This report provides an overview of the current state of the German twin family panel TwinLife, including information on design and sample characteristics as well as a selection of assessed constructs. TwinLife is a register-based longitudinal panel of four birth cohorts of German-speaking monozygotic and dizygotic same-sex twin pairs, and their core family members, including parents and siblings, but also partners and children of twins, if available. Using address data provided by a representative set of residents’ registration offices from across Germany, twin families were identified through a multistage process. The twins were born in 1990−1993, 1997−1998, 2003−2004, and 2009−2010. They were about 5, 11, 17, and 23 years old at the time of the first survey. Designed to investigate the development of social inequalities over the life course, the TwinLife currently covers an observation period from 2014 to 2025. In the first wave, the panel included data on 4096 twin families from all parts of Germany, covering the full range of key socioeconomic indicators. Over the years, the core TwinLife project has been expanded by several satellite projects, such as the molecular genetic TwinSNPs and the TwinLife Epigenetic Change Satellite (TECS) project. TwinLife provides a unique database with a wide-ranging potential for research, since it combines a longitudinal panel with an extended twin family design, adding genetic, epigenetic and additional biological data as well as the possibility to match geographical information.

Twin studies provide a powerful framework to identify genetic and environmental influences on health and development, particularly during infancy, childhood, and adolescence. Although twin research has expanded globally, evidence from South Asia remains limited and poorly characterized. This systematic review aimed to synthesize health research involving infant, child, and adolescent twins conducted in South Asia and to identify methodological patterns and research gaps relevant to future twin research in the region. A systematic review was conducted according to a registered protocol (PROSPERO: CRD42018105704) using the PICOS framework. Multiple international and regional databases were searched from inception to September 2025. Studies recruiting twins aged 0–19 years within the South Asian Association for Regional Cooperation (SAARC) countries were included. Study selection, data extraction, and quality appraisal were conducted independently, with study quality assessed using Joanna Briggs Institute critical appraisal tools. Results were synthesized narratively due to heterogeneity across study designs and outcomes. Eighteen studies were included from 1245 identified records. Most studies focused on twin pregnancy and neonatal outcomes (61%), followed by dentistry, ocular health, and gut microbiota. Twin pregnancies were consistently associated with increased risks of low birth weight, preterm birth, and perinatal morbidity and mortality. A limited number of studies applied classical twin designs to investigate genetic and environmental contributions. Twin research involving infants, children, and adolescents in South Asia remains limited in scope and methodological depth. Strengthening twin registries, methodological standardization, and collaborations is essential to advance genetically informative research and enhance the global representativeness of twin studies.

Twin-to-twin transfusion syndrome (TTTS) affects 10–15% of monochorionic diamniotic (MCDA) twin pregnancies and is associated with high perinatal mortality if left untreated. Selective Fetoscopic Laser Photocoagulation (SFLP) is the current standard of care for advanced-stage TTTS. This retrospective cohort study aimed to assess survival outcomes following SFLP and evaluate the learning curve associated with its implementation. It included 300 MCDA twin pregnancies with TTTS treated by SFLP from October 2018 to December 2024. Patients were chronologically divided into three equal groups (n = 100 per group) based on the order of procedure to assess outcome trends over time. Primary outcome was dual survival at birth; secondary outcomes included survival of at least one fetus, gestational age at delivery, birth weight, and neonatal complications. Multivariable logistic regression was used to assess clinical and procedural factors associated with dual survival, while CUSUM analysis was employed to evaluate the learning curve. Among 300 cases, dual survival was achieved in 57% of pregnancies. In multivariable logistic regression, clinical factors, including Quintero stage, placental orientation, and gestational age, were not significantly associated with outcomes (p > .05). The treatment cohort remained the sole independent predictor of survival, with significantly lower odds of success in Group 1 (OR 0.041; 95% CI [0.004, 0.429]) and Group 2 (OR 0.051; 95% CI [0.005, 0.492]) compared to Group 3 (p < .01). CUSUM analysis demonstrated that procedural competence, defined by sustained adequate survival rates, was achieved after the 141st procedure. This study demonstrates improved survival outcomes in TTTS cases treated with SFLP over time, confirming a significant institutional learning curve. The findings highlight the importance of surgical experience and procedural standardization. In resource-limited settings, establishing structured training and mentorship is essential to optimize outcomes and safely expand access to fetal therapy.

The aim of this study was to evaluate fetal survival and neonatal outcomes following fetoscopic laser ablation (FLA) for twin-to-twin transfusion syndrome (TTTS). This retrospective study included 106 TTTS pregnancies treated with FLA at Harapan Kita Women and Children Hospital, from January 2016 to January 2025. Maternal and neonatal data were collected from medical records. Fetal survival, gestational age, and neurodevelopmental outcomes were assessed. Quintero staging was used to classify TTTS severity. Statistical analysis used the Mann‒Whitney-U test for continuous variables and the chi square test for categorical variables. Among the 106 pregnancies, 93 (87.73%) had at least one surviving fetus, and 82 (77.35%) had both fetuses that survived the procedure. The highest proportion of double fetal survival was observed in Quintero stage 2 (35/82, 42.68%), whereas double fetal death was most frequently observed in stage 3 (7/13, 53.84%) and stage 4 (4/13, 30.76%) (p = .003). Neonates surviving to 28 days were born at a significantly higher gestational age than nonsurvivors (32.25 ± 2.95 vs. 27.62 ± 2.68 weeks, p < .001; OR 1.70, 95% CI [1.40, 2.10]). The neonatal survival rate at 28 days was 44.87%. Brain injury was observed in 52.17% of the screened infants and long-term neurodevelopmental follow-up was documented in 8 infants, with 2 showing delayed milestones. Quintero stage at the time of the procedure, prematurity, and growth restriction were significantly associated with neonatal survival outcomes. Early detection, timely intervention, and long-term follow-up are essential to optimize outcomes and address potential developmental delays.

This study aims to explore the significance of discrepancies in crown-rump length (CRL) and abnormal cord insertion in predicting selective fetal growth restriction (sFGR) in monochorionic (MC) twin pregnancies. Special attention was paid to determining the potential value of these factors in early pregnancy for predicting sFGR. A retrospective analysis included 131 pregnant women diagnosed with MC twin pregnancies. CRL was measured using a GE E8 color Doppler ultrasound instrument. Groups were compared based on the discrepancy in CRL (≥6 mm difference group, n = 26; <6 mm normal group, n = 105) and the presence of abnormal cord insertion (abnormal group, n = 27; normal group, n = 104). The study found no significant association between CRL discrepancy and postnatal birth-weight discordance suggestive of sFGR (p > .05). In contrast, abnormal cord insertion was significantly associated with an increased occurrence of sFGR-related outcome (p < .001). Specifically, sFGR-related outcome occurred in 12 of 27 cases (44.4%) in the abnormal cord insertion group, compared with 7 of 104 cases (6.7%) in the normal cord insertion group. In this retrospective MC twin cohort, abnormal cord insertion was associated with an increased risk of postnatal birth-weight discordance suggestive of sFGR, whereas CRL discrepancy showed limited discriminative ability. Because of the retrospective design, limited number of outcome events, and use of postnatal rather than standardized antenatal Doppler-based sFGR criteria, these findings should be interpreted as exploratory and require validation in larger prospective studies.

Twin registries worldwide increasingly function as large-scale research infrastructures, enabling standardized phenotyping across the lifespan, integration of biological and environmental data streams, and international cross-cohort collaborative research and replications. This development is also taking place in Germany. The GERman Twin Registry Under Development (GERTRUD; www.gertrud.info) was established in 2022 as the first nationwide research platform for recruiting twins and higher order multiples of all ages within Germany to support the large-scale genetically informative psychological, sociological, health, and neuroscience twin research at national and international levels. GERTRUD is being developed as a modular infrastructure that supports classical and extended twin family designs, combining annual core survey waves with optional embedded modules for intensive phenotyping (e.g., neuroimaging, smartphone-based assessments), biosampling, and linkage of participants’ residential context to external geographic datasets via geospatial information systems (GIS). To operate within Germany’s stringent data protection landscape, GERTRUD implements project-specific pseudonymisation, role-based access control, and contract-governed remote analysis access. This article describes GERTRUD’s governance and legal–technical framework, its multisource data architecture, and the potential for collaboration across Germany and internationally. Examples of early data implementations further illustrate that the continuously collected multimodal twin data constitute a critical asset, essential for successful harmonization, replication, and collaborative and integrative behavioral genetics research.

Self-concept in adolescence plays a key role in psychological adjustment, yet its development in twins remains underexplored — especially in Central Europe. While previous studies suggest minor differences between monozygotic (MZ) and dizygotic (DZ) twins, cultural and contextual influences may affect how twins perceive themselves. This mixed-method study examined 336 Czech adolescent twins (168 twin pairs; 32 MZ, 136 DZ) aged 11–15 years. Participants completed three standardized instruments: the Behavioral and Psychological Adjustment Questionnaire (BPQA), the School Performance and Adjustment Scale (SPAS), and the Rosenberg Self-Esteem Scale (RSES). Quantitative analyses included t tests, Analyses of Variance (ANOVAs), and Pearson correlations. To triangulate findings, focus group interviews were conducted with three twin pairs and analyzed thematically. No statistically significant differences were found between MZ and DZ twins across BPQA, SPAS, and RSES measures. However, small, nonsignificant trends suggested slightly higher emotional self-concept scores in MZ twins. A modest gender difference emerged: girls reported more internalizing symptoms. Correlational analysis showed that lower behavioral difficulties were associated with higher school adjustment and self-esteem. Thematic analysis from focus groups confirmed key patterns such as the emotional safety of twinship and the struggle for individuality. Zygosity does not appear to be a major predictor of self-concept in early adolescence. Instead, self-perception in twins is shaped by a complex interplay of mental health, family dynamics, and peer context. Future research should compare twin self-concept development across cultures to further examine sociocultural influences.

Monozyotic (MZ) twins reared together are typically raised in the same religion, as are all children in a family. However, a young pair of MZ male twins, raised together, but with different religions, was identified and raises interesting issues that warrant consideration. This case, as well as those of reared-apart MZ twins who adopt different religions due to their family background or other circumstances, are summarized. A review of recent and current twin research follows. The studies included here concern language development in a pair of dizygotic (DZ) female twins, the second case of MZ twins with presumed Hallermann-Streiff syndrome, and the use of digital twins to advance asthma research. The final entry in the research section describes parents’ responses to their twin children’s misdiagnosed zygosity. Human interest stories involving twins — some entertaining, but all informative — include a pen that saved nontwin Holocaust children by assigning them as twins, identical twin Jeopardy winners, twin hostages freed from Gaza, loss of a twin son, fraternal twin mountaineers, twin sailors in the Sydney to Hobart Yacht Race, and the twin Patron Saints of Cobblers.

Over the past years, the Italian Twin Registry (ITR) has joined networks and consortiums for genetic-epidemiological research, and several opportunities have arisen for national and international collaborations involving data and tool sharing, as well as cohorts’ harmonization. These networking efforts have led ITR to face complex ethico-legal challenges within the framework provided by the General Data Protection Regulation (GDPR) since 2018 but have also represented the occasion to address critical aspects in ITR organization and activities, looking for long-term solutions. This article describes some examples of: ITR international collaborations implying data and tools sharing; ITR national collaborations aimed to replicate general-population findings; ITR integration with national and regional administrative records; ethico-legal criticalities experienced by ITR in multicohort collaborations, and initiatives to overcome the GDPR bottleneck. Regarding collected data, ITR currently contains information on 30,362 twins of different ages and geographical areas, of whom 7797 are being asked to reconfirm their enrollment, having turned 18 or over. In the coming period, these figures may increase rapidly, thanks to the ongoing access to the National Register of the Resident Population, for which ITR is working to solve privacy-related legal issues. This will enable a systematic enrolment of large and representative twin cohorts, making ITR a valuable tool for genetic-epidemiological research and for the monitoring of relevant health outcomes in the Italian population.

Neonates are highly susceptible to infection, a major cause of neonatal death, given their immature immune system. Comprehensive studies examining multiple immune-response-related proteins in relation to neonatal infection are scarce. We conducted a nested case-control study within the Shenzhen Baoan Birth and Twin (SZBBTwin) cohort, measuring 92 immune-response-related proteins in cord plasma of 149 twins (including 34 discordant twin pairs) with proximity extension assay. All twins were followed for clinical diagnoses of infection from birth until 27 days of age. Wilcoxon rank-sum test was used to determine differentially abundant proteins (DAPs) between infected and noninfected neonates, the predictive performance of which was evaluated by receiver operating characteristic curves, and their functions and pathways were annotated through enrichment analysis. Logistic regression was used to assess the associations between levels of proteins and risk of neonatal infection. Finally, five DAPs (ITGA11, FCRL6, DDX58, SH2D1A, and EDAR) were identified for neonatal infection, and the area under the curve of the five DAPs achieved 0.835 for infection prediction. Enrichment analysis indicated that five DAPs were mainly involved in immune function and cell binding, and they were mainly enriched in the nuclear factor kappa-B pathway. A higher level of ITGA11 was associated with an increased risk of neonatal infection in all twins (OR 3.00; 95% CI [1.33, 6.78]) and discordant twin pairs (OR 5.50; 95% CI [1.20, 25.23]). In conclusion, multiple immune-response-related proteins in cord plasma, particularly ITGA11, are associated with the risk of neonatal infection in twins.

Twin research informs our thinking about possible human reproductive cloning (HRC). Other human models for assessing the benefits and difficulties of HRC are look-alike parent-child and sibling pairs. An in-depth interview with an extraordinary mother-daughter pair is provided. This section is followed by reviews of research on infantile pyknocytosis in a dizygotic (DZ) twin, monozygotic (MZ) twin discordance for hemimicrencephaly, unusual dental development in a twin, and ‘gaze fingerprint signatures’. The final section presents human interest stories of identical twin executives at odds, the loss of identical twin Jim Whittaker, identical twin artists, and twin wisdom captured in stone.

Record linkage projects provide powerful opportunities to enrich cohort data with administrative and clinical information from national registries. The Netherlands Twin Register (NTR) is a large population-based twin-family cohort with longitudinal data collection that has established multiple record linkage initiatives to expand research on genetic and environmental determinants of health, disease, behaviour, development, and fertility. In this article we summarize and discuss NTR record linkage projects. Clinical and pathology registries have provided essential data on chorionicity, reproductive history, neonatal thyroid function, and cancer diagnoses, facilitating studies on prenatal environment, disease heritability, and polygenic risk prediction for diseases. Linkage with health insurance data has enabled validation of medication use and health research. Collaborations with Statistics Netherlands allow linkage to nationwide population-based data via secure infrastructures such as Open Data Infrastructure for Social Science and Economic Innovations (ODISSEI), supporting genomewide association studies of, for example, healthcare expenditure. Finally, linkage to environmental exposure datasets has permitted exposomewide analyses of health and wellbeing. Together, these projects illustrate the feasibility, scientific value, and challenges of record linkage in the Dutch context, highlighting its role in advancing twin research, genetic epidemiology, and population health studies.

A pair of young monozygotic male twins affected with Feingold syndrome were briefly described in previous issues of Twin Research and Human Genetics. An opportunity to meet the twins and their mother occurred in March 2026. An overview of the condition and interview with the twins’ mother is presented, as are implications for future research and practice. Next, research summaries of pseudoamonoamniotic twins, selective fetal growth restriction in monoamniotic twins, twin authorship attribution, and twins discordant for clubfoot are provided. This article concludes with human interest stories involving twins with different fathers, biological relatedness in a married couple, the birth of identical quadruplets, and twins switching places.

In behavioral genetics, divorce is typically analyzed as an individual-level outcome, even though marital dissolution can only be experienced by couples. In this article, we discuss how assortative mating complicates the study of couple-shared outcomes because individual-level effects can be confounded by effects of the spouse. We then show how chain-linking affines (i.e., in-laws) provides sufficient information to estimate spousal similarity for couple-shared outcomes, which we incorporate into an extended twin model that we use to test for sex differences and assortative mating for individuals’ liability to divorce. We linked the Norwegian twin register to the Norwegian population register and constructed 124,544 extended family units (1196 units with monozygotic twins) comprising 353,210 marriages entered between 1983 and 2008. We found that divorce was significantly correlated among affines, and that female relatives were more highly correlated than male relatives. The extended twin model estimated a strong correlation (r = .60, SE = .10) between female and male familial factors. Couples’ liability to divorce was attributed to 18% (SE = 5%) female and 10% (SE = 3%) male familial factors, with an additional 16% (SE = 4%) accounted for by their correlation. Estimates from a classic twin model were considerably higher. These findings show that spousal similarity is an important source of variation in divorce liability and that failing to model it can inflate estimates of individual-level effects. Overall, the analytical framework offers a blueprint for dissecting any couple-shared outcome into sex-specific and assortative components.

Populations of Arab ancestry remain underrepresented in global twin studies. These populations are described by distinct genetic characteristics, high consanguinity, and strong familial relatedness. To contribute to diversity in genetics and twin research, the Qatar Twin Registry (QTR) was established as the first national twin registry in the Arab world and the second in the Middle East and North Africa. The QTR is a prospective and retrospective cohort study developed by Qatar University in collaboration with the Qatar Precision Health Institute (QPHI). The registry integrates with Qatar’s national biobank and whole-genome sequencing infrastructure to enable large-scale twin-based research. Participants contribute comprehensive health, developmental, psychosocial, and lifestyle information, along with biological samples collected through clinic-based assessments and home visits. QTR marks a milestone in regional genetic and epidemiological research. Leveraging and linking to Qatar’s national biobank and genomic infrastructure, it establishes a sustainable framework for investigating heritability, gene–environment interactions, and complex diseases in an underrepresented Arab population, contributing to global precision medicine efforts.

The Murcia Twin Registry (MTR) has steadily expanded over two decades and has become a key resource for twin research in the Mediterranean region. The registry currently includes data from 3971 individual twins born between 1940 and 1999, as well as an associated biobank containing samples from 1586 participants. Its primary research focus is on health and health-related behaviors within a public health framework, covering areas such as lifestyle, health promotion, quality of life, and environmental factors. Across multiple waves of data collection, the MTR has compiled extensive and wide-ranging phenotypic data. These data can be further expanded and have strong potential for record linkage with other health databases, particularly those of the regional public health care system, including both primary and inpatient care. Efforts are also underway to establish record linkage with additional sources of information, such as the educational system. In the near future, the registry aims to expand its biobank and continue the collection of longitudinal data, as well as increasing the ability to collect additional data that could enrich the information from participants in the register.