Over the previous decades, twinning rates worldwide have increased remarkably. This study aimed to describe changes in multiple birth rates and related demographic characteristics, such as maternal age and the level of education of parents in South Korea from 1981 to 2019. This study analyzed birth data obtained from the Korean Statistical Information Service. From 1981 to 2019, the total twinning rate increased from 5 to 22.5 pairs per 1000 births. This rapid increase was due to an increase in dizygotic twin births, which mainly occurred in mothers aged between 30 and 39 years. The average maternal age of multiples increased from 26.06 years in 1981 to 33.98 years in 2019, suggesting that a delay in childbearing contributed to the increase in the twinning rate. The percentage of mothers of multiples who completed a college or higher degree (CHD) increased by 1000% from 1981 to 2019, indicating that a sharp increase in the level of education of females in part explains the delay in childbearing. The percentages of individuals who completed a CHD were higher among parents of multiples than among those of singletons in recent years when public funding arrangements for fertility treatments were available. This result suggested that completion of higher education may be associated with increased use of assisted reproductive technology (ART) independent of the financial status of couples. Taken together, the analysis suggested that increased maternal age, ART and the increased number of females who completed CHD may be responsible for the remarkable increase in the rates of multiple births in South Korea in the last four decades.

The purpose of our study was to elucidate the functions of miR-30c-5p on adenomyosis for exploring novel treatment strategies. We first detected the expression of miR-30c-5p in clinical adenomyotic tissues and isolated endometrial cells from adenomyotic tissues. Next, gain and loss-of-function assays were performed to detect the effect of miR-30c-5p on adenomyotic endometrial cells. Further, luciferase assay and real-time polymerase chain reaction as well as western blot were conducted to investigate the potential target of miR-30c-5p; and transwell assay, wound-healing assay and CCK-8 assay were used to evaluate the effects of miR-30c-5p and its target on regulating biological functions of adenomyotic endometrial cells. Our results found that miR-30c-5p was down-regulated in both adenomyosis tissues and adenomyotic epithelial cells, which correlated with dysmenorrhea, longer duration of symptoms and more menstrual bleeding. Moreover, the overexpression of miR-30c-5p inhibited the proliferation, migration and invasion of adenomyotic epithelial cells, where miR-30c-5p knockdown had an opposite effect. Furthermore, we confirmed mitogen-activated protein kinase 1 (MAPK1) was one of the direct targets of miR-30c-5p, indicating its important role in miR-30c-5p-mediated suppression of proliferation, invasion and migration in adenomyotic epithelial cells. This study showed that the interaction of miR-30c-5p with MAPK1 can regulate the proliferation, invasion and migration in adenomyotic epithelial cells.

Twin studies of physical exercise for Asian twins are sparse. This study aimed to examine genetic and environmental influences on frequency of vigorous exercise (FVE) in South Korean twins, with a special emphasis on sex effects. Telephone interviews on FVE were administered to 1757 twins (mean age = 19.05 years, SD = 3.01 years). Tetrachoric correlations were significantly different between monozygotic (MZ) and dizygotic (DZ) twins in males (.40 vs. .12), but they were similar in females (.44 vs. .45), suggesting the importance of genetic factors in FVE in males and that of common environmental factors in females. A scalar sex-limitation model incorporating age as a modifier was applied to data. The results revealed that genetic, common and individual environmental influences did not vary significantly with age, but differed across two sexes, confirming twin correlational analyses. In the best-fitting model, additive genetic and individual environmental influences on FVE were, respectively, .35 (95% CI [.26, .39]) and .65 (95% CI [.61, .74]) in males, and common and individual environmental influences were, respectively, .45 (95% CI [.35, .53]) and .55 (95% CI [.47, .65]) in females. These results contrasted starkly with recent findings from a large sample of Chinese adult twins (age >18 years), in which most variance (≥95%) of vigorous physical activity was attributable to common environmental influences in both sexes. Replications in other Asian samples are clearly needed.

Twin studies can help us understand the relative contributions of genes and environment to phenotypic trait variation, including attentional and brain activation measures. In terms of applying methodologies such as electroencephalography (EEG) and eye tracking, which are key methods in developmental neuroscience, infant twin studies are almost nonexistent. Here, we describe the Babytwins Study Sweden (BATSS), a multi-method longitudinal twin study of 177 MZ and 134 DZ twin pairs (i.e., 622 individual infants) covering the 5−36 month time period. The study includes EEG, eye tracking and genetics, together with more traditional measures based on in-person testing, direct observation and questionnaires. The results show that interest in participation in research among twin parents is high, despite the comprehensive protocol. DNA analysis from saliva samples was possible in virtually all participants, allowing for both zygosity confirmation and polygenic score analyses. Combining a longitudinal twin design with advanced technologies in developmental cognitive neuroscience and genomics, BATSS represents a new approach in infancy research, which we hope to have impact across multiple disciplines in the coming years.

The objective of this study was to analyze differences in birth weight and overweight/obesity in a Shanghai twin cohort. We also wanted to study their association and explore possible risk factors for the discordance of overweight/obesity within twins. This was an internal case–control study designed for twins. The 2012 Shanghai Twin Registration System baseline survey data of a total of 3417 twin pairs were statistically analyzed using SPSS22 software. Results show that the body mass index (BMI) of the Shanghai twin population increased with age. Twins with a high birth weight had a higher BMI and a higher rate of overweight and obesity; 0- to 6-year-old twins, male twins and dizygotic (DZ) twins had higher rates of overweight/obesity than other groups. The greater the discordant birth weight rate of twins, the more obvious the difference in BMI (p < .05). There was a significant difference in overweight/obesity between twins with a relative difference of birth weight ≥15% in DZ twins (p < .05). DZ twins, male twins and 0- to 6-year-old twins were more likely to be discordant in overweight/obese than others. The discordant birth weight within twins was not a risk factor for discordant overweight/obesity. However, attention should be paid to childhood obesity, and appropriate interventions should be made at the appropriate time. Genetics may play an important role in the occurrence and development of overweight/obesity. In conclusion, discordant growth and development in the uterus early in life may not lead to discordant weight development in the future.

The aim of the present study was to compare the rate of preterm birth (PTB) and growth from birth to 18 years between twins conceived by in vitro fertilization (IVF) and twins conceived by spontaneous conception (SC) in mainland China. The retrospective cohort study included 1164 twins resulting from IVF and 25,654 twins conceived spontaneously, of which 494 from IVF and 6338 from SC were opposite-sex twins. PTB and low birth weight (LBW), and growth, including length/height and weight, were compared between the two groups at five stages: infancy (0 year), toddler period (1–2 years), preschool (3–5 years), primary or elementary school (6–11 years), and adolescence (10–18 years). Few statistically significant differences were found for LBW and growth between the two groups after adjusting for PTB and other confounders. Twins born by IVF faced an increased risk of PTB compared with those born by SC (adjusted odds ratio [aOR] 8.21, 95% confidence interval [CI] [3.19, 21.13], p < .001 in all twins and aOR 10.12, 95% CI [2.32, 44.04], p = .002 in opposite-sex twins). Twins born by IVF experienced a similar growth at five stages (0–18 years old) when compared with those born by SC. PTB risk, however, is significantly higher for twins conceived by IVF than those conceived by SC.

Adult socioeconomic status (SES) has been consistently associated with body mass index (BMI), but it is unclear whether it is linked to BMI independently of childhood SES or other potentially confounding factors. Twin studies can address this issue by implicitly controlling for childhood SES and unmeasured confounders. This co-twin control study used cross-sectional data from Twins Research Australia’s Health and Lifestyle Questionnaire (N = 1918 twin pairs). We investigated whether adult SES, as measured by both the Index of Relative Socioeconomic Disadvantage (IRSD) and the Australian Socioeconomic Index 2006 (AUSEI06), was associated with BMI after controlling for factors shared by twins within a pair. The primary analysis was a linear mixed-effects model that estimated effects both within and between pairs. Between pairs, a 10-unit increase in AUSEI06 was associated with a 0.29 kg/m2 decrease in BMI (95% CI [−.42, −.17], p < .001), and a 1-decile increase in IRSD was associated with a 0.26 kg/m2 decrease in BMI (95% CI [−.35, −.17], p < .001). No association was observed within pairs. In conclusion, higher adult SES was associated with lower BMI between pairs, but no association was observed within pairs. Thus, the link between adult SES and BMI may be due to confounding factors common to twins within a pair.

Accurate zygosity determination is a fundamental step in twin research. Although DNA-based testing is the gold standard for determining zygosity, collecting biological samples is not feasible in all research settings or all families. Previous work has demonstrated the feasibility of zygosity estimation based on questionnaire (physical similarity) data in older twins, but the extent to which this is also a reliable approach in infancy is less well established. Here, we report the accuracy of different questionnaire-based zygosity determination approaches (traditional and machine learning) in 5.5 month-old twins. The participant cohort comprised 284 infant twin pairs (128 dizygotic and 156 monozygotic) who participated in the Babytwins Study Sweden (BATSS). Manual scoring based on an established technique validated in older twins accurately predicted 90.49% of the zygosities with a sensitivity of 91.65% and specificity of 89.06%. The machine learning approach improved the prediction accuracy to 93.10%, with a sensitivity of 91.30% and specificity of 94.29%. Additionally, we quantified the systematic impact of zygosity misclassification on estimates of genetic and environmental influences using simulation-based sensitivity analysis on a separate data set to show the implication of our machine learning accuracy gain. In conclusion, our study demonstrates the feasibility of determining zygosity in very young infant twins using a questionnaire with four items and builds a scalable machine learning model with better metrics, thus a viable alternative to DNA tests in large-scale infant twin studies.

Formation of identity and differentiation is crucial in the development process of adolescents and young people. Despite many studies on identity status and self-differentiation in adolescence and early adulthood stages, this issue has been less examined among twins. This study was conducted to compare self-differentiation and identity statuses in twins and nontwins. The sample of the study consisted of 128 identical twins, 176 nonidentical twins and 170 nontwins aged from 13 to 30 years and living in Mashhad in Iran, who completed the Objective Measure of Ego Identity Status and Self-Differentiation questionnaires. The results showed that the self-differentiation of identical and nonidentical twins was significantly higher than nontwins. The results also showed that level of Foreclosure identity, Moratorium identity and Achieved identity was similar among identical twins, nonidentical twins and nontwins, but significantly different in terms of Diffusion identity. Further, the results showed that twins younger than 18 years had scores higher than nontwins and twins over 18 years in terms of Diffusion identity. Findings revealed that females were significantly lower than males for Moratorium identity. The adolescence period and co-twins may be the reason for the high levels of Diffusion identity and self-differentiation. Also, it seems that identity development and differentiation are related to emotional and cognitive development.

Ultrasound determination of chorionicity in the first trimester has a high accuracy, but it is associated with some pitfalls. This report presents changes in ultrasound findings during a monochorionic pregnancy with chorionic membrane folding (CMF). The patient was a 32-year-old woman, gravida 2 para 0. Her transvaginal ultrasonography identified two gestational sacs (GSs) and two embryos at 7 weeks of gestation. At 9 weeks’ gestation, an ultrasound image showed a lambda sign at both sides and the interruption of chorionic membranes, resulting in the diagnosis of a monochorionic diamniotic (MCDA) twin pregnancy with CMF. At 11 weeks’ gestation, an ultrasound image showed a lambda sign at one portion of the septum and a T sign at another portion. This change suggested that the folded chorionic membrane had partially flattened. At 35 weeks’ gestation, an emergency cesarean section was performed. Two healthy male neonates were delivered. Histological placental examination confirmed that the intertwin membrane was composed of two amniotic membranes without a folded chorionic membrane, confirming the diagnosis of a MCDA twin pregnancy. This case presents two important ultrasound chorionicity findings: a monochorionic pregnancy with CMF can show two GSs and a lambda sign and the CMF can flatten or change during the pregnancy.

Selective intrauterine growth restriction (sIUGR) in monochorionic twin pregnancies is associated with greater morbidity and mortality for both fetuses when compared to singleton and dichorionic pregnancies. This retrospective cohort study aimed to assess the perinatal outcomes of monochorionic twin pregnancies affected by this disorder and conducted expectantly, by analyzing the results according to the end-diastolic flow in the umbilical artery Doppler of the smaller twin (type I: persistently forward/type II: persistently absent or reversed/type III: intermittently absent or reversed). Seventy-five monochorionic diamniotic twin pregnancies with sIUGR were included in this study. sIUGR was defined by estimated fetal weight below the 3rd centile for gestational age, or below the 10th centile, when associated with at least one of the following three criteria: abdominal circumference below the 10th percentile, umbilical artery pulsatility index of the smaller twin above the 95th percentile, or estimated fetal weight discordance of 25% or more. Perinatal outcomes were analyzed from the prenatal period to hospital discharge and included perinatal death, neurological injury, retinopathy of prematurity (ROP), bronchopulmonary dysplasia (BPD), necrotizing enterocolitis (NEC), and sepsis. The mortality rate was 1.33% in this cohort. The overall morbidity rate was lower in type I twin pregnancies. In conclusion, this study shows that sIUGR type I has lower morbidity than types II and III in expectant management.

The objective of this study was to comprehensively assess fetal hemodynamic adaptions to occlusive procedures. Twin pregnancies complicated with acardiac twin and hydrops fetalis of the pump twin were recruited. The occlusive procedures — either alcoholization, radiofrequency ablation, coil embolization or occlusive glue — were performed under ultrasound guidance. Various hemodynamic parameters were assessed before, shortly after, then every 6 h for 48 h and 2−4 weeks after the procedures. Seven pregnancies were recruited. The median (range) gestational age of intervention was 21 (17−26) weeks of gestation. Before the procedures, all cases showed normal cardiac function. Just after the procedures, all cases showed an increase in Tei index and isovolumic relaxation time but returned to preocclusion levels within 6−48 h, except for two cases that were persistently high. Increased preload and poor shortening fraction were observed in two cases, leading to heart failure, with one recovery and one death in utero. Five out of the seven cases got through the critical period with a gradual return to normal hemodynamics, ending with the disappearance of hydrops and successful outcomes. It was concluded that the occlusive procedure could aggravate the overworked heart, leading to heart failure. Preocclusion preload index and Tei index may predict risk of heart failure due to the occlusion. This small series strongly suggests that the occlusion should be performed before the deterioration of cardiac function.

The processes that give rise to monozygotic (MZ) twins remain elusive. This article describes various theories of MZ twinning that have been examined over the years, although they continue to be speculative. It has also been impossible to know if a singleton began life as an MZ twin; however, a critical technological breakthrough can now reveal this important birth history information with a high degree of certainty. The section that follows presents reviews of current research regarding rare twin loss, development of a twin registry, twins’ communicative delays and DNA testing for vanishing twins. The article concludes with a survey of newsworthy twins, namely identical twins discordant for COVID vaccination, the world’s oldest identical twins, an Olympic athletic stand-in and fraternal twin football players.

There are few studies on twins in Ecuador and Latin America. It requires a better understanding of perinatal conditions, especially from an ethnic perspective. This work aims to assess perinatal factors related to twin pregnancy in Ecuadorian Mestizo individuals. We performed an epidemiological, observational and cross-sectional study at the Hospital San Francisco and Hospital Nueva Aurora in Quito, Ecuador, from November 2019 to January 2020. It included 203 newborns from twin pregnancies, including mothers with and without pathological history. The average gestational age was 31 weeks, and the APGAR score at first minute was 6.86, with significant differences. Regarding the metabolic balance, the mean pH was 6.14; and bicarbonate was 11.57, with significant differences. Twins had intrauterine growth restriction in 6.9% of cases, with significant differences (p = .003); 81.4% required supplemental oxygen, with significant differences (p = .002); 93% required noninvasive mechanical ventilation (NIMV), with significant differences (p = . 003); 93% required inotropic and sedation, with substantial differences; 69% required antibiotics (≥21 days), with significant differences (p = .014); and 17.2% needed between 8 to 14 days of hospitalization, and 51% more than 28 days, with significant differences. The studied mothers’ demographic profile was mostly Mestizos, with an average age of 32 years, and 93% had a poverty status. Most of the twins were diamniotic monochorial and were discordant twins. It found jaundice, premature anemia and sepsis in 100% of twins and hyaline membrane disease in 89.66% of twins. Twins of women with relevant prenatal care had more premature births (30.4 ± 2.6 weeks), more acid−base imbalance, APGAR at ≥7 min in 90% of cases, and patent ductus arteriosus in all. There was also a greater need for double intensive phototherapy than twins of healthy women.

We report a very rare case of monochorionic dizygotic twins conceived spontaneously. The fetuses were sex-discordant in ultrasonography despite being monochorionic twins. After birth, the girl and boy showed normal phenotypes but they showed blood chimerism in karyotype and blood group type.