Genetic and environmental influences on the tracking of body size from birth to early adulthood

OBJECTIVE: This study identified genetic and environmental influences on the tracking of body size from birth to 16 to 18.5 years of age. RESEARCH METHODS AND PROCEDURES: Longitudinal information was collected from a nationally representative sample of Finnish twin adolescents (birth cohorts 1975 to 1979) and their parents through questionnaires mailed when the twins were ages 16 and 18.5 years old. The sample included 702 monozygotic, 724 same-sex dizygotic, and 762 opposite-sex dizygotic sets of twins. The measures used were length, weight, ponderal index (kilograms per cubic meters), and gestational age at birth, and height, weight, and body mass index (kilograms per square meters) at 16 to 18.5 years of age. The changes in genetic and environmental influences on body size from birth to early adulthood were analyzed by quantitative genetic modeling. RESULTS: The twins who had a higher weight or ponderal index at birth were taller and heavier in early adulthood, whereas those who were longer at birth were taller, but not heavier, later in life. Adult height was affected more by the birth size than body mass index. In the genetic modeling analyses, the genetic factors accounting for the variation of body size became more apparent with age, and both genetic and environmental influences on stature had a sizable carry-over effect from birth to late adolescence, whereas for relative weight, the influences were more age-specific. DISCUSSION: The genetic and environmental architecture of body size changes from birth to adulthood. Even in monozygotic twins who share their genetic background, the initially larger twin tended to remain larger, demonstrating the long-lasting effects of fetal environment on final body size.     

Plasma cholesterol variation in the National Heart, Lung and Blood Institute Twin Study

Plasma cholesterol was measured in the fifth decade of the life of 249 pairs of monozygotic (MZ) and 262 pairs of dizygotic (DZ) World War II veteran twins and 70% of the same cohort 10 years later. There were no significant differences between the mean cholesterol values for MZ and DZ twins, and the within DZ pair mean squares were significantly larger than the within MZ pair mean squares for all of the cholesterol variables measured. However, the DZ twins were found to have greater total variance, positive skewness, and leptokurtosis than the MZ twins for total and high-density lipoprotein cholesterol, and the total/high-density ratio. Comparisons with published data revealed that the variance of DZ twins was similar to that of singletons while the MZ twins have smaller total variance, perhaps owing to a missing component of variation. Hypotheses for the source of the differences in the zygosity distributions are proposed including environmental influences (pre- or post-natal and within- or among-families), genetic differences, and selection at the time of induction into the armed services. Because of the differences in total variance of the two zygosities it is difficult to know which estimates of genetic variance or heritability have the least bias. However, these data provide clues that may lead to further understanding of sources of plasma cholesterol variation that could be important to the future understanding of risk for coronary heart disease.     

Genetic and environmental influences on variation in balance performance among female twin pairs aged 21-82 years

Genetic and environmental influences on variation in balance performance were measured in 93 monozygous and 83 dizygous female twin pairs aged 21-82 years (mean age, 50.5 years) in Melbourne, Australia, between 1999 and 2003. The authors administered clinical (Lord's Balance Test and Step Test) and laboratory tests of static and dynamic balance from the Chattecx Balance System with and without distractor tasks. The authors conducted factor analysis and estimated genetic and environmental variance components and heritability (defined as additive genetic variance as a proportion of all variance, after adjustment for age) using a multivariate normal model with the statistical package FISHER. Three factors were identified and adjusted for age. Heritability was 46% (standard error (SE), 9) for the "sensory balance tests" factor and 30% (SE, 9) for the "static and dynamic perturbations" factor. For both factors, the remaining variance was attributed to unique environmental effects. There was no evidence that genetic factors influenced variation in the "dynamic weight shift tests" factor, with environmental effects shared by twins accounting for 38% (SE, 7) of variance. Neither genetic nor environmental proportions of variance differed significantly between twin subgroups by age (50 years). An age-related decline in performance measures was found across the whole sample. These results imply that balance impairments may have a heritable element.     

童年中期双生子下肢长相关指标的遗传学分析

目的分析遗传与环境因素对童年中期双生子儿童下肢长相关指标的影响,并分析其中年龄和性别的作用。方法对呼和浩特市和包头市5~8岁171对双生子儿童下肢长相关指标进行测量,使用Mx软件拟合最佳模型,计算校正年龄前后各指标遗传与环境方差组分。结果各指标最佳拟合模型均为ACES,校正年龄后遗传度分别为:身高:男43%,女60%);坐高:男62%,女62%;下肢长:男50%,女62%;下肢长指数:男18%,女36%。身高、下肢长和下肢长指数的遗传度存在一定性别差异。结论童年中期下肢长相关指标发育受环境因素的影响较大,应通过加强营养与锻炼及控制相关疾病等改善下肢长度发育,促使儿童生长发育潜力得到更好的发挥。     

Predictors of late-onset smoking and cessation over 10 years.

PURPOSE: To identify predictors of smoking onset and cessation between early (age 13 years) and late adolescence (age 18 years) and between late adolescence (age 18 years) and young adulthood (age 23 years). METHODS: We employed logistic regression to predict smoking initiation and cessation for an ethnically diverse sample of 3056 adolescents recruited from 30 West Coast schools in 1985 and observed from age 13 to age 23 years. Fifty-six percent of the sample was female. Predictors tapping sociodemographic characteristics, environmental influences, attitudes and beliefs about smoking, bonds with school, and problem behavior were measured at age 13 years (older teenager models) and at age 18 years (young adult models). RESULTS: Robust predictors of both initiation and cessation across the two developmental periods included doing poorly in middle/high school and prior smoking behavior. Predictors common to three of the four models included being young for one's grade cohort and intending to smoke in the next 6 months. Early deviant behavior and drinking fostered initiation among older teenagers, but problem behavior as an older teenager did not predict young adult initiation. Smokers who had few or no high school friends who smoked and felt able to resist prosmoking pressures at age 18 years were more likely to quit by age 23 years. Being female predicted initiation by age 18 years; being African-American, Hispanic, or Asian inhibited it. CONCLUSIONS: The strong association of prior smoking behavior and intentions with later smoking status among both adolescents and young adults underscores the importance of starting smoking prevention early and continuing it through high school. Such programs might also consider the greater vulnerability of females, youth who are young for their grade cohort, and those who are doing poorly in school.     

中国6省市中小学生近视流行现状及其影响因素分析

目的 分析6省市部分中小学生近视流行现状及其可能的影响因素。方法 采用多阶段整群抽样方法按全国6个行政区划分别抽取1个省或直辖市,每个城市抽取中学(初中一、二年级和高中一、二年级)和小学(一至六年级)各10所。共收回问卷60 334份,有效问卷57 904份,有效率为96.0%。按《全国学生体质与健康调研检测细则》要求检查视力及开展问卷调查,采用一般统计描述、Pearson χ²检验、二分类logistic多因素方法分析中小学生近视流行现状及影响因素。结果 6省市调查的中小学生近视患病率为55.7%,其中女生(59.7%)高于男生(51.9%),差异有统计学意义(P <0.01);6～8岁组、10～12岁组、13～15岁组和16～18岁组近视患病率分别为35.8%、58.9%、73.4%和81.2%,呈随年龄增长患病率有明显上升趋势,差异有统计学意义(P <0.001)。父母近视、计算机显示屏与眼的距离、读写时胸部距桌边沿不足一拳(10 cm)、眼距离书本不足一尺(约30 cm)、写字时手指距离笔尖一寸(3 cm)、每天睡眠时间、最近1周平均每天户外活动时间、下午学校安排体育活动的时间、家中电视机的大小、是否限制看电视/计算机的时间为近视的影响因素。结论 调查地区中小学生近视患病率仍处于较高水平并受遗传因素、户外活动时间、用眼行为等因素的综合影响。     

Heritability of body mass index in pre-adolescence, young adulthood and late adulthood

Increased body mass index (BMI) is a worldwide health issue. Individual differences in the susceptibility to increased BMI could be related to genes or environment. We performed a systematic review of genetic studies on BMI in pre-adolescence, young adulthood and late adulthood. We searched PubMed and EMBASE with heritability, body mass index, BMI, weight, height, anthropometry and twins as search terms. Studies reporting intra-pair correlations of healthy twin pairs that were raised together were included. This resulted in the inclusion of 8,179 monozygotic (MZ) and 9,977 dizygotic (DZ) twin pairs from twelve published studies in addition to individual participant data for 629 MZ and 594 DZ pairs from four twin registries. Structural equation modelling with intra-pair twin correlations showed that the heritability of BMI remained high over all age categories ranging from 61% (95% CI 54-64%) to 80% (95% CI 76-81%) for male and female subjects combined, while unique environmental influences increased from 14% (95% CI 13-15%) to 40% (95% CI 37-43%) with increasing age. Heritability of BMI remains consistently high over different age categories. Environmental changes over time do not seem to have as big a relative impact on an individual's weight as previously reported, suggesting a mainly genetic influence on variation in BMI over the years.     

Nongenetic influences of obesity on other cardiovascular disease risk factors: an analysis of identical twins.

The importance of genetic influences on obesity has been emphasized recently. We conducted matched co-twin analyses of 250 pairs of White, male, monozygotic twins from the National Heart, Lung, and Blood Institute (NHLBI) Twin Study. Entirely in the absence of genetic influences, obesity was significantly associated with systolic and diastolic blood pressures; one-hour, post-load glucose; total, LDL-, and HDL-cholesterol; and triglycerides among these 42-55 year old men. Similar results were obtained in longitudinal analyses of weight change during adulthood (from mean age of 20 to mean age of 48 years) and risk factor status at middle-age. These results indicate that behaviors and environmental exposures that occur later in life are responsible, at least in part, for the associations between adult obesity and cardiovascular disease risk, supporting the appropriateness of weight reduction efforts during adulthood.     

Heritability of the risk factors characteristic for the metabolic syndrome: a twin study

Both genetic and environmental factors play role in the pathogenesis of the metabolic syndrome. The magnitude of genetic and environmental influences on the components of metabolic syndrome may vary in different populations. Aims: The present study was aimed to determine the effects of genetic and environmental factors on risk factors characteristic for the metabolic syndrome. Methods: A total of 101 (63 monozygotic and 38 dizygotic) adult twin pairs (n=202; mean age: 43.3±15.8 years) were investigated. Medical history was recorded and physical examination was carried out for each subject. Fasting venous blood samples were used for measuring laboratory parameters. The presented estimates include the heritability structural equation (A-C-E) model results. In Model-1, all presented parameters are age- and gender- corrected. In Model-2, parameters were corrected for age, gender, body mass index and waist circumference. Results: Heritability in waist circumference (as well as in other anthropometric parameters such as weight and height) was high (Model-1: 71.0-88.1%). Similarly, genetic factors had the highest proportion of total phenotypic variance in systolic and diastolic blood pressure (Model-2: 57.1% and 57.7%, respectively). Based on the results of Model-2, unique environmental factors dominate alterations in serum triglycerides values (55.9%) while shared environmental factors proved to be substantial in alterations of HDL-cholesterol and fasting blood glucose values (58.1% and 57.1%, respectively). Comparing the results of Model-1 and Model-2, the difference in A-C-E model varied from 0.0% to 17.1%, indicating that only a minor proportion of genetic and environmental influences can be explained by the effects of anthropometric parameters. Conclusions: Among adult Hungarian healthy people, genetic factors have substantial influence on waist circumference and blood pressure values while environmental factors dominate alterations in serum triglycerides, HDL-cholesterol and fasting blood glucose values. The different heritability of individual risk factors challenges the original unifying concept of the metabolic syndrome. The results may be useful for establishing and implementing primary cardiovascular prevention both at individual and population levels.     

Genetic and environmental influences on eating patterns of twins aged >/=50 y.

BACKGROUND: Clinicians and researchers could benefit from a greater understanding of the role of genetic and environmental factors in human eating behavior. OBJECTIVE: Our aim was to estimate the relative influence of genetic and environmental factors on habitual eating patterns in middle-aged and elderly men and women. DESIGN: Male and female twins (n = 4640) aged >/=50 y completed a mailed version of the National Cancer Institute food-frequency questionnaire. Factor analysis was performed to identify eating patterns among respondents. Estimates of genetic, common environmental (shared by family members), and specific environmental (unique to an individual) influences were obtained for food use, serving size, and consumption frequency by comparing monozygotic and dizygotic twin-pair groups with structural equation analysis. RESULTS: Two independent eating patterns were identified: the first consisted of items high in fat, salt, and sugar, and the second reflected healthful eating habits. Although the influence of environmental factors was larger, between 15% and 38% of the total variation in pattern 1 and between 33% and 40% in pattern 2 were explained by genetic influences. Models accounting for sex differences in genetic and environmental estimates fit the data significantly better for food use and serving size of foods in eating pattern 1 and for food use in eating pattern 2. CONCLUSION: Although 60-85% of the variability in eating patterns was associated with environmental factors, genetic influences were also apparent and there was some evidence of sex specificity. These findings may be important in crafting dietary interventions and predicting adherence to these interventions.     

襄樊市低年级小学生单纯性肥胖的遗传和环境相关因素分析

目的:了解襄樊市低年级小学生单纯性肥胖的流行情况,探讨影响儿童肥胖的遗传和环境相关因素,为制定地方性儿童肥胖干预策略提供科学依据。方法:采用分层整群随机抽样的方法,收集襄樊城区及所辖市县1、2、3年级小学生身高、体重资料;自编问卷调查肥胖相关影响因素,并进行非条件Logistic回归分析。结果:2532名7～9岁儿童肥胖检出率为8.65%,其中男童为11.10%,女童为5.96%。男童以9岁组肥胖率最高,总体上随年龄增长而增加;女童肥胖率以7岁组最高,总体上随年龄增长而降低。共有17项因素进入Logistic多因素回归方程,包括4项遗传因素和13项环境因素,其中关联强度较大的因素为监护人持有胖就是健康的观念、食欲好、父母双方均肥胖(OR值分别为13.622、9.314、3.412);高出生体重、父母肥胖、不偏食、进食速度快、运动时间短、暴饮暴食、喜食油炸食品和肉类食物、不吃早餐、服从家长对食物的提示、母亲文化程度、广告传媒影响等也与儿童肥胖相关(OR值均1)。结论:儿童肥胖与遗传因素和环境因素关系密切,应在小学低年级阶段采取健康教育、饮食调整、运动锻炼等综合措施进行群体干预,预防单纯性肥胖。     

Familial aggregation in the presence of temporal trends

Models for assessing temporal trends in familial aggregation are described for both cross-sectional and longitudinal family data. Simultaneous linear structural equations on latent variables are used to model the dependence among family members. The coefficients of the equations are assumed to be parametric functions of time, so that quite complex temporal trends in familial aggregations can be accommodated. Variable family sizes and missing data values pose no problem as the parameters of the models are estimated via maximum likelihood techniques. One of the models is applied to systolic blood pressure data in 542 Japanese-American nuclear families. The results indicate limited evidence for temporal variation in the genetic expression, but that there is substantial temporal variation in environmental influences, which appear to peak at middle age.     

A new model for the resolution of cultural and biological inheritance in the presence of temporal trends: application to systolic blood pressure

A contemporary path model for the resolution of cultural and biological inheritance is extended to incorporate temporal variation in family resemblance. Specifically, the genetic and environmental effects, like all other parameters of the model, are allowed to vary over an individual's age according to some specific mathematical functions. In the computer program BETREND a library of such functions is incorporated. Data on systolic blood pressure in 542 Japanese-American nuclear families were analyzed. This new methodology detected significant temporal variation in cultural inheritance, a result that was not found previously by using static models which could only distinguish between adult and childhood heritabilities. Cultural heritability, estimated to be 10% at birth, increases to a maximum of 28% at age 36, at which time it declines until it eventually reaches 10% at age 49. Although not statistically significant, there was some evidence for temporal trends in genetic heritability as well. Ignoring trends, the genetic heritability was estimated as .30. On the other hand, trends in sibling environment were clearly nonsignificant. This model provides an objective method of testing the significance of temporal trends in familial resemblance by using multifactorial models, of resolving varying gene expression and transient environmental effects as possible sources of generating the observed temporal variation, and of estimating continuous changes in heritability with age.     

A Discordant Monozygotic Twin Approach to Testing Environmental Influences on Sexual Dysfunction in Women

The present study explored the causal role played by putative environmental factors on variation in female sexual dysfunction (FSD) by investigating FSD discordant monozygotic (MZ) twins, which permits a control over genetic confounders. In a population-based sample of female twins aged 25–69 years (M = 55 years), MZ twins discordant for recent and lifelong FSD were selected. Sample sizes varied depending on the specific sexual problem (N = 33–90 pairs). The Female Sexual Function Index (FSFI) score was used to discriminate cases from controls. Once genetic factors were controlled for, relationship satisfaction emerged as the strongest independent predictor for recent and lifelong FSD, being associated with FSFI dimensions measuring desire, arousal, and lubrication problems. The association with orgasm problems was especially strong (OR 7.1, 95 % CI: 1.9–25.3) as was the association with sexual dissatisfaction (OR 5.1, 95 % CI: 2.1–12.1). Furthermore, obsessive–compulsive symptomatology was weakly associated with desire problems (OR 1.5, 95 % CI: 1.4–1.8) and anxiety-sensitivity with orgasm problems (OR 1.1, 95 % CI: 0.9–1.3). Negligible effects were found for personality factors and small effects for self-reported abusive experiences. These data indicate, for the first time, that in women at identical genetic risk, relationship factors play a key role in the development of sexual problems. These findings require replication in prospective designs which can provide additional powerful tests of the direction of causality between interpersonal factors and later sexual dysfunction.     

AIDS-free time and survival of an injecting drug users HIV seroconvertors cohort

OBJECTIVE: To analyze AIDS free time, survival and the pre-AIDS survival for a injecting drug users cohort (IDU) of HIV seroconvertors. SUBJECT AND METHODS: Interval for seroconversion was available for 276 IDUs from Centers for AIDS Information and Prevention (CIPS) recruited between 1987 and until June of 1996. AIDS diagnosis and vital status dates were obtained by follow up at hospitals, mortality and AIDS registries, and CIPS visits. The end of follow up was December of 1996. Seroconversion date was estimated as the middle point between last HIV- and first HIV+. Kaplan-Meier extension and Cox regression for truncated data were fitted to estimate AIDS-free and survival times and to observe differences by sex, age consumption time and year of seroconversion. Weibull, and Log-normal parametric models were fitted to estimate median and percentiles of AIDS-free and survival times distribution. RESULTS: 34 cases have been identified as AIDS, 24 as deaths, 9 of them being before AIDS. 63.5% of the individuals were AIDS-free 7 years after seroconversion, and the probability of death was 25.50. Pre-AIDS mortality is around 8.7%. There were not significant differences by sex, age, consumption time and year of seroconversion. Log-normal model fitted better estimating an AIDS-free median time of 10.93 years and 13.67 survival years. CONCLUSION: The incubation period of HIV infection in a cohort of seroconvertors in our environment was around 11 years, not different from that observed out of the Mediterranean area as Holland, Scotland or United States     

Genetic and environmental influences on body-fat measures among African-American twins

OBJECTIVE: To investigate the genetic and environmental influences on body-fat measures including waist circumference (WC), waist-to-hip ratio (WHR), and body mass index (BMI) among African-American men and women. RESEARCH METHODS AND PROCEDURES: Measurements were taken as part of the Carolina African American Twin Study of Aging. This sample currently comprises 146 same-sex African-American twins with an average age of 50 years (range, 22 to 88 years). This analysis included 26 monozygotic and 29 dizygotic men and 45 monozygotic and 46 dizygotic women. Maximum likelihood quantitative genetic analysis was used. RESULTS: In men, additive genetic effects accounted for 77% of the variance in WC, 59% in WHR, and 89% in BMI. In women, additive genetic effects accounted for 76% of the variance in WC, 56% in WHR, and 73% in BMI. The remaining variance in both men and women was attributed to unique environmental effects (WC, 21%; WHR, 36%; BMI, 11% in men and WC, 22%; WHR, 38%; BMI, 27% in women) and age (WC, 2%; WHR, 5% in men and WC, 2%; WHR, 6% in women). When BMI was controlled in the analysis of WC and WHR, it accounted for a portion of the genetic and environmental variance in WHR and over one-half of the genetic and environmental variance in WC. DISCUSSION: There are both genetic and environmental influences on WC, WHR, and BMI, and independent of BMI, there are genetic and environmental effects on WC and WHR among both genders. The results from this African-American twin sample are similar to findings among white twin samples.     

Variability in aflatoxin-albumin adduct levels and effects of hepatitis B and C virus infection and glutathione S-transferase M1 and T1 genotype

Exposure to aflatoxin B1 (AFB1), an important cofactor in the etiology of hepatocellular carcinoma in Taiwan, is influenced by dietary and other factors. The present study examined the intraindividual variability in AFB1-albumin adducts, the most reliable long-term biomarker of AFB1 exposure, and whether the baseline or follow-up adduct levels and the intraindividual variability in adduct levels are modified by endogenous and environmental factors. The study measured AFB1-albumin adduct levels among 264 healthy male residents of three townships (Hu-Hsi, Ma-Kung, and Pai-Hsa) of Penghu Islets, Taiwan, at two different time points with a median interval of 1.68 years (range 1.00-3.17 years). There was a generalized reduction in the adduct levels, with the median values being 22.1 pmol/mg (range 5.0-355.8 pmol/mg) at time 1 and 14.3 pmol/mg (range 5.0-205.2 pmol/mg) at time 2. This intraindividual variability in adduct levels was inversely associated with the age of subjects and the time interval between the two blood draws. The variability in adduct levels was lower among subjects in Hu-Hsi and Pai-Hsa townships as compared to those in Ma-Kung. No significant association was observed for the intraindividual variability in AFB1-albumin adducts with regard to the season when blood was drawn. There was also no significant association between intraindividual variability and hepatitis B surface antigen, anti-hepatitis C virus (anti-HCV), glutathione S-transferase (GST) M1, or GSTT1 status. In conclusion, we found substantial intraindividual variability in the AFB1 exposure (as determined by AFB1-albumin adducts) in Taiwan, which was probably more likely related to dietary or other environmental influences rather than to endogenous factors (e.g., hepatitis B/C viral infection or GST M1/T1 genetic status).     

Genetic and environmental influences on birth weight, birth length, head circumference, and gestational age by use of population-based parent-offspring data

Familial correlations in birth weight and gestational age have been explained by fetal and maternal genetic factors, mainly in studies on offspring of twins. The aim of the present intergenerational study was to estimate and compare fetal and maternal genetic effects and shared sibling environmental effects on birth weight and gestational age and also on crown-heel length and head circumference. The authors used path analysis and maximum likelihood principles to estimate these effects and, at the same time, to adjust for covariates. Parent-offspring data were obtained from the Medical Birth Registry of Norway from 1967 to 2004. For the analysis of birth weight and crown-heel length, 101,748 families were included; for gestational age, 91,617 families; and for head circumference, 77,044 families. Assuming no cultural transmission and random mating, the authors found that fetal genetic factors explained 31% of the normal variation in birth weight and birth length, 27% of the variation in head circumference, and 11% of the variation in gestational age. Maternal genetic factors explained 22% of the variation in birth weight, 19% of the variation in birth length and head circumference, and 14% of the variation in gestational age. Relative to the proportion of explained variation, fetal genes were most important for birth length and head circumference.     

Stability of the association between birth weight and childhood overweight during the development of the obesity epidemic

OBJECTIVE: To assess whether changes in the birth weight distribution or changes in the association of birth weight with the later risk of childhood overweight have contributed to the development of the obesity epidemic. RESEARCH METHODS AND PROCEDURES: A Danish population-based cohort study of 124,615 girls and 128,346 boys (ages 6 to 13 years), born between 1936 and 1983, were studied. Birth weight and annual measurements of height and weight were obtained from school health records. Overweight was defined by BMI in relation to internationally accepted criteria. The relative risk of being overweight by birth weight was calculated separately for each age, sex, and time period. RESULTS: The birth weight distribution remained relatively stable over time. Compared with children with a birth weight of 3.0 to 3.5 kg, the risk of overweight increased consistently with each increase in birth weight category among girls and boys and at all ages between 6 and 13 years. Furthermore, the association between birth weight and increased risk of overweight in childhood remained stable across a 48-year period. DISCUSSION: The increase in the prevalence of overweight could not be explained by time trends in the distribution of birth weight or by changes in the association between birth weight and the later risk of overweight over time. This implies that, unless the prenatal environment influences the later risk of overweight without increasing birth weight, the environmental influences contributing to the obesity epidemic in children of school age operate in the early postnatal period.     

Genetic and Environmental Influences on Chest Circumference during Infancy: A Longitudinal Study of Japanese Twins

Background: Chest circumference (CC) is suggested to be a good indicator of early life nutrition, but little is known on the heritability of CC. Thus we analysed the effects of genetic and environmental factors on the development of CC in Japanese infants. Methods: CC was measured longitudinally from birth until 1 year of age in a cohort of 211 monozygotic and 160 dizygotic complete Japanese twin pairs born in 1989–2002. The data were analysed using applications of structural linear equation modelling for twin data. Results: No sex‐specific differences in the variance components were found. Environmental factors unique to each twin explained the major part of the variation of CC at birth whereas environmental factors shared by co‐twins were more important at 1–3 months of age. From 3 months of age, the effect of genetic factors become steadily stronger and they explained the majority of the variation at 1 year of age. Strong genetic continuity in the development of CC was found, but also new sets of genes were activated during the first year of life. The origin of the environmental part of the variation could be tracked before 3 months of age. A substantial part of common and specific environmental factors affecting CC affected also birthweight. Conclusions: CC is sensitive to intrauterine environmental factors, but these effects diminish during the first year of life, at least if postnatal environment is good. CC can be a useful indicator when identifying newborns who have suffered suboptimal pre‐natal conditions.