Assessing the size of twins at birth using charts developed for singletons may over diagnose small for gestational age in this sub-population. The study aimed to produce international, twin-specific, newborn size normative charts by gestational age and sex. This longitudinal observational study in eight geographically diverse settings prospectively collected data between May 2009 and August 2013 from healthy pregnant women and their newborn twins. The participants were enrolled as part of the INTERGROWTH-21st study, and recruited based on World Health Organization recommendations for evaluation of anthropometric measures. All the women met, in addition to the underlying population characteristics of low perinatal risk, strict individual criteria for a population at low risk of impaired fetal growth. Newborn weight, length and head circumference measures were collected independently in duplicate by two trained anthropometrists within 12 hours of birth using identical equipment and protocols at all sites. From 1034 multiple pregnancies, after exclusions of condition such as smoking, high maternal BMI, and congenital malformations, the final sample was 864 twin newborns. Most of the twins were below the 50th centile of the INTERGROWTH-21st standards for singletons. We present international newborn size normative charts for twins using the same methodological approach adopted to construct the singleton standards.

The Painel USP de Gêmeos (University of São Paulo Twin Panel) is, based at the Instituto de Psicologia da Universidade de São Paulo. It was formally established in 2017 to advance research on fundamental psychological processes through twin study designs. Our relatively new registry comprises a volunteer sample of 8839 twin individuals, 70% of whom live in Brazil’s Southeast, the region with the highest twinning birth rate (10.64‰) of the country, within a national population of 213 million. Our collaborative research group has expanded to include partners from psychology, dentistry, and medicine at USP, as well as other Brazilian institutions, such as the Universidade Federal do Rio Grande do Norte, Universidade Federal da Bahia, and Universidade Federal do Espírito Santo. We are advancing biobehavioral research in Brazil through innovative methodologies, interdisciplinary collaboration, and international partnerships. All twin participants contribute to multiple studies associated with four datasets employing the same hierarchical identification system for participants and families: the Biorepository, the Physiological and Physical Repository, the Behavioral Repository, and Fonoteca Cesar Ades (FOCA). Future directions include expanding our twin registry across the five regions of Brazil, our research partnerships, promoting genetic literacy, and fostering public engagement.

Twin fetuses show a physiologically slower growth rate in the third trimester compared to singletons. Therefore, it has been suggested that the anthropometric evaluation of twins at birth should be performed using twin-specific charts. To be reliable, anthropometric charts need to fulfil certain methodological criteria to systematically review studies that have developed neonatal twin-specific anthropometric charts and assess their methodological quality. A comprehensive search was conducted across Cinahl, Embase, Global Index Medicus, PubMed, Scopus, Web of Science, Google Scholar, and relevant preprint repositories, followed by a predefined snowballing search. Two reviewers independently screened studies for eligibility, selecting those that have developed cross-sectional anthropometric charts for twins at birth, published since January 1990. Articles were excluded if they did not use anthropometric measurements at birth to construct charts. Two reviewers independently extracted data and performed quality assessment with a 16-item grading system. Sixty-eight studies were included, with all but one constructed reference, not standard, charts. Most studies did not meet the quality criteria: only 9% were prospectively designed, 34% reliably estimated gestational age, 19% reported standardized instruments, 28% described measurement techniques, and 26% did not stratify centiles by sex. This review reveals considerable methodological limitations in existing twin-specific neonatal anthropometric charts.

Obesity represents a major global public health concern. Body fat percentage (BF%) is a key indicator for assessing adiposity and provides a more precise estimation of obesity-related health risks compared to the traditional body mass index (BMI). Accumulating evidence suggests that BF% is influenced by both genetic and environmental factors. However, most genetic studies on BF% have been conducted in European and American population, with limited data available from Chinese cohorts. To address this gap, a classical twin study was conducted using data from the Qingdao Twin Registry in China to estimate the heritability of BF% adjusted for age, sex, and BMI. This study included Han Chinese twins registered in the Qingdao Twin Registry. This study included 344 middle and old-aged Chinese twin pairs (217 monozygotic and 127 dizygotic). comprising 327 males and 361 females. The median age of participants was 50 (interquartile range [IQR]:12) years, with BF% of 27.6 (11.4) %. Model fitting indicated that the best-fitting model was AE model. The additive genetic effect (A) accounted for 54% (95% CI [44, 59) of the total variance, while unique environmental effect (E) contributed 46% (95% CI [37, 56]). In conclusion, this twin-based study provides robust evidence for a moderate genetic contribution (heritability = 54%) to BF% in a middle- and old-aged Qingdao population.

The most accurate confidence intervals for estimates of heritability are based on Fisher’s Z-transformation. Using these methods, Loehlin and Nichol’s (1976) analysis is confirmed, viz., that it is pointless to estimate heritabilities from the classical twin method unless one is prepared to recruit upwards of 800 twin pairs for study. Even then the assumptions of that method are so improbable as to leave reasonable doubt about the true value of H. Estimates of H from the correlation of monozygotic twins reared apart (MZA twins), on the other hand, is remarkably more stable than that obtained by comparing MZ with DZ correlations. Moreover, estimates of H based on the MZA design rest upon more reasonable (and often testable) assumptions.

Twin children are more likely to die than singletons. This is an additional burden in sub-Saharan African (SSA) countries, as child mortality levels are already higher than anywhere else. This article provides estimates of under-5 mortality rates (U5MRs) for twins and singletons in SSA from 1986 to 2016. It describes the geographical variations and changes over time. It also describes the variation of twins’ excess mortality according to age from 0 to 5 years. Additionally, it analyzes the factors associated with twins’ excess mortality. We used data from 156 national surveys from 42 countries. We estimated U5MRs for twins and single children and built a Cox model to analyze factors associated with excess mortality among twins. Although child mortality has declined on the continent, twins’ excess mortality remains very high. U5MRs are, on average, 3 times higher among twins than singletons. The Cox model shows that all other things being equal, the adjusted hazard ratio of under-5 mortality (U5M) is 3.2 (2.9−3.3; p < .001) times higher among twins than singletons. The main factors associated with excess mortality risks among twins are biomedical and nutritional features, such as low birth weight, non-use of cesarean section delivery, and lack of breastfeeding. Health policy makers in SSA should be aware of the vulnerability of twins, and interventions to prevent their early deaths should be considered.

Sense of humor is a universal human trait, enjoyed daily across cultures. However, little is known about the factors that shape individual differences in humor, particularly what contributes to developing a great sense of humor. While previous studies have identified a significant genetic component for various humor attributes, such as humor appreciation and humor styles, no study has looked at the heritability of humor production ability. This study is the first to assess the genetic and environmental influences on humor production ability using a twin study design. Participants included 448 pairs of monozygotic twins and 196 pairs of dizygotic twins (median age 66 years, mostly female) from the Twins UK registry. Twins self-assessed their humor ability, rated the funniness of their co-twin, and completed an objective humor production task by composing funny captions for captionless cartoons. Additionally, they completed a short cognitive ability test and reported their overall health. Findings revealed that self-rated humor ability was influenced by both additive genetic and nonshared environmental factors. In contrast, objective humor production showed no evidence of additive genetic effects. Instead, all individual differences were shaped by shared and nonshared environmental influences, though a small genetic effect cannot be ruled out. These results suggest that humor production may be more complex and difficult to assess than other cognitive abilities. The study also presents intriguing implications for the evolutionary basis of humor.

Threat sensitivity, an individual difference construct reflecting variation in responsiveness to threats of various types, predicts physiological reactivity to aversive stimuli and shares heritable variance with anxiety disorders in adults. However, no research has been conducted yet with youth to examine the heritability of threat sensitivity or evaluate the role of genetic versus environmental influences in its relations with mental health problems. The current study addressed this gap by evaluating the psychometric properties of a measure of this construct, the 20-item Trait Fear scale (TF-20), and examining its phenotypic and genotypic correlations with different forms of psychopathology in a sample of 346 twin pairs (121 monozygotic), aged 9–14 years. Analyses revealed high internal consistency and test-retest reliability for the TF-20. Evidence was also found for its convergent and discriminant validity in terms of phenotypic and genotypic correlations with measures of fear-related psychopathology. By contrast, the TF-20’s associations with depressive conditions were largely attributable to environmental influences. Extending prior work with adults, current study findings provide support for threat sensitivity as a genetically-influenced liability for phobic fear disorders in youth.

Since the 1970s, twin birth rates have increased sharply in developed countries. In Africa, where the rate is the highest globally, its evolution and variation are poorly understood. This article aims to estimate the twinning rate in sub-Saharan African (SSA) countries throughout 1986–2016 and analyze its spatial, temporal, and ethnic variations. It also seeks to identify social and demographic factors associated with a high probability of twin births and outline a forecast of the twinning rate. We used data from 174 Demographic and Health Surveys from 42 countries. We supplemented them with the UN World Population Prospects (WPP). The twinning rate was calculated by reporting the number of twin births per thousand total births. We used logistic regression to analyze the factors associated with twin births. We projected the twinning rate based on WPP. The overall SSA twinning rate is 17.4 per 1000, but it has changed very little over time, and we expect it will grow a little between 2015 and 2050, increasing at most from 17.4 per 1000 to 18.4 per 1000. We also show significant differences in the twinning rate in SSA according to mother ethnicity. Most ethnic groups with high twinning rates belong to the large Bantu ethnic family. SSA remains the ‘land of twins’, with the twinning rate changing slowly. However, specific health policies must target twin births in SSA to address the public health challenges they present.

Despite the decline in mortality rates among children in developing countries, disparities persist between countries, particularly between twins and singletons. This study employed data from nine Demographic and Health Surveys in the Dominican Republic and Haiti to estimate and compare mortality rates for twins and singletons in categories of the under-5 age group (neonatal, postneonatal, and child mortality) and examine the factors associated with excess mortality among twins. From 1996 to 2013, the under-5 mortality rate (U5MR) for singletons in the Dominican Republic declined from 56‰ (95% CI [47, 64) to 30‰ (22–39) and from 108‰ (53–164) to 53‰ (16–89) among twins. In Haiti, between 1994 and 2016, the U5MR declined from 121‰ (109–133) to 77‰ (68–80) for singletons and from 432‰ (327–538) to 204‰ (149–260) among twins. The adjusted risk of neonatal death for twins is 1.4 (1.0–1.9) times higher than for singletons in the Dominican Republic, compared to a risk of 4.3 (3.5–5.3) times higher in Haiti. In the post-neonatal period, the mortality risk for twins in the Dominican Republic was 1.8 (1.0–3.1) times higher than that for singletons, 2.9 (2.3–3.8) in Haiti. The risk of death for twins was not significantly different from that for singletons in both the Dominican Republic and Haiti at ages 1–4 years. Low birth weight, lack of breastfeeding, absence of, or inadequate, antenatal care, noncesarean section birth, and high birth order were associated with excess mortality among twins in both countries.

Vanishing twin syndrome is a miscarriage of multiples where one or more fetuses ‘vanishes’ (i.e., partial or full resorption or via calcification) during pregnancy, often before detection via ultrasound. It affects 30−50% of multifetal pregnancies, with most cases resulting in full resorption within the first trimester. Despite its recognition since 1945 and rising occurrence in both natural and assisted pregnancies, standardized clinical guidelines remain inadequate, leading to inconsistent diagnosis, counseling, and communication from healthcare providers. This study examines the experiences of mothers and gestational carriers diagnosed with VTS, focusing on patient-provider communication, risks, and symptom disclosure. A global online survey collected qualitative and quantitative data from 153 participants across 17 countries. Results show that most patients with formal diagnoses experienced negative interactions with healthcare providers, with an average sentiment score of −0.7 (on a scale from −2 to 2). Over 53.4% rated their communication experience as −1, and the average satisfaction score for the amount of information received was 3.5/10. Additionally, 43% of respondents were not informed about chorionicity, a key factor affecting fetal outcomes. Significant discrepancies in care were observed across different countries. The findings highlight major gaps in patient-provider communication and inconsistent clinical practices regarding VTS. Addressing these issues through improved education, clearer protocols, and standardized guidelines could enhance patient experiences and decision-making. Future research should focus on provider training and evidence-based strategies to improve the management of VTS and other types of miscarriage and death of multiples during pregnancy and postpartum.

We evaluated the prevalence and risk factors for child maltreatment in multiples aged 3 years and compared them to singletons in Japanese population-based data. Records on child maltreatment and health check-ups at 3 years of age from 17,125 singletons, 488 twins and 18 triplets were collected from a Public Health Center between April 2007 and March 2011. The associations of child maltreatment with potential risk factors were analyzed using the logistic regression model. Out of all children, 76 (4.31 per 1000) children had documented maltreatment including 69 (4.03 per 1000) singletons and seven (14.31 per 1000) twins. All of the cases in twins were physical abuse (100%) and nearly half of the cases (43%) included emotional abuse. Among twins, 86% of the biological mothers were suspected. The alleged perpetrators of twins showed a significantly higher rate of maternal depression compared to those of singletons. After adjusting the results for a number of potential biological and social risk factors, twins or triplets had a higher risk for maltreatment than singletons (OR 3.39, 95% CI [1.17, 9.83]). Healthcare providers should be aware that a multiple birth can place considerable stress on a family leading to child maltreatment and should provide appropriate support and intervention for mothers with multiples.