Body size of twins compared with siblings and the general population: from birth to late adolescence

Co-recovery of physical size and cognitive ability from infancy to adolescence: A twin study

This study tested phenotypic and biometric associations between physical and cognitive catch-up growth in a community sample of twins (n = 1285, 51.8% female, 89.3% White). Height and weight were measured at up to 17 time points between birth and 15 years, and cognitive ability was assessed at up to 16 time points between 3 months and 15 years. Weight and length at birth were positively associated with cognitive abilities in infancy and adolescence (r's = .16-.51). More rapid weight catch-up growth was associated with slower, steadier cognitive catch-up growth. Shared and nonshared environmental factors accounted for positive associations between physical size at birth and cognitive outcomes. Findings highlight the role of prenatal environmental experiences in physical and cognitive co-development.

The effects of intrauterine growth on physical and intellectual development of one-year-old infants: a study on monochorionic twins with selective intrauterine growth restriction

This study aimed to evaluate physical and intellectual development of one-year-old infants of monochorionic twins with selective intrauterine growth restriction (sIUGR). A total of 31 pairs of sIUGR twins ageing 1 year old were included in the study. Each pair of sIUGR twins was divided into low birthweight-twin group (L-twin group) and high birthweight-twin group (H-twin group) according to twins' birthweight. The differences in height, weight, head circumstance and body mass index (BMI) in each stage were statistically significant for all measures from birth until 1 year old (p < .05), and there was a disappointed catch-up growth in lighter twins. Psychomotor development index (PDI) and mental development index (MDI) at 1 year old were significantly different between the two groups (p < .05). Stepwise regression analysis showed that the effects of weight on both PDI and MDI were statistically significant (p < .05). Intrauterine growth inconsistencies in monochorionic twins with sIUGR persist until the first year of life and affect low-birthweight infants' physical and intellectual development.Impact StatementWhat is already known on this subject? Selective intrauterine growth restriction in monochorionic twins increases the risks of intrauterine foetal demise, preterm birth, caesarean delivery and adverse neonatal outcomes, especially in the smaller foetus.What do the results of this study add? Previous studies have concentrated on the clinical management of sIUGR, while little attention has been paid to the growth and development of twins after birth. Given the adverse neurobiological effects of suboptimal nutrition on the brain development, it is important to determine whether IUGR causes long-term cognitive deficits and physical retardation. The current study has assessed the physical and intellectual development of one-year-old infants of monochorionic twins with sIUGR.What are the implications of these findings for clinical practice and/or further research? Intrauterine growth inconsistencies in monochorionic twins with sIUGR persist until the first year of life and affect low-birthweight infants' physical and intellectual development. Further research on the longer-term effects of sIUGR is needed.

Longitudinal Child Growth Patterns in Twins and Singletons in the Upstate KIDS Cohort

OBJECTIVES

To investigate childhood growth patterns in twins and to determine whether they show the same signs of excess growth as singletons born small-for-gestational age (SGA), which may confer future cardiometabolic risk.

STUDY DESIGN

In the Upstate KIDS cohort of infants delivered from 2008 through 2010, we compared height, weight, and body mass index (BMI) z-scores at 0-3 and 7-9 years of age, as well as risk of rapid weight gain (RWG) in infancy and overweight/obesity beginning at 2 years, among appropriate-for-gestational age (AGA) twins (n = 1121), AGA singletons (n = 2684), and two groups of SGA twins: uncertain SGA twins (<10th percentile for birthweight by a singleton reference but >10th% by a population-based twin birthweight reference; n = 319) and true SGA twins (<10th% by a population-based twin reference; n = 144).

RESULTS

Compared with AGA twins, both SGA twin groups had lower weight and BMI z-scores at both time points. By 7-9 years, both groups caught up in height with AGA twins. Compared with AGA singletons, z-score differences decreased between 0-3 and 7-9 years for uncertain SGA and true SGA twins, though true SGA twins had the lowest z-scores for all measures. During infancy, twins were more likely to display RWG compared with AGA singletons (RR = 2.06 to 2.67), which may reflect normal catch-up growth, as no twin group had higher prevalence of overweight/obesity at either time point.

CONCLUSIONS

Though twins had lower height, weight, and BMI z-scores at birth and into toddlerhood, differences were reduced by 7-9 years, with no evidence of pathological growth and no group of twins showing elevated risk of overweight/obesity.

Nonlinear Catch-Up Growth in Height, Weight, and Head Circumference from Birth to Adolescence: A Longitudinal Twin Study

Owing to high rates of prenatal complications, twins are, on average, substantially smaller than population norms on physical measurements including height, weight, and head circumference at birth. By early childhood, twins are physically average. This study is the first to explore the process of catch-up growth by fitting asymptotic growth models to age-standardized height, weight, and head circumference measurements in a community sample of twins (n = 1281, 52.3% female) followed at up to 17 time points from birth to 15 years. Catch-up growth was rapid over the first year and plateaued around the population mean by early childhood. Shared environmental factors accounted for the majority of individual differences in initial physical size (57.7-65.5%), whereas additive genetic factors accounted for the majority of individual differences in the upper asymptotes of height, weight, and head circumference (73.4-92.6%). Both additive genetic and shared environmental factors were associated with variance in how quickly twins caught up. Gestational age and family SES emerged as important environmental correlates of physical catch-up growth.

Maximizing the value of twin studies in health and behaviour

In the classical twin design, researchers compare trait resemblance in cohorts of identical and non-identical twins to understand how genetic and environmental factors correlate with resemblance in behaviour and other phenotypes. The twin design is also a valuable tool for studying causality, intergenerational transmission, and gene-environment correlation and interaction. Here we review recent developments in twin studies, recent results from twin studies of new phenotypes and recent insights into twinning. We ask whether the results of existing twin studies are representative of the general population and of global diversity, and we conclude that stronger efforts to increase representativeness are needed. We provide an updated overview of twin concordance and discordance for major diseases and mental disorders, which conveys a crucial message: genetic influences are not as deterministic as many believe. This has important implications for public understanding of genetic risk prediction tools, as the accuracy of genetic predictions can never exceed identical twin concordance rates.

Nonlinear Catch-Up Growth in Height, Weight, and Head Circumference from Birth to Adolescence: A Longitudinal Twin Study

Owing to high rates of prenatal complications, twins are, on average, substantially smaller than population norms on physical measurements including height, weight, and head circumference at birth. By early childhood, twins are physically average. This study is the first to explore the process of catch-up growth by fitting asymptotic growth models to age-standardized height, weight, and head circumference measurements in a community sample of twins ( n  = 1,281, 52.3% female) followed at up to 17 time points from birth to 15 years. Catch-up growth was rapid over the first year and plateaued around the population mean by early childhood. Shared environmental factors accounted for the majority of individual differences in initial physical size (57.7%-65.5%), whereas additive genetic factors accounted for the majority of individual differences in the upper asymptotes of height, weight, and head circumference (73.4%-92.6%). Both additive genetic and shared environmental factors were associated with variance in how quickly twins caught up. Gestational age and family SES emerged as important environmental correlates of physical catch-up growth.

Physical growth and intelligence development of discordant dizygotic twins from birth to preschool age: a prospective cohort study

Background

The majority of studies are limited to adverse perinatal outcomes and poor cognitive abilities in the short term in discordant monochorionic twins.

Methods

To determine whether small and large discordant dizygotic twins differ in physical growth and intelligence development and weight and height from birth up to 6 years of age were measured in 34 dizygotic twin pairs with ≥ 20% birth weight discordance. Mental developmental index (MDI) and psychomotor developmental index (PDI) were calculated at 1 year, while the Wechsler Intelligence Scale for Children-IV (WISC-IV) full-scale intelligence quotient (IQ) was assessed at the age of 6.

Results

The difference in height and weight in each stage differed significantly from birth to 72-months-old (P < 0.05), although there was disappointing catch-up growth in smaller twins. PDI but not MDI at 1 year of age was significantly different between the two groups (P < 0.05), and smaller twins experienced higher psychomotor retardation rates (P < 0.05). Also, the influence of height and weight on PDI was statistically significant (P < 0.05). No significant difference was detected in the WISC-IV full-scale IQ at the age of 6; however, the full-scale IQ may be affected by the history of suffocation and the S/D value (P = 0.011, P = 0.022).

Conclusions

Intrauterine fetal growth and development lead to birth weight differences in twins and sustain an impact on the children’s physical growth in height and weight from birth to preschool age, causing psychomotor developmental differences at 1 year of age. However, the differences in psychomotor development decrease gradually by the age of 6.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13052-022-01354-y.

Infant Motor Milestones and Childhood Overweight: Trends over Two Decades in A Large Twin Cohort

BACKGROUND

Poor motor skill competence may influence energy balance with childhood overweight as a result. Our aim was to investigate whether the age of motor milestone achievement has changed over the past decades and whether this change may contribute to the increasing trend observed in childhood overweight.

METHODS

Motor skill competence was assessed in children from the Young Netherlands Twin Register born between 1987 and 2007. Follow-up ranged from 4 up to 10 years. Weight and height were assessed at birth, 6 months, 14 months, and 2, 4, 7, and 10 years.

RESULTS

Babies born in later cohorts achieved their motor milestones 'crawling', 'standing', and 'walking unassisted' later compared to babies born in earlier cohorts (N = 18,514, p < 0.001). The prevalence of overweight at age 10 was higher in later cohorts (p = 0.033). The increase in overweight at age 10 was not explained by achieving motor milestones at a later age and this persisted after adjusting for gestational age, sex, and socioeconomic status.

CONCLUSION

Comparing children born in 1987 to those born in 2007, we conclude that children nowadays achieve their motor milestones at a later age. This does not however, explain the increasing trend in childhood overweight.

Testing Bidirectional Associations Between Childhood Aggression and BMI: Results from Three Cohorts

OBJECTIVE

This study examined the prospective, potentially bidirectional association of aggressive behavior with BMI and body composition across childhood in three population-based cohorts.

METHODS

Repeated measures of aggression and BMI were available from the Generation R Study between ages 6 and 10 years (N = 3,974), the Netherlands Twin Register (NTR) between ages 7 and 10 years (N = 10,328), and the Swedish Twin Study of Child and Adolescent Development (TCHAD) between ages 9 and 14 years (N = 1,462). In all samples, aggression was assessed with the Child Behavior Checklist. Fat mass and fat-free mass were available in the Generation R Study. Associations were examined with cross-lagged modeling.

RESULTS

Aggressive behavior at baseline was associated with higher BMI at follow-up in the Generation R Study (β = 0.02, 95% CI: 0.00 to 0.04), in NTR (β = 0.04, 95% CI: 0.02 to 0.06), and in TCHAD (β = 0.03, 95% CI: -0.02 to 0.07). Aggressive behavior was prospectively associated with higher fat mass (β = 0.03, 95% CI: 0.01 to 0.05) but not fat-free mass. There was no evidence that BMI or body composition preceded aggressive behavior.

CONCLUSIONS

More aggressive behavior was prospectively associated with higher BMI and fat mass. This suggests that aggression contributes to the obesity problem, and future research should study whether these behavioral pathways to childhood obesity are modifiable.

Prevalence of dieting and fear of weight gain across ages: a community sample from adolescents to the elderly

OBJECTIVES

The current study aimed to define the prevalence of dieting and fear of weight gain among men and women across the entire lifespan and identify factors associated with them.

METHODS

Data were available for 31,636 participants (60.2% women; age 13-98 years) from the Netherlands Twin Register. Dieting and fear of weight gain were described by age and sex. Associations with BMI, exercise behavior, urbanization and educational attainment were examined by regression analyses in 19,294 participants.

RESULTS

Dieting was most frequently reported by 35- to 65-year-old women (56.6-63%), and 45- to 65-year-old men (31.7-31.9%). Fear of weight gain was most prevalent in women between 16 and 25 (73.2-74.3%), and in 25- to 55-year-old men (43.2-46.1%). In addition to sex and BMI, dieting and fear of weight gain were associated with each other. Furthermore, fear was associated with the age × sex interaction and educational attainment.

CONCLUSIONS

Dieting and fear of weight gain is common during the entire lifespan for women, but is also endorsed by a substantial number of men. Given the low rate of overweight in young women, the high levels of fear of weight gain are striking.

The genetic architecture of body mass index from infancy to adulthood modified by parental education

OBJECTIVE

A higher prevalence of obesity in lower socioeconomic classes is common in Western societies. This study examined the role of gene-environment interactions in the association between parental education and body mass index (BMI) from infancy to the onset of adulthood.

METHODS

Parentally reported BMI from 1 to 13 and self-reported BMI from 14 to 20 years of age were collected in 16,646 complete Dutch twin pairs and analyzed by genetic twin modeling.

RESULTS

At 7 to 8 years of age, children whose parents had middle or low educational levels had more excess weight than the children of more highly educated parents, and the difference increased until 18 to 20 years of age. The major part of the BMI variation was explained by additive genetic factors (a(2)  = 0.55-0.85), but environmental factors common for co-twins also played a significant role, especially from 3 to 7-8 years of age (c(2)  = 0.15-0.29). The genetic variation in BMI was higher in children whose parents had middle or low educational levels compared with children whose parents had a high educational level.

CONCLUSIONS

The interaction between genetic factors and the childhood social environment may contribute to the formation of socioeconomic differences in obesity.

Arithmetic, reading and writing performance has a strong genetic component: A study in primary school children

Even children attending the same primary school and taught by the same teacher differ greatly in their performance. In the Netherlands, performance at the end of primary school determines the enrollment in a particular level of secondary education. Identifying the impact of genes and the environment on individual differences in educational achievement between children is important. The Netherlands Twin Register has collected data on scores of tests used in primary school (ages 6 to 12) to monitor a child's educational progress in four domains, i.e. arithmetic, word reading, reading comprehension and spelling (1058 MZ and 1734 DZ twin pairs), and of a final test (2451 MZ and 4569 DZ twin pairs) in a large Dutch cohort. In general, individual differences in educational achievement were to a large extent due to genes and the influence of the family environment was negligible. Moreover, there is no evidence for gender differences in the underlying etiology.

Zygosity Differences in Height and Body Mass Index of Twins From Infancy to Old Age: A Study of the CODATwins Project

A trend toward greater body size in dizygotic (DZ) than in monozygotic (MZ) twins has been suggested by some but not all studies, and this difference may also vary by age. We analyzed zygosity differences in mean values and variances of height and body mass index (BMI) among male and female twins from infancy to old age. Data were derived from an international database of 54 twin cohorts participating in the COllaborative project of Development of Anthropometrical measures in Twins (CODATwins), and included 842,951 height and BMI measurements from twins aged 1 to 102 years. The results showed that DZ twins were consistently taller than MZ twins, with differences of up to 2.0 cm in childhood and adolescence and up to 0.9 cm in adulthood. Similarly, a greater mean BMI of up to 0.3 kg/m2 in childhood and adolescence and up to 0.2 kg/m2 in adulthood was observed in DZ twins, although the pattern was less consistent. DZ twins presented up to 1.7% greater height and 1.9% greater BMI than MZ twins; these percentage differences were largest in middle and late childhood and decreased with age in both sexes. The variance of height was similar in MZ and DZ twins at most ages. In contrast, the variance of BMI was significantly higher in DZ than in MZ twins, particularly in childhood. In conclusion, DZ twins were generally taller and had greater BMI than MZ twins, but the differences decreased with age in both sexes.

Neurodevelopmental problems and extremes in BMI

Background. Over the last few decades, an increasing number of studies have suggested a connection between neurodevelopmental problems (NDPs) and body mass index (BMI). Attention deficit/hyperactivity disorder (ADHD) and autism spectrum disorders (ASD) both seem to carry an increased risk for developing extreme BMI. However, the results are inconsistent, and there have been only a few studies of the general population of children. Aims. We had three aims with the present study: (1) to define the prevalence of extreme (low or high) BMI in the group of children with ADHD and/or ASDs compared to the group of children without these NDPs; (2) to analyze whether extreme BMI is associated with the subdomains within the diagnostic categories of ADHD or ASD; and (3) to investigate the contribution of genetic and environmental factors to BMI in boys and girls at ages 9 and 12. Method. Parents of 9- or 12-year-old twins (n = 12,496) were interviewed using the Autism-Tics, ADHD and other Comorbidities (A-TAC) inventory as part of the Child and Adolescent Twin Study in Sweden (CATSS). Univariate and multivariate generalized estimated equation models were used to analyze associations between extremes in BMI and NDPs. Results. ADHD screen-positive cases followed BMI distributions similar to those of children without ADHD or ASD. Significant association was found between ADHD and BMI only among 12-year-old girls, where the inattention subdomain of ADHD was significantly associated with the high extreme BMI. ASD scores were associated with both the low and the high extremes of BMI. Compared to children without ADHD or ASD, the prevalence of ASD screen-positive cases was three times greater in the high extreme BMI group and double as much in the low extreme BMI group. Stereotyped and repetitive behaviors were significantly associated with high extreme BMIs. Conclusion. Children with ASD, with or without coexisting ADHD, are more prone to have low or high extreme BMIs than children without ADHD or ASD.

Growth parameters of discordant preterm twins during the first year of life

OBJECTIVE

To evaluate and compare growth parameters (weight, length and head circumference) of discordant preterm twins during the first year of life.

METHODS

Retrospective data were collected on 78 pairs of >10% discordant preterm twins. Data regarding short-term neonatal outcome were recorded. Growth parameters were recorded at birth and again at 1 year of age, as collected by phone interview.

RESULTS

At 1 year of age, the gap was significantly reduced between the group of smaller twins and the group of larger twins on all the parameters studied. Discordance in weight decreased from a mean of 22% at birth (calculated from the weight of the larger twin) to 8.9%, in length from 6.5% to 2.5% and in head circumference from 5.5% to 1.3% at 1 year of age. While length and head circumference (HC) z-scores improved in both the large and the small siblings, weight z-score decreased significantly among the large twins and decreased mildly among the small twins.

CONCLUSIONS

The gaps in growth parameters between the smaller twins of preterm discordant twins and their larger siblings are significantly reduced during the first year of life. The impact of the differential growth patterns between the two siblings should be further evaluated.

Development of body mass index of Japanese triplets from birth until the onset of puberty

We aimed to analyze the characteristics of development of relative weight in Japanese triplets from birth until 12 years of age. Data were collected through a mailed questionnaire sent to mothers of triplets asking for information recorded in medical records. Altogether we had information on 1,061 triplet children of 354 mothers born between 1978 and 2006. For these births, data on triplets' height and weight growth, gestational age, sex, parity, and maternal age at delivery were obtained from records in the maternal and child health handbooks and records from the schools where children receive health check-ups. In addition, information on maternal height and weight was obtained. Triplets have a lower ponderal index at birth and lower body mass index (BMI) compared with the general population until 12 years of age, except for the period during 1 and 3 years of age. Moreover, birth weight had the strongest contribution to BMI of triplets until 6 years of age. After 9 years of age, maternal BMI was a significant factor affecting BMI of triplets.

Adult body height of twins compared with that of singletons: a register-based birth cohort study of Norwegian males

In the present study, we evaluated whether childhood differences in body height between singletons and twins persist into adulthood. Data from the Medical Birth Register of Norway were linked with data from the Norwegian National Conscript Service. This study used data on the 457,999 males who were born alive and without physical anomalies in single or twin births in Norway during 1967-1984 and who were examined at the mandatory military conscription (age 18-20 years; 1985-2003). For sibling comparisons, the authors selected the 1,721 sibships of full brothers that included at least 1 male born in a single birth and at least 1 male born in a twin birth (4,520 persons, including 2,493 twins and 2,027 singletons). An analysis of the total study population using generalized estimating equations showed that the twins were 0.6 cm (95% confidence interval: 0.4, 0.7) shorter than were the singletons after adjustment for a series of background factors. The fixed-effects regression analysis of the sibships that included both twins and singletons showed that the twins were 0.9 cm (95% confidence interval: 0.6, 1.2) shorter than were their singleton brothers. The study suggests that male twins born in Norway during 1967-1984 were slightly shorter in early adulthood than were singletons.

The Young Netherlands Twin Register (YNTR): longitudinal twin and family studies in over 70,000 children

The Netherlands Twin Register (NTR) began in 1987 with data collection in twins and their families, including families with newborn twins and triplets. Twenty-five years later, the NTR has collected at least one survey for 70,784 children, born after 1985. For the majority of twins, longitudinal data collection has been done by age-specific surveys. Shortly after giving birth, mothers receive a first survey with items on pregnancy and birth. At age 2, a survey on growth and achievement of milestones is sent. At ages 3, 7, 9/10, and 12 parents and teachers receive a series of surveys that are targeted at the development of emotional and behavior problems. From age 14 years onward, adolescent twins and their siblings report on their behavior problems, health, and lifestyle. When the twins are 18 years and older, parents are also invited to take part in survey studies. In sub-groups of different ages, in-depth phenotyping was done for IQ, electroencephalography , MRI, growth, hormones, neuropsychological assessments, and cardiovascular measures. DNA and biological samples have also been collected and large numbers of twin pairs and parents have been genotyped for zygosity by either micro-satellites or sets of short nucleotide polymorphisms and repeat polymorphisms in candidate genes. Subject recruitment and data collection is still ongoing and the longitudinal database is growing. Data collection by record linkage in the Netherlands is beginning and we expect these combined longitudinal data to provide increased insights into the genetic etiology of development of mental and physical health in children and adolescents.

Twins, tissue, and time: an assessment of SNPs and CNVs

With the desire to assess genetic variation across the lifespan in large-scale collaborative projects, one question is whether inference of copy number (CN) is sensitive to the source of material for deoxyribonucleic acid (DNA) analysis (e.g., blood and buccal) and another question is whether CN is stable as individual sage. Here, we address these questions by applying Affymetrix 6.0 single nucleotide polymorphism (SNP)micro-arrays to 1,472 DNA samples from 710 individuals from the Netherlands Twin Register, including twin and non-twin individuals (372 with buccal and blood derived DNA and 388 with longitudinal data).Similar concordance for CN and genotype inference between samples from the same individual [or from the monozygotic (MZ) co-twins] was found for blood and buccal tissues. There was a small but statistically significant decrease in across-tissue concordance compared with concordance of samples from the same tissue type. No temporal effect was seen on CN variation from the 388 individuals sampled at two time points ranging from 1 to 12 years apart. The majority of our individuals were sampled at age younger than 20 years. Genotype concordance was very high (~ > 99%) between co-twins from 43 MZ pairs. For75 dizygotic (DZ) pairs, ~was ~65%. CN estimates were highly consistent between co-twins from MZ pairs for both deletions (f?2 ~ 90%) and duplications (~ ~ 86%). For DZ, these were similar for within-individual comparisons, but naturally lower between co-twins (~ ~ 50-60%). These results suggest that DNA from buccal samples perform as well as DNA from blood samples on the current generation of micro-array technologies.

Brain SCALE: brain structure and cognition: an adolescent longitudinal twin study into the genetic etiology of individual differences

From childhood into adolescence, the child's brain undergoes considerable changes in both structure and function. Twin studies are of great value to explore to what extent genetic and environmental factors explain individual differences in brain development and cognition. In The Netherlands, we initiated a longitudinal study in which twins, their siblings and their parents are assessed at three year intervals. The participants were recruited from The Netherlands Twin Register (NTR) and at baseline consisted of 112 families, with 9-year-old twins and an older sibling. Three years later, 89 families returned for follow-up assessment. Data collection included psychometric IQ tests, a comprehensive neuropsychological testing protocol, and parental and self-ratings of behavioral and emotional problems. Physical maturation was measured through assessment of Tanner stages. Hormonal levels (cortisol, luteinizing hormone, follicle-stimulating hormone, testosterone, and estrogens) were assessed in urine and saliva. Brain scans were acquired using 1.5 Tesla Magnetic Resonance Imaging (MRI), which provided volumetric measures and measures of cortical thickness. Buccal swabs were collected for DNA isolation for future candidate gene and genome-wide analysis studies. This article gives an overview of the study and the main findings. Participants will return for a third assessment when the twins are around 16 years old. Longitudinal twin-sibling studies that map brain development and cognitive function at well-defined ages aid in the understanding of genetic influences on normative brain development.

Comparison of naturally conceived and IVF-DZ twins in the Netherlands Twin Registry: a developmental study

In a large set of twin pairs, we compared twins born after IVF to naturally conceived twins with respect to birth characteristics, growth, attainment of motor milestones, and emotional and behavioral problems. Twin families were registered with the Netherlands Twin Register. We included 1534 dizygotic (DZ) twins born after IVF, 5315 naturally conceived (NC) DZ twins, and 1504 control NC DZ twins who were matched to the IVF twins based on maternal age, maternal educational level, smoking during pregnancy, gestational age, and offspring sex. Data were obtained by longitudinal surveys sent to fathers, mothers, and teachers at ages 1, 2, 3, 7, 10, and 12 years. Results showed no differences in growth, in attainment of motor milestones, and in behavioral development between IVF and matched NC twins. It can be concluded that for nearly all aspects, development in IVF and NC children is similar.

Ophthalmic phenotypes and the representativeness of twin data for the general population

PURPOSE

To compare the distributional parameters for a series of ocular biometric traits between twins and their singleton siblings, to evaluate the generalizability of twin data, as used in heritability analyses to the general population.

METHODS

A series of birth, anthropometric, and 13 ocular biometric traits were selected for analysis: interpupillary distance (IPD), visual acuity (logMAR), spherical equivalent refractive error, corneal curvature, axial length, anterior chamber depth (ACD), central corneal thickness (CCT), intraocular pressure (IOP), optic disc, cup and rim areas, and measures of retinal vessel caliber; central retinal arteriolar equivalent (CRAE), and central retinal venular equivalent (CRVE). Structural equation modeling was used to test the assumption that the means and variances for each trait did not differ between twins and their siblings.

RESULTS

Significant differences in log-likelihood for birth weight and gestational age were observed between twins and siblings, with the latter being both heavier and closer to full-term at birth. Siblings were also found to have larger IPD and axial length, and better visual acuity compared with their twin counterparts. Refractive error, corneal curvature, ACD, CCT, optic disc parameters, and retinal vascular calibers did not differ significantly between the two groups.

CONCLUSIONS

Twins are representative of the general population for some but not all measures of ocular biometry. Consequently, care should be taken when extrapolating twin data for these traits in heritability and other genetic studies. Birth weight differences between twins and siblings do not appear to account for the differences in ocular biometry observed in this study.

The Netherlands Twin Register biobank: a resource for genetic epidemiological studies

In 2004 the Netherlands Twin Register (NTR) started a large scale biological sample collection in twin families to create a resource for genetic studies on health, lifestyle and personality. Between January 2004 and July 2008, adult participants from NTR research projects were invited into the study. During a home visit between 7:00 and 10:00 am, fasting blood and morning urine samples were collected. Fertile women were bled on day 2-4 of the menstrual cycle, or in their pill-free week. Biological samples were collected for DNA isolation, gene expression studies, creation of cell lines and for biomarker assessment. At the time of blood sampling, additional phenotypic information concerning health, medication use, body composition and smoking was collected. Of the participants contacted, 69% participated. Blood and urine samples were collected in 9,530 participants (63% female, average age 44.4 (SD 15.5) years) from 3,477 families. Lipid profile, glucose, insulin, HbA1c, haematology, CRP, fibrinogen, liver enzymes and creatinine have been assessed. Longitudinal survey data on health, personality and lifestyle are currently available for 90% of all participants. Genome-wide SNP data are available for 3,524 participants, with additional genotyping ongoing. The NTR biobank, combined with the extensive phenotypic information available within the NTR, provides a valuable resource for the study of genetic determinants of individual differences in mental and physical health. It offers opportunities for DNA-based and gene expression studies as well as for future metabolomic and proteomic projects.