This present study investigated the parental characteristics of multiple births using national birth data in Japan. This study included birth data from Vital Statistics: Occupational and Industrial Aspects every five fiscal years from 1995 to 2020. The multiple birth rates were defined as the number of live-birth deliveries with multiple fetuses (e.g., twins, triplets) per total live-birth deliveries. Parental ages, nationalities, occupations and household occupation (occupation of the top earner of the household) were considered as parental characteristics. The multiple birth rates were calculated based on parental characteristics for each year, and a log-binomial regression model was used to assess the association between parental characteristics and multiple births. The multiple birth rate for Japanese mothers consistently exceeded that for non-Japanese mothers over the years, and the rate increased progressively from manual workers to lower non-manual workers and then to upper non-manual workers for both maternal and paternal occupations. The regression results indicated that the risk ratio (RR) for multiple births among non-Japanese mothers was significantly lower than that among Japanese mothers. Moreover, concerning household occupation, the RRs of self-employed individuals, full-time employees at smaller companies, others, and the unemployed were significantly lower than those of full-time employees at larger companies. Furthermore, the RRs of lower non-manual and manual workers were significantly lower than those of upper non-manual workers in maternal and paternal occupations. The results suggested an association between multiple births and parental socioeconomic status in Japan.

Much progress has been made in twin research since our last special issue on twin registries (Hur, Y.-M., & Craig, J. M. (2013). Twin Research and Human Genetics, 16, 1–12.). This special issue provides an update on the state of twin family registries around the world. This issue includes 61 papers on twin family registries from 25 countries, of which 3 describe consortia based on collaborations of several twin family registries. The articles included in this issue discuss the establishment and maintenance of twin registries, recruitment strategies, methods of zygosity assessment, research aims and major findings from twin family cohorts, as well as other important topics related to twin studies. The papers amount to approximately 1.3 million monozygotic, dizygotic twins and higher order multiples and their family members who participate in twin studies around the world. Nine new twin family registries have been established across the world since our last issue, which demonstrates that twin registers are increasingly important in studies of the determinants and correlates of complex traits from disease susceptibility to healthy development.

Objective: The aim of this study was to compare the neurodevelopmental outcome at 12–18 months’ corrected age between multiples and singleton preterm infants. Methods: We designed a prospective study of preterm infants (≤32 weeks gestation) born and hospitalized in the neonatal intensive care unit between November 2008 and November 2009, whose assessments were performed at 12–18 months’ corrected age. Neurodevelopmental impairment was defined as the presence of any one of the following: moderate or severe cerebral palsy, severe bilateral hearing loss or bilateral blindness, mental developmental index score, or psychomotor developmental index score less than 70. Results were compared for both multiples and singleton infants. Results: One hundred and fifty-nine multiples and 211 singleton infants were assessed at 12–18 months’ corrected age. The neurodevelopmental outcome including all parameters at 12–18 months’ corrected age in multiples was not significantly different from singleton preterm infants. Conclusions: Multiple gestation in preterm infants is not associated with an increased risk of neurodevelopmental impairment at 12–18 months’ corrected age compared with singleton preterm infants. For further information, long term and high participation in neurodevelopmental follow-up and evaluation at pre-school age will be needed.

Multiple birth remains prevalent, with prematurity and subsequent disability being common complications. However, little is known of the experiences of mothers living with the combined circumstances of multiple birth, prematurity, and special needs. This paper reports an exploratory study using the qualitative technique of thematic analysis, to describe and interpret the experiences of 10 mothers of prematurely born multiple birth children with diverse special needs. Mothers were shocked to learn they were carrying multiple fetuses, including those who underwent in vitro fertilization with dual embryo transfer. Most experienced protracted concern over one or more babies’ survival during pregnancy, and prescribed bed rest was frequently associated with increased anxiety and other adverse psychological effects. Some experienced disenfranchised grief, such as those grieving the opportunity to bear a single child. The contrast with experiences of mothers of healthy, term singletons caused considerable distress. Feelings of detachment and unreality were common in the immediate postpartum period, possibly due to transient depersonalization. Having more than one newborn created practical and psychological problems during the neonatal period, particularly when infants were separated due to differences in medical status. The extent to which hospitals accommodated the multiple birth relationship varied and significantly affected mothers’ postnatal experiences. Mothers often felt guilty, particularly regarding inequality of care and attention they were able to provide to each child. This was especially problematic for multiples discordant for special needs status. The presence of one normally developing child complicated adjustment to the other's difficulties, and several experienced chronic sorrow. Serious maternal mental health difficulties were common but not universal. Available formal supports were generally perceived as inadequate, addressing some, but not all, of the mothers’ needs. Further work is needed to advance understanding of the relationships between mothers and their multiples, and to explore the implications of special needs within multiple birth families.

Assisted reproductive techniques and fertility enhancing therapies have increased multiple births and, therefore, the risk of prematurity and its developmental consequences. Parent intervention is an effective source of compensation for the cognitive effects of prematurity. We hypothesized that relative to parents of preterm singletons, parents of preterm twins are less able to provide such enhancing care, resulting in a developmental disadvantage for preterm twins. Maternal–infant interactions of premature singletons (n =22; birth weight =1668 ± 350 g, gestational age =32.3 ± 2.1 weeks) and premature twins (n = 8; birth weight = 1618 ± 249 g; gestational age = 32.0 ± 2.6 weeks) with comparable demographic and medical status were observed at home at 1 and 8 months corrected age using a 30 min checklist of developmentally facilitative behavior. Mental (MDI) and psychomotor (PDI) indices of the Bayley Scales of Infant Development and Caldwell Home Observations for Measurement of the Environment (HOME) inventories were administered (18 months corrected age). Compared with mothers of premature singletons, mothers of premature twins exhibited fewer initiatives (P < 0.001) and responses (P < 0.01) and were less responsive to positive signals (P < 0.01) and crying (P < 0.01). Unprompted by the infant, twin mothers lifted or held (P < 0.05), touched (P < 0.01), patted (P < 0.05) or talked (P < 0.01) less. Singleton MDIs surpassed twins (119.4 ± 7.7 vs 103.6 ± 7.7; P < 0.01). Maternal verbal behavior and the acceptance of child factor (HOME), both favoring singletons, correlated with MDI (R-square = 0.46, P < 0.0002). Mothers of premature twins exhibited fewer initiatives and responses toward offspring than did mothers of premature singletons. Maternal behavior was predictive of cognitive development. Twin Research (2000) 3, 234–241.

Dizygotic twinning rates have changed over time, which has been seen as a sign of a decline in fecundity. Since a woman's birthweight has been shown to be a marker of her fecundity, maternal birthweight may correlate with subsequent twinning rates. In the Danish National Birth Cohort (1996–2002), we examined if maternal birthweight, and whether she was born at term or preterm, correlated with her probability of multiple birth. For 20,719 live born infants, we had self-reported information about maternal birthweight, collected during the first wave of the 7-year follow-up, and information on multiple births from record linkage. The association between maternal birthweight and multiple births was investigated by use of logistic regression and presented as odds ratios (ORs) with 95% confidence intervals (CIs). Compared to women born at term with a birthweight of 3001–4000 g, women with a birthweight > 4500 g appeared to have higher chance of multiple birth while women with a birthweight of 4001–4500 had a lower chance, especially if the analysis was restricted to women with a BMI < 25 (Adjusted ORs 2.3 [1.0–5.4] and 0.4 [0.2–0.9] respectively). Odds ratios for dizygotic twinning were of the same magnitude. In women with a BMI ≥ 25, no obvious pattern was present. Our findings do not indicate that twinning is a fecundity indicator. Women with a birthweight that may indicate a pregnancy complicated with gestational diabetes had the highest rate of multiple birth. These findings are new and should be put to a critical test in other data sources.

Fifty-two Australian couples who had experienced the death of at least one member of a multiple birth (twin or higher order), with at least one survivor of that birth, were interviewed about their experiences at the time of the death, and since. This study compared parents' coping after the twins' deaths using the Beck Depression Inventory II, Perinatal Grief Scale, and unstructured interviews with some structured queries. Parents provided information on the influence of family, community and medical staff. According to retrospective reports, mothers experienced significantly more depression and grief than fathers at the time of loss. Both parents found the death of their twins grievous, but fathers, unlike mothers, were not encouraged to express their emotions. Although parents generally agreed about what helped them cope, fathers believed that they should be able to cope regardless of their grief. The strength of parents' spiritual beliefs had increased significantly since their loss, and there was some evidence that depressed and grieving mothers turned to spiritual support. Parents whose children died earlier reported levels of depression similar to those reported by parents whose children died later. To date, this is the largest study of grief in couples who have experienced the death of a twin and who have a surviving twin or higher order multiple.

This study investigated the degree of risk of handicap in twins, triplets, quadruplets and quintuplets and associated factors, and examined the clustering tendency of handicaps. The sample was recruited from the Kinki University Twin and Higher Order Multiple Birth Registry. This panel consisted of 705 pairs of twins (1410 twins), 96 sets of triplets (287 triplets excluding 1 infant death), 7 sets of quadruplets (27 quadruplets excluding 1 infant death), and 2 sets of quintuplets (10 quintuplets), all of whom were born after 1977. The incidence of handicap was 3.7% in twins, 8.7% in triplets, 11.1% in quadruplets, and 10.0% in quintuplets. The risk of prodicing at least 1 handicapped child was approximately 1 in 13 pairs of twins (7.4%), 1 in 4 or 5 sets of triplets (21.6%), and 1 in 2 sets of quadruplet and quintuplets (50%). There was a significantly higher clustering tendency of handicaps in twins and triplets compared with the expected frequency calculated from the incidence rate of handicap. Four significant risk factors for subsequent handicap were found by logistic regression: gestation number, shortening of gestational age, premature rupture of the membrane, and toxemia during pregnancy.

The rate of multiple births and the incidence of congenital anomalies in Tokyo Metropolitan Hospitals were studied during the period 1979-1990. The number of twins was 968 pairs (8.23 per 1,000 deliveries) and of triplets 18 sets (15.3 per 100,000 deliveries) among 117,672 deliveries including 1,587 stillbirths after 16 weeks gestation. Multiple birth rates increased yearly. Stillbirth rates in twins and triplets were 5.5% and 16.7% respectively, which were both significantly higher than that in singletons (1.3%). The number of congenital anomalies was 42 in 1,936 twins (2.17%), 2 in 54 triplets (3.7%) and 1721 in 116,686 singletons (1.47%). The most common defects in twins were those of the cardiovascular system (0.72% in twins vs 0.52% in singletons) and of the musculoskeletal system (0.72% in twins vs 0.50% in singletons), followed by upper respiratory tract and/or mouth conditions (0.67% in twins vs 0.35% in singletons), all of which had no significant difference in frequency between twins and singletons. Though some anomalies had a significantly higher frequency in twins than in singletons, the concordance rate in the like-sexed twins was very low.