[Associations between mothers' economic insecurity and mood and thoughts on life with children at infants' 3-month check-ups: A cross-sectional study using data from infant health check-ups].

Objectives　We aimed to clarify the relationship between economic insecurity and the psychological profiles of mothers raising infants by analyzing data from three-month health check-ups in relation to the birth population in order to generate basic data that can be used to consider support for families facing economic insecurity.Methods　The study area was a neighborhood in a major Japanese city. The survey focused on 1013 mothers who had received health check-ups for children aged 18 months between November 2017 and October 2019. The data included in the analysis were data from the children's three-month health check-ups and survey data from 908 mothers who responded to the questionnaire and consented to the use of their health check-up data. After excluding data from potential participants who were not mothers or had multiple births, data from 847 participants were analyzed (valid response rate: 93.3%).　The objective variables were mothers' mood and thoughts about life with their children at the time of the three-month check-up as the mothers' psychological profiles. The explanatory variable was the presence or absence of maternal economic insecurity, and logistic regression analysis was conducted, adjusted for the children's sex and birth order, presence or absence of maternal counselors, and the mothers' educational attainment.Results　A total of 60 (7.1%) mothers were economically insecure. Of the mothers' moods, 122 (14.4%) reported feeling anxious, followed by 36 (4.3%) who reported feeling lonely. Of their thoughts on life with their children, 776 (91.6%) mothers reported that they enjoyed it, and 567 (66.9%) reported that they were happy to be parents. On the other hand, 157 (18.5%) reported feeling frustrated, and 75 (8.9%) reported that the lack of time for themselves was painful. Economically insecure mothers had an odds ratio of 5.59 (95% confidence interval, 2.49-12.55) for feeling lonely, 4.77 (2.67-8.54) for feeling anxious, and 2.70 (1.50-4.86) for feeling frustrated, all significantly higher than in mothers not facing economic insecurity.Conclusion　Economic insecurity among mothers at the time of the three-month check-up was associated with the psychological states of loneliness, anxiety, and frustration about living with their children. It was suggested that to solve economically insecure mothers' problems, they need support, including connecting them with social welfare services, so that they will be able to raise their children in a more stable environment.

Changing genetic architecture of body mass index from infancy to early adulthood: an individual based pooled analysis of 25 twin cohorts.

BACKGROUND: Body mass index (BMI) shows strong continuity over childhood and adolescence and high childhood BMI is the strongest predictor of adult obesity. Genetic factors strongly contribute to this continuity, but it is still poorly known how their contribution changes over childhood and adolescence. Thus, we used the genetic twin design to estimate the genetic correlations of BMI from infancy to adulthood and compared them to the genetic correlations of height. METHODS: We pooled individual level data from 25 longitudinal twin cohorts including 38,530 complete twin pairs and having 283,766 longitudinal height and weight measures. The data were analyzed using Cholesky decomposition offering genetic and environmental correlations of BMI and height between all age combinations from 1 to 19 years of age. RESULTS: The genetic correlations of BMI and height were stronger than the trait correlations. For BMI, we found that genetic correlations decreased as the age between the assessments increased, a trend that was especially visible from early to middle childhood. In contrast, for height, the genetic correlations were strong between all ages. Age-to-age correlations between environmental factors shared by co-twins were found for BMI in early childhood but disappeared altogether by middle childhood. For height, shared environmental correlations persisted from infancy to adulthood. CONCLUSIONS: Our results suggest that the genes affecting BMI change over childhood and adolescence leading to decreasing age-to-age genetic correlations. This change is especially visible from early to middle childhood indicating that new genetic factors start to affect BMI in middle childhood. Identifying mediating pathways of these genetic factors can open possibilities for interventions, especially for those children with high genetic predisposition to adult obesity.

[Perceptual changes among public health nurses brought on through a newly adopted system for providing continuous support in the municipality: Modelled at its core on Finnish practice for maternity and child health clinics (Neuvola)].

Objectives　This study targeted public health nurses in a Japanese municipality that had newly adopted the core components of the Finnish system to provide continuous support to families with children until preschool. It sought to analyze changes in their perceptions for activities in maternal and child health care before and after the adoption.Methods　Relevant data were collected through semi-structured focus group interviews, following an interview guide, conducted from September to October 2020. The study protocol was approved by the Ethics Committee of Osaka City University in Japan.Results　A total of 12 public health nurses participated. Prior to the adoption of the system modelled on Finnish practice, these nurses were dedicated to addressing the risks regarding families with children (especially those at high risk) through continuous engagement. However, they were hesitant to proactively engage with families at lower risk as non-continuous, one-off engagements resulted in a patchwork response. After the system was adopted, the nurses became aware of the trust cultivated with families under their care, which included those at lower risk, that enabled them to respond to the changing needs flexibly. The nurses recognized that they had acquired the capability to notice the subtle signs of changes, engage more proactively with the families under their care, and deliver the necessary preventive interventions at an early stage. They also demonstrated joy and satisfaction derived from the growth of children and mothers under their care, as well as keen awareness of the need to improve their professional skills, even though they were busy.Conclusion　These findings indicate that the continuous support system modelled on the core components from Finland enables public health nurses to proactively engage with families at lower risk and deliver preventive interventions at an early stage. The system also motivated Japanese public health nurses who found greater pleasure in their work.

[Relationship between economic disparity and eating habits of 3-year-old children].

Objectives　This study aimed to clarify the relationship between economic disparity and eating habits of young children to explore the support that should be provided to families with young children.Methods　We conducted an anonymous self-administered questionnaire survey of parents who brought their children to the 3-year-old health checkup in four wards in City A. Questionnaire items examined the children's eating habits and the childrearing environment, such as the parents' socioeconomic status. Of 1,150 parents asked to participate in the survey, 616 responded (response rate: 53.6%). Invalid questionnaires (e.g., with missing answers) were eliminated. The remaining 498 parents (effective response rate: 80.8%) were classified into two groups to examine the association of household financial status with their 3-year-old eating habits: the relative poverty group and the non-relative poverty group. The groups were divided according to the poverty line of the Comprehensive Survey of Living Conditions. Statistical analysis was performed using Fisher's exact test and the Mann-Whitney U test. Eating habits that were identified to be associated with relative poverty were further tested using logistic regression analysis.Results　Analysis of the children's eating habits in the two groups revealed that, compared with the non-relative poverty group, there was a higher proportion of 3-year-olds who ate vegetables less than 6 days a week (P=0.003) and who consumed snacks 6 days or more per week (P=0.034) in the relative poverty group. The associations of relative poverty with vegetable intake less than 6 days a week and snack consumption 6 days or more a week remained significant after adjusting for parents' age, highest educational attainment, and subjective views on the everyday economic situation. Regarding the childrearing environment, the relative poverty group had a higher proportion of parents who were younger than 30 years of age (P<0.001) and in single-parent households (P=0.007). The relative poverty group had a higher proportion of parents whose highest education level was high school (P<0.001). Furthermore, the subjective view of the everyday economic situation was more negative in the relative poverty group (P<0.001).Conclusion　Economic disparity was related to the eating habits of 3-year-old children. The study results indicate that identifying families with financial difficulties during pregnancy and childbirth is necessary to support them early on so that young children can acquire healthy eating habits.

Genetic and environmental influences on human height from infancy through adulthood at different levels of parental education.

Genetic factors explain a major proportion of human height variation, but differences in mean stature have also been found between socio-economic categories suggesting a possible effect of environment. By utilizing a classical twin design which allows decomposing the variation of height into genetic and environmental components, we tested the hypothesis that environmental variation in height is greater in offspring of lower educated parents. Twin data from 29 cohorts including 65,978 complete twin pairs with information on height at ages 1 to 69 years and on parental education were pooled allowing the analyses at different ages and in three geographic-cultural regions (Europe, North America and Australia, and East Asia). Parental education mostly showed a positive association with offspring height, with significant associations in mid-childhood and from adolescence onwards. In variance decomposition modeling, the genetic and environmental variance components of height did not show a consistent relation to parental education. A random-effects meta-regression analysis of the aggregate-level data showed a trend towards greater shared environmental variation of height in low parental education families. In conclusion, in our very large dataset from twin cohorts around the globe, these results provide only weak evidence for the study hypothesis.

[Childcare burdens perceived by mothers with an inter-pregnancy interval less than 12 months].

Objectives　The aims of this study were to investigate perceived childcare burdens among mothers with an inter-pregnancy interval of less than 12 months, and to identify the factors associated with those perceptions.Methods　Anonymous, self-administered questionnaires on inter-pregnancy intervals, the Edinburgh postnatal depression scale (EPDS), and the childcare emotion scale were distributed to mothers who visited one of three health and welfare centers in City A for three-month well-baby checkups. Six hundred eighty-five subjects were classified into four groups according to their inter-pregnancy interval (<12 months apart; ≥12 months but <24 months apart; ≥24 months apart; and a group with no siblings), and they was used to analyze associations between the timing of pregnancy and the mothers' perceptions of their childcare burdens. Chi-squared tests, a one-way analysis of variance, the Kruskal-Wallis test, and linear regression analyses were used for analysis.Results　The inter-pregnancy interval was <12 months in 35 of the 685 respondents, ≥12 months but <24 months in 114 respondents, and ≥24 months in 194 respondents. There were 342 respondents in the no-siblings group. Linear regression analyses disclosed some factors associated with the subordinate items of the childcare emotion scale. "Feeling restricted by childcare duties" is significantly correlated to inter-pregnancy interval (P=.032), family composition (P=.014), time of sleeping (P=.010), and night wake-up time (P=.001). "Feeling of burden because of the child's attitude and behavior" was significantly related to inter-pregnancy interval (P<.001), mother's age (P=.003), time of sleeping (P=.009), and night wake-up time (P=.002). "Anxiety about the child's growth" was significantly related to inter-pregnancy interval (P<.001), mother's age (P=.016), and gestational age (P<.001). In addition, the group with gestation intervals of less than 12 months had significantly higher numbers of women in single-parent households (P=.005), who were unmarried (P=.007), had only a junior high school education (P=.0027), and were 24 years old or younger (P<.001).Conclusion　The pregnancy interval is significantly related to the "Feeling restricted by childcare duties," "Feeling of burden because of the child's attitude and behavior," and "Anxiety about the child's growth," even after adjusting the influence of other factors. In addition, the results of this study demonstrated that mothers in the <12 months group were more likely to be in a single-parent household, unmarried, with low educational attainment, and were 24 years old or younger. The results suggest that greater support is needed for these mothers.

The New West Japan Twins and Higher Order Multiple Births Registry.

The new West Japan Twins and Higher Order Multiple Births Registry was established by recruiting young twins and multiple births and by referrals from public health centers in the 1990s. The participants included in the survey comprised over 7800 twins and 4241 higher order multiples, and their families. Specifically, the present registry contains one of the largest triplet samples in the world. For these twins and multiples, data on year of delivery, mode of delivery, gestational age, intrapartum complications, longitudinal physical measures, motor milestones, cerebral palsy and feeding methods were obtained from records in the Maternal and Child Health Handbooks and schools. Participating mothers were asked to indicate family structure, parental educational history, maternal sleeping time, maternal health condition, maternal and paternal age at multiple delivery, complications during pregnancy, handedness of multiples and age at menarche of multiples. However, the zygosity differed among the various collaborating public health centers according to factors such as the time of investigation. Follow-up questionnaires have been mailed out every 3-4 years for longitudinal studies. This article describes the goals of this registry, recruitment of multiples and the focus of the study. The goals of this registry are not only to conduct research on human genetics and maternal and child health, but also to contribute to providing appropriate information for families with multiples.

Parental Education and Genetics of BMI from Infancy to Old Age: A Pooled Analysis of 29 Twin Cohorts.

OBJECTIVE: The objective of this study was to analyze how parental education modifies the genetic and environmental variances of BMI from infancy to old age in three geographic-cultural regions. METHODS: A pooled sample of 29 cohorts including 143,499 twin individuals with information on parental education and BMI from age 1 to 79 years (299,201 BMI measures) was analyzed by genetic twin modeling. RESULTS: Until 4 years of age, parental education was not consistently associated with BMI. Thereafter, higher parental education level was associated with lower BMI in males and females. Total and additive genetic variances of BMI were smaller in the offspring of highly educated parents than in those whose parents had low education levels. Especially in North American and Australian children, environmental factors shared by co-twins also contributed to the higher BMI variation in the low education level category. In Europe and East Asia, the associations of parental education with mean BMI and BMI variance were weaker than in North America and Australia. CONCLUSIONS: Lower parental education level is associated with higher mean BMI and larger genetic variance of BMI after early childhood, especially in the obesogenic macro-environment. The interplay among genetic predisposition, childhood social environment, and macro-social context is important for socioeconomic differences in BMI.

Association between birth weight and educational attainment: an individual-based pooled analysis of nine twin cohorts.

BACKGROUND: There is evidence that birth weight is positively associated with education, but it remains unclear whether this association is explained by familial environmental factors, genetic factors or the intrauterine environment. We analysed the association between birth weight and educational years within twin pairs, which controls for genetic factors and the environment shared between co-twins. METHODS: The data were derived from nine twin cohorts in eight countries including 6116 complete twin pairs. The association between birth weight and educational attainment was analysed both between individuals and within pairs using linear regression analyses. RESULTS: In between-individual analyses, birth weight was not associated with educational years. Within-pairs analyses revealed positive but modest associations for some sex, zygosity and birth year groups. The greatest association was found in dizygotic (DZ) men (0.65 educational years/kg birth weight, p=0.006); smaller effects of 0.3 educational years/kg birth weight were found within monozygotic (MZ) twins of both sexes and opposite-sex DZ twins. The magnitude of the associations differed by birth year in MZ women and opposite-sex DZ twins, showing a positive association in the 1915-1959 birth cohort but no association in the 1960-1984 birth cohort. CONCLUSION: Although associations are weak and somewhat inconsistent, our results suggest that intrauterine environment may play a role when explaining the association between birth weight and educational attainment.

Triplets, birthweight, and handedness.

The mechanisms behind handedness formation in humans are still poorly understood. Very low birthweight is associated with higher odds of left-handedness, but whether this is due to low birthweight itself or premature birth is unknown. Handedness has also been linked to development, but the role of birthweight behind this association is unclear. Knowing that birthweight is lower in multiple births, triplets being about 1.5 kg lighter in comparison with singletons, and that multiples have a higher prevalence of left-handedness than singletons, we studied the association between birthweight and handedness in two large samples consisting exclusively of triplets from Japan (n = 1,305) and the Netherlands (n = 947). In both samples, left-handers had significantly lower birthweight (Japanese mean = 1,599 g [95% confidence interval (CI): 1,526-1,672 g]; Dutch mean = 1,794 g [95% CI: 1,709-1,879 g]) compared with right-handers (Japanese mean = 1,727 g [95% CI: 1,699-1,755 g]; Dutch mean = 1,903 g [95% CI: 1,867-1,938 g]). Within-family and between-family analyses both suggested that left-handedness is associated with lower birthweight, also when fully controlling for gestational age. Left-handers also had significantly delayed motor development and smaller infant head circumference compared with right-handers, but these associations diluted and became nonsignificant when controlling for birthweight. Our study in triplets provides evidence for the link between low birthweight and left-handedness. Our results also suggest that developmental differences between left- and right-handers are due to a shared etiology associated with low birthweight.

Genetic and environmental factors affecting birth size variation: a pooled individual-based analysis of secular trends and global geographical differences using 26 twin cohorts.

Background: The genetic architecture of birth size may differ geographically and over time. We examined differences in the genetic and environmental contributions to birthweight, length and ponderal index (PI) across geographical-cultural regions (Europe, North America and Australia, and East Asia) and across birth cohorts, and how gestational age modifies these effects. Methods: Data from 26 twin cohorts in 16 countries including 57 613 monozygotic and dizygotic twin pairs were pooled. Genetic and environmental variations of birth size were estimated using genetic structural equation modelling. Results: The variance of birthweight and length was predominantly explained by shared environmental factors, whereas the variance of PI was explained both by shared and unique environmental factors. Genetic variance contributing to birth size was small. Adjusting for gestational age decreased the proportions of shared environmental variance and increased the propositions of unique environmental variance. Genetic variance was similar in the geographical-cultural regions, but shared environmental variance was smaller in East Asia than in Europe and North America and Australia. The total variance and shared environmental variance of birth length and PI were greater from the birth cohort 1990-99 onwards compared with the birth cohorts from 1970-79 to 1980-89. Conclusions: The contribution of genetic factors to birth size is smaller than that of shared environmental factors, which is partly explained by gestational age. Shared environmental variances of birth length and PI were greater in the latest birth cohorts and differed also across geographical-cultural regions. Shared environmental factors are important when explaining differences in the variation of birth size globally and over time.

Birth size and gestational age in opposite-sex twins as compared to same-sex twins: An individual-based pooled analysis of 21 cohorts.

It is well established that boys are born heavier and longer than girls, but it remains unclear whether birth size in twins is affected by the sex of their co-twin. We conducted an individual-based pooled analysis of 21 twin cohorts in 15 countries derived from the COllaborative project of Development of Anthropometrical measures in Twins (CODATwins), including 67,850 dizygotic twin individuals. Linear regression analyses showed that boys having a co-twin sister were, on average, 31 g (95% CI 18 to 45) heavier and 0.16 cm (95% CI 0.045 to 0.274) longer than those with a co-twin brother. In girls, birth size was not associated (5 g birth weight; 95% CI -8 to -18 and -0.089 cm birth length; 95% CI -0.202 to 0.025) with the sex of the co-twin. Gestational age was slightly shorter in boy-boy pairs than in boy-girl and girl-girl pairs. When birth size was standardized by gestational age, the magnitude of the associations was attenuated in boys, particularly for birth weight. In conclusion, boys with a co-twin sister are heavier and longer at birth than those with a co-twin brother. However, these differences are modest and partly explained by a longer gestation in the presence of a co-twin sister.

Associations between birth size and later height from infancy through adulthood: An individual based pooled analysis of 28 twin cohorts participating in the CODATwins project.

BACKGROUND: There is evidence that birth size is positively associated with height in later life, but it remains unclear whether this is explained by genetic factors or the intrauterine environment. AIM: To analyze the associations of birth weight, length and ponderal index with height from infancy through adulthood within mono- and dizygotic twin pairs, which provides insights into the role of genetic and environmental individual-specific factors. METHODS: This study is based on the data from 28 twin cohorts in 17 countries. The pooled data included 41,852 complete twin pairs (55% monozygotic and 45% same-sex dizygotic) with information on birth weight and a total of 112,409 paired height measurements at ages ranging from 1 to 69 years. Birth length was available for 19,881 complete twin pairs, with a total of 72,692 paired height measurements. The association between birth size and later height was analyzed at both the individual and within-pair level by linear regression analyses. RESULTS: Within twin pairs, regression coefficients showed that a 1-kg increase in birth weight and a 1-cm increase in birth length were associated with 1.14-4.25 cm and 0.18-0.90 cm taller height, respectively. The magnitude of the associations was generally greater within dizygotic than within monozygotic twin pairs, and this difference between zygosities was more pronounced for birth length. CONCLUSION: Both genetic and individual-specific environmental factors play a role in the association between birth size and later height from infancy to adulthood, with a larger role for genetics in the association with birth length than with birth weight.

Maternal subjective well-being and preventive health care system in Japan and Finland.

Background: Maternal well-being is an important issue not only for mothers but also for their offspring and whole families. This study aims to clarify differences in subjective well-being for mothers with infants and associated factors by comparing Japanese and Finnish mothers. Methods: In Finland, 101 mothers with infants who received health check-ups at child's age 4 months participated in the study. In Japan, 505 mothers with infants who should receive health check-ups at child's age 4 months and, whose age, age of the infant and number of children matched with the Finnish mothers were selected. The factors associated with maternal subjective well-being were explored by the linear regression analysis. All Finnish mothers had individual infant health check-ups by nurses in Child Health Clinics nearly monthly. The same nurse was responsible for following up the family throughout the years. All Japanese participants received group health check-up once at child's age 3 to 4 months, and a nurse did not cover same child and their mother. Results: Finnish mothers showed significantly better subjective well-being compared with Japanese mothers. Whereas 85% of Finnish mothers responded that they had obtained childcare information from public health nurses, significantly fewer Japanese mothers indicated the same response (8%). Linear regression analyses disclosed that mothers' subjective well-being was associated with country, mothers' stress and age. Conclusion: Finnish mothers had better subjective well-being than Japanese mothers. Our results may indicate that the Finnish health care system supports mothers better than the Japanese health care system does.

Characteristics of stepfamilies and maternal mental health compared with non-stepfamilies in Japan.

BACKGROUND: Stepfamilies remain poorly understood in Japanese society, and the support needs of stepfamily mothers are unclear. This study aimed to identify characteristics of stepfamilies and maternal mental health as compared with non-stepfamilies in Japan to utilize as a primary resource for providing effective support through community-based health care for stepfamilies. METHODS: From December 2011 to July 2012, we conducted this questionnaire survey with mothers at 3- and 4-month checkups for infants. The response rate was 75.1%. The sample for analysis included responses of 2246 mothers, excluding single mothers. RESULTS: Respondents comprised 47 (2.1%) stepfamilies and 2199 (97.9%) non-stepfamilies. There were significantly higher rates of parents with not more than a high school education and ≥3 children among stepfamilies compared with non-stepfamilies. Stepfamily mothers had significantly higher rates of feeling a lack of economic resources, absence of participation in childbirth education classes, smoking during pregnancy, and unplanned pregnancy. Furthermore, they also had significantly higher rates of depression and a lack of confidence in the parent role. Maternal depression was associated with factors such as maternal age, self-perceived health, stress level, confidence in breastfeeding, confidence in the parent role, and number of children. CONCLUSIONS: These findings suggest that stepfamilies exhibit many characteristics related to social disadvantage and problems with community-based health care in Japan. Healthcare providers should be aware of stepfamily mothers' support needs and should put in place a support system for stepfamilies. Moreover, compared with non-stepfamily mothers, stepfamily mothers have a significantly higher prevalence of depression. However, stepfamily composition does not necessarily increase the risk of maternal depression. Therefore, healthcare providers should put in place a system for obtaining more thorough information about stepfamilies and conduct an early assessment to identify their support needs.

Education in Twins and Their Parents Across Birth Cohorts Over 100 years: An Individual-Level Pooled Analysis of 42-Twin Cohorts.

Whether monozygotic (MZ) and dizygotic (DZ) twins differ from each other in a variety of phenotypes is important for genetic twin modeling and for inferences made from twin studies in general. We analyzed whether there were differences in individual, maternal and paternal education between MZ and DZ twins in a large pooled dataset. Information was gathered on individual education for 218,362 adult twins from 27 twin cohorts (53% females; 39% MZ twins), and on maternal and paternal education for 147,315 and 143,056 twins respectively, from 28 twin cohorts (52% females; 38% MZ twins). Together, we had information on individual or parental education from 42 twin cohorts representing 19 countries. The original education classifications were transformed to education years and analyzed using linear regression models. Overall, MZ males had 0.26 (95% CI [0.21, 0.31]) years and MZ females 0.17 (95% CI [0.12, 0.21]) years longer education than DZ twins. The zygosity difference became smaller in more recent birth cohorts for both males and females. Parental education was somewhat longer for fathers of DZ twins in cohorts born in 1990-1999 (0.16 years, 95% CI [0.08, 0.25]) and 2000 or later (0.11 years, 95% CI [0.00, 0.22]), compared with fathers of MZ twins. The results show that the years of both individual and parental education are largely similar in MZ and DZ twins. We suggest that the socio-economic differences between MZ and DZ twins are so small that inferences based upon genetic modeling of twin data are not affected.

Differences in genetic and environmental variation in adult BMI by sex, age, time period, and region: an individual-based pooled analysis of 40 twin cohorts.

Background: Genes and the environment contribute to variation in adult body mass index [BMI (in kg/m2)], but factors modifying these variance components are poorly understood.Objective: We analyzed genetic and environmental variation in BMI between men and women from young adulthood to old age from the 1940s to the 2000s and between cultural-geographic regions representing high (North America and Australia), moderate (Europe), and low (East Asia) prevalence of obesity.Design: We used genetic structural equation modeling to analyze BMI in twins ≥20 y of age from 40 cohorts representing 20 countries (140,379 complete twin pairs).Results: The heritability of BMI decreased from 0.77 (95% CI: 0.77, 0.78) and 0.75 (95% CI: 0.74, 0.75) in men and women 20-29 y of age to 0.57 (95% CI: 0.54, 0.60) and 0.59 (95% CI: 0.53, 0.65) in men 70-79 y of age and women 80 y of age, respectively. The relative influence of unique environmental factors correspondingly increased. Differences in the sets of genes affecting BMI in men and women increased from 20-29 to 60-69 y of age. Mean BMI and variances in BMI increased from the 1940s to the 2000s and were greatest in North America and Australia, followed by Europe and East Asia. However, heritability estimates were largely similar over measurement years and between regions. There was no evidence of environmental factors shared by co-twins affecting BMI.Conclusions: The heritability of BMI decreased and differences in the sets of genes affecting BMI in men and women increased from young adulthood to old age. The heritability of BMI was largely similar between cultural-geographic regions and measurement years, despite large differences in mean BMI and variances in BMI. Our results show a strong influence of genetic factors on BMI, especially in early adulthood, regardless of the obesity level in the population.

Association between birthweight and later body mass index: an individual-based pooled analysis of 27 twin cohorts participating in the CODATwins project.

Background: There is evidence that birthweight is positively associated with body mass index (BMI) in later life, but it remains unclear whether this is explained by genetic factors or the intrauterine environment. We analysed the association between birthweight and BMI from infancy to adulthood within twin pairs, which provides insights into the role of genetic and environmental individual-specific factors. Methods: This study is based on the data from 27 twin cohorts in 17 countries. The pooled data included 78 642 twin individuals (20 635 monozygotic and 18 686 same-sex dizygotic twin pairs) with information on birthweight and a total of 214 930 BMI measurements at ages ranging from 1 to 49 years. The association between birthweight and BMI was analysed at both the individual and within-pair levels using linear regression analyses. Results: At the individual level, a 1-kg increase in birthweight was linearly associated with up to 0.9 kg/m2 higher BMI (P < 0.001). Within twin pairs, regression coefficients were generally greater (up to 1.2 kg/m2 per kg birthweight, P < 0.001) than those from the individual-level analyses. Intra-pair associations between birthweight and later BMI were similar in both zygosity groups and sexes and were lower in adulthood. Conclusions: These findings indicate that environmental factors unique to each individual have an important role in the positive association between birthweight and later BMI, at least until young adulthood.

Erratum to: Characteristics of stepfamilies and maternal mental health compared with non-stepfamilies in Japan.

[This corrects the article DOI: 10.1186/s12199-017-0658-z.].