses, which was confirmed morphologically in placenta examination.Conclusion. A timely diagnosis and correction of emerging complications allowed prolongation of pregnancy in hospital conditions to 33–34 weeks. Both boys were live-born with Apgar score 7–8.

Foeto-foetal transfusion syndrome in dichorionic diamniotic twins: a clinical case

Background. Management of multiple gestation complicated by the foeto-foetal transfusion syndrome is among most intricate modern obstetric issues. The syndrome develops in 10–20% of monochorionic diamniotic twins leading to 80–100% mortality in one or both twins if left uncorrected, especially in early syndrome cases. Although foeto-foetal transfusion usually develops with monochorionic placentae, there are notable exceptions of vascular placental anastomoses reported with dichorionic monozygotic twins. The disease supposedly entails from an imbalanced blood interflow between dichorionic twins due to placental vascular anastomoses.Clinical Case Description. Patient S., 32 yo, visited perinatal diagnostics at the Territorial Perinatal Centre of the Children’s Territorial Clinical Hospital with a preliminary diagnosis: 22 weeks and 5 days’ pregnancy. Dichorionic diamniotic twins. Threat of extremely preterm birth. Two caesarean uterine scars. Gestational diabetes mellitus.Medical files: patient history, pregnancy calendar. Pregnancy was regularly monitored with main ultrasound foetometry, foeto-foetal transfusion dynamics control and Doppler velocimetry.Ultrasonographic signs of abnormal haemodynamics underlying the foeto-foetal transfusion syndrome were detected at the first scan at 11–13 weeks 6 days’ term as collar space thickening in one foetus. Hydramnios in one foetus and oligohydramnios in the other were diagnosed at 28 weeks’ gestation conclusively indicating the foeto-foetal transfusion syndrome. This evidence suggested the formation of placental anastomoses, which was confirmed morphologically in placenta examination.Conclusion. A timely diagnosis and correction of emerging complications allowed prolongation of pregnancy in hospital conditions to 33–34 weeks. Both boys were live-born with Apgar score 7–8.

Chorionicity and Heritability Estimates from Twin Studies: The Prenatal Environment of Twins and Their Resemblance Across a Large Number of Traits

There are three types of monozygotic (MZ) twins. MZ twins can either share one chorion and one amnion, each twin can have its own amnion, or MZ twins can—like dizygotic twins—each have their own chorion and amnion. Sharing the same chorion may create a more similar/dissimilar prenatal environment and bias heritability estimates, but most twin studies do not distinguish between these three types of MZ twin pairs. The aim of this paper is to investigate the effect of chorion sharing on the similarity within MZ twin pairs for a large number of traits. Information on chorion status was obtained for the Netherlands twin register (NTR) by linkage to the records from the database of the dutch pathological anatomy national automated archive (PALGA). Record linkage was successful for over 9000 pairs. Effect of chorion type was tested by comparing the within-pair similarity between monochorionic (MC) and dichorionic (DC) MZ twins on 66 traits including weight, height, motor milestones, child problem behaviors, cognitive function, wellbeing and personality. For only 10 traits, within-pair similarity differed between MCMZ and DCMZ pairs. For traits influenced by birth weight (e.g. weight and height in young children) we expected that MC twins would be more discordant. This was found for 5 out of 13 measures. When looking at traits where blood supply is important, we saw MCMZ twins to be more concordant than DCMZ’s for 3 traits. We conclude that the influence on the MZ twin correlation of the intra-uterine prenatal environment, as measured by sharing a chorion type, is small and limited to a few phenotypes. This implies that the assumption of equal prenatal environment of mono- and DC MZ twins, which characterizes the classical twin design, is largely tenable.Electronic supplementary materialThe online version of this article (doi:10.1007/s10519-015-9745-3) contains supplementary material, which is available to authorized users.

What makes them split? Identifying risk factors that lead to monozygotic twins after in vitro fertilization

To identify the incidence, risk factors, and obstetric/perinatal outcomes associated with monozygotic twins (MZTs) after IVF. Nested case-control. University-based center. The IVF cycles eventuating in pregnancy from 2000-2009. None. The MZT incidence, chorionicity/zygosity, pregnancy/neonatal outcome. Of 6,223 gestations, 131 MZTs were diagnosed (2.1% incidence; 2.0% in autologous and 2.7% in donor IVF cycles), 10 were dichorionic, and 121 were monochorionic. Controlling for all risk factors, young oocyte age, extended culture (noncleavage embryos transferred on/after day 4), and year of IVF treatment cycle were significantly associated with MZT. When assessing factors associated with specific MZT placentation, day 3 assisted hatching correlated more with dichorionic MZT, whereas extended culture and advanced day 5 embryonic stage correlated with monochorionic MZT. Comparing monozygotic to dizygotic multigestation outcomes, MZT fared worse; however, once controlling for triplet gestation, only gestational age at delivery remained significantly compromised in the monozygotic group. After IVF the incidence of MZT is high, with young oocyte age, year of treatment, and extended culture (or embryo stage at transfer) conferring greatest risk. Regarding MZT type, assisted reproductive technology (ART) procedures may influence the timing of embryonic splitting (i.e., division in early embryonic development may be influenced by zona pellucida [ZP] manipulation whereas later splitting may occur during delayed implantation). Poor obstetric/perinatal outcome is significantly impacted by the presence of an "extra" fetus, as high-order multiple gestation concurrent with an MZT conveyed the worst prognosis.

Difference in nuchal translucency between monozygotic and dizygotic spontaneously conceived twins

To assess nuchal translucency (NT) thickness and differences in inter-twin NT thickness among naturally conceived monozygotic and dizygotic twins. Enrolled women prospectively underwent NT measurement for both twins between 11 and 14 weeks of gestational age. NT thickness and inter-twin NT difference were recorded and compared among dizygotic twins, monozygotic dichorionic twins, and monozygotic monochorionic twins. This study obtained analyzable data for 174 monozygotic twins and 107 dizygotic twins. Of the 174 monozygotic twins, 105 were monochorionic, and 69 were dichorionic. Monozygotic monochorionic twin pregnancies had a significantly higher mean fetal NT thickness (1.48 +/- 0.62 mm) than either dizygotic twin pregnancies (1.34 +/- 0.50 mm) or monozygotic dichorionic twin pregnancies (1.31 +/- 0.48 mm). Mean inter-twin NT difference was significantly lower in monozygotic dichorionic twin pregnancies (0.37 +/- 0.19 mm) than in dizygotic twin pregnancies (0.57 +/- 0.22 mm) or monochorionic twin pregnancies (0.65 +/- 0.43 mm).Irrespective of twin zygosity, NT thickness and inter-twin NT difference were larger in monochorionic twins than in dichorionic twins (1.48 +/- 0.62 mm versus 1.33 +/- 0.49 mm, and 0.65 +/- 0.43 mm versus 0.49 +/- 0.23 mm, respectively). During first-trimester ultrasonographic screening for Down syndrome, monochorionic twin pregnancies have the greatest NT thickness and monozygotic dichorionic twin pregnancies have the lowest inter-twin NT difference.

Curves of Placental Weights of Live-Born Twins

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.

Curves of Placental Weights of Live-Born Twins

The 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.

The risk of monozygotic twinning in an in vitro fertilization cycle (IVF) may be intrisinically related to oocyte quality

Objective: To determine the prevalence of monozygotic twinning (MZT) in IVF pregnancies. Additionally, to evaluate what clinical and laboratory factors may predispose embryos to the development of MZT gestations. Design: IRB-approved retrospective review of 4700 IVF cycles done between October 1999 and March 2003. Materials and Methods: The records of all patients attempting conception through IVF were reviewed. Parameters selected for evaluation included age, ovarian response to stimulation, number of oocytes retrieved, number of oocytes which fertilized, number of embryos still developing on day 3, distribution of embryo quality on day 3, number of embryos still developing on day 5 (for those patients whose embryos were placed in extended culture), the distribution of blastocyst quality on day 5 (where appropriate), day of transfer, the number of embryos transferred, number of clinical implantations, number of ongoing implantations, and overall ongoing pregnancy rate. Data were analyzed by complex Chi-square tests. Results: A total of 12912 embryos were transferred during 4700 treatment cycles during the study interval. There were a total of 2758 pregnancies with a total of 4483 implantations being identified. There were 69 documented monozygotic twins ( 2.5% of pregnancies and 1.5% of implantations) documented during the study interval. All were identified in the first trimester and as such were all monochorionic. Dichorionic monozygotic twins would not be evident in the clinical data and as such would not be captured in this analysis. Several clinical parameters were evaluated in relationship to the prevalence of MZT and the following findings were documented: a)The prevalence of MZT increased with increasing number of oocytes retrieved. ( eg in those case 1–5 oocytes retrieved - MZT occured in 3/436 (0.7%); in those cases where > 20 oocytes retrieved - MZT occured in 25/1014 pregnancies (2.5%) [p < 0.02].b)MZT rates were negatively related with age with the youngest patients having the highest prevalence of MZT [p < 0.01].c)MZT was significantly related to the number of high quality embryos on day 3 (independent of day of transfer) [p < 0.02].d)MZT was higher with day 3 transfer of good quality embryos than in day 3 transfer of lower quality embryos [p < 0.02].e)MZT was higher with blastocyst transfer than with day 3 transfers [p < 0.0001].f)MZT was higher following transfer of high quality blastocysts in comparison to the transfer of lower quality blastocysts [p < 0.02].g)MZT was higher following the transfer of blastocysts with high quality inner cell masses in comparison to those with lesser quality inner cell masses [p < 0.02]. Conclusion: The prevalence of MZT is relatively low, but remains higher than that typically reported in the general population. While the risk of MZT was higher in those patients undergoing blastocyst transfer, these data indicate that the risk relates principally to the quality of the embryos and not the culture system or developmental day of transfer. In essentially all comparisons, higher quality embryos as judged by traditional morphological criteria on day 3 or day 5 had significantly higher MZT rates than did those with lower quality. This strongly suggests that the risk for MZT relates prinicipally to the patient and the intrinsic quality of the oocytes provided, and not to the culture system employed or the duration of in vitro culture.

Special twin environments, genetic influences and their effects on the handedness of twins and their siblings.

It has been suggested that twinning may influence handedness through the effects of birth order, intra-uterine crowding and mirror imaging. The influence of these effects on handedness (for writing and throwing) was examined in 3657 Monozygotic (MZ) and 3762 Dizygotic (DZ) twin pairs (born 1893-1992). Maximum likelihood analyses revealed no effects of birth order on the incidence of left-handedness. Twins were no more likely to be left-handed than their singleton siblings (n = 1757), and there were no differences between the DZ co-twin and sibling-twin covariances, suggesting that neither intra-uterine crowding nor the experience of being a twin affects handedness. There was no evidence of mirror imaging; the co-twin correlations of monochorionic and dichorionic MZ twins did not differ. Univariate genetic analyses revealed common environmental factors to be the most parsimonious explanation of familial aggregation for the writing-hand measure, while additive genetic influences provided a better interpretation of the throwing hand data.

The Influence of Zygosity and Chorion Type on Fat Distribution in Young Adult Twins Consequences for Twin Studies

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.

The influence of zygosity and chorion type on fat distribution in young adult twins consequences for twin studies.

An 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 (< or = 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.

Birthweight in liveborn twins: the influence of the umbilical cord insertion and fusion of placentas.

To assess the relation of umbilical cord insertion and fusion of placentas with birthweight in monozygotic monochorionic, monozygotic dichorionic, and dizygotic twins. A prospective population study. 4529 liveborn twin pairs whose birthweight was recorded, placentas were examined, and site of umbilical cord insertion was determined after delivery. Infants with a peripheral cord insertion weighed 150g less (P < 0.001) than infants with a central cord insertion. Dizygotic infants had a significantly (P < 0.001) higher incidence of central cord insertion (82%) than monozygotic dichorionic (65%) and monozygotic monochorionic (53%) infants. Monozygotic dichorionic infants with fused placentas and a peripheral cord insertion weighed on average 300g less (P < 0.01) than infants with separate placentas and a central cord insertion. In dizygotic infants, fusion of the placentas did not affect birthweight. Umbilical cord insertion is associated with birthweight in all types of twins. Fusion of the placentas only affects the birthweight of monozygotic dichorionic, not that of dizygotic infants. This might be due to the greater proximity of implantation of monozygotic dichorionic twins. Dizygotic twins weighed more than monozygotic twins only when placentas were fused and cord insertion was peripheral. Hence, the difference between the mean birthweights of dizygotic, monozygotic dichorionic, and monozygotic monochorionic infants may originate from the least favourable antenatal situation, namely fused placentas with a peripheral cord insertion, which occurs most frequently in monozygotic twins.

Birthweight in liveborn twins: the influence of the umbilical cord insertion and fusion of placentas

Objective To assess the relation of umbilical cord insertion and fusion of placentas with birthweight in monozygotic monochorionic, monozygotic dichorionic, and dizygotic twins. Design A prospective population study. Population 4529 liveborn twin pairs whose birthweight was recorded, placentas were examined, and site of umbilical cord insertion was determined after delivery. Results Infants with a peripheral cord insertion weighed 150g less ( P<0.001) than infants with a central cord insertion. Dizygotic infants had a significantly ( P<0.001) higher incidence of central cord insertion (82%) than monozygotic dichorionic (65%) and monozygotic monochorionic (53%) infants. Monozygotic dichorionic infants with fused placentas and a peripheral cord insertion weighed on average 300g less ( P<0.01) than infants with separate placentas and a central cord insertion. In dizygotic infants, fusion of the placentas did not affect birthweight. Conclusions Umbilical cord insertion is associated with birthweight in all types of twins. Fusion of the placentas only affects the birthweight of monozygotic dichorionic, not that of dizygotic infants. This might be due to the greater proximity of implantation of monozygotic dichorionic twins. Dizygotic twins weighed more than monozygotic twins only when placentas were fused and cord insertion was peripheral. Hence, the difference between the mean birthweights of dizygotic, monozygotic dichorionic, and monozygotic monochorionic infants may originate from the least favourable antenatal situation, namely fused placentas with a peripheral cord insertion, which occurs most frequently in monozygotic twins.

X-chromosome inactivation is mostly random in placental tissues of female monozygotic twins and triplets.

Patterns of X-chromosome inactivation in chorion, amnion, and cord from 79 pairs of twins were examined. Seven sets of triplets were included in the analysis, both as twin pairs and triplets. Twins were stratified as dizygotic (DZ), monozygotic (MZ), monochorionic, and dichorionic and were selected for birth weight discordance, discordance for congenital anomalies, twin-twin transfusion syndrome, and various patterns of vascular anastomosis. X-inactivation was predominantly symmetric. Chorion was the most likely tissue to show asymmetric X-inactivation and was found most frequently in MZ dichorionic twins. There was no correlation of X-inactivation pattern with the selected clinical criteria. This study does not confirm that asymmetric X-inactivation in embryonic tissues is a common phenomenon in female twins, including monozygotic twins.

Intrapair differences in personality and cognitive ability among young monozygotic twins distinguished by chorion type.

We evaluated placentation effects on behavioral resemblance of 44 pairs of monozygotic (MZ) twin children. Tested at ages 4-6, the twins' zygosity and placental type had been determined at their delivery. The sample included 23 monochorionic (MC) and 21 dichorionic (DC) MZ twin pairs: DC-MZ twins result from separation of blastomeres within 72 h of ovulation; MC-MZ twins arise from later duplication of the inner cell mass. Twins were individually administered the McCarthy Scales of Cognitive Ability, while their mothers separately rated each cotwin on an individualized 280-item form of the Personality Inventory for Children (PIC). Absolute differences between MC-MZ cotwins were smaller than those between DC-MZ cotwins for all 20 PIC scales, significantly so for 3 of 4 factor scales, 8 of 12 clinical scales, and 2 of 4 validity/screening scales from the PIC; in contrast, no consistent differences in intrapair resemblance of mono- and dichorionic MZ twins were found for the McCarthy Scales. The chorion differences found in the PIC data cannot be due to genetic differences, because all pairs are monozygotes; nor are they associated with differences in parity, gestational age, birth weight, maternal education, palmar dermatoglyphic asymmetry, or maternal knowledge of chorion type. We interpret our findings as suggestive evidence that variation in timing of embryological division, with effects on MZ twins' placental vasculature, has significant consequences for some dimensions of their behavioral development, as well.

Some perinatal characteristics of monozygotic twins who are dichorionic

Zygosity testing of all multiple births allowed the identification of a subgroup of 42 monozygotic twin pairs who have dichorionic placentas, fused and separate. Perinatal outcomes of this group were compared with 110 pairs of monochorionic monozygotic twins and 148 pairs of dizygotic twins. Dichorionic monozygotic twins had the lowest incidence of preterm birth, perinatal mortality, and birth weight discordance. There was an excess of like-sexed over unlike-sexed pairs among the dizygotic twins.