Do genetic factors contribute to the association between birth weight and blood pressure?

Developmental origins of cardiometabolic health outcomes in twins: A systematic review and meta-analysis

BACKGROUND AND AIMS

Studies of twins can reduce confounding and provide additional evidence about the causes of disease, due to within-pair matching for measured and unmeasured factors. Although findings from twin studies are typically applicable to the general population, few studies have taken full advantage of the twin design to explore the developmental origins of cardiometabolic health outcomes. We aimed to systematically review the evidence from twin studies and generate pooled estimates for the effects of early-life risk factors on later-life cardiometabolic health.

METHODS AND RESULTS

An initial search was conducted in March 2018, with 55 studies of twins included in the review. Risk of bias was assessed using the Newcastle-Ottawa Scale, and eligible studies were included in a meta-analysis, where pooled estimates were calculated. Twenty-six studies analysed twins as individuals, and found that higher birthweight was associated with lower SBP (β = -2.02 mmHg, 95%CI: -3.07, -0.97), higher BMI (β = 0.52 kg/m², 95%CI: 0.20, 0.84) and lower total cholesterol (β = -0.07 mmol/L, 95%CI: -0.11, -0.04). However, no associations were reported in studies which adjusted for gestational age. Few of the included studies separated their analyses into within-pair and between-pair associations.

CONCLUSIONS

Early-life risk factors were associated with cardiometabolic health outcomes in twin studies. However, many estimates from studies in this review were likely to have been confounded by gestational age, and few fully exploited the twin design to assess the developmental origins of cardiometabolic health outcomes.

Genetic and environmental determinants of population variation in interleukin-6, its soluble receptor and C-reactive protein: insights from identical and fraternal twins

Interleukin-6 and C-reactive protein are commonly assessed biomarkers linked to illness, obesity, and stressful life events. However, relatively little is known about their heritability. By comparing Caucasian twins from the Midlife in the US project (MIDUS), we estimated the heritability of IL-6, its soluble receptor, and CRP. Based on the hypothesis that adiposity might contribute more to IL-6 than to sIL-6r, we fit heritability models quantifying the extent to which each reflected genetic and environmental factors shared with obesity. Genetic influences on IL-6 and its receptor proved to be distinct. Further, the appearance of a heritable basis for IL-6 was mediated largely via shared paths with obesity. Supporting this conclusion, we confirmed that when unrelated adult controls are carefully matched to twin participants on BMI, age, gender and socioeconomic indices, their IL-6 is similar to the corresponding twins. In contrast, the effect of BMI on CRP was split between shared genetics and environmental influences. In conclusion, IL-6 is strongly affected by factors associated with obesity accounting for its lability and responsiveness to diet, life style and contemporaneous events.

Familial aggregation and childhood blood pressure

There is growing concern about elevated blood pressure (BP) in children. The evidence for familial aggregation of childhood BP is substantial. Twin studies have shown that a large part of the familial aggregation of childhood BP is due to genes. The first part of this review provides the latest progress in gene finding for childhood BP, focusing on the combined effects of multiple loci identified from the genome-wide association studies on adult BP. We further review the evidence on the contribution of the genetic components of other family risk factors to the familial aggregation of childhood BP including obesity, birth weight, sleep quality, sodium intake, parental smoking, and socioeconomic status. At the end, we emphasize the promise of using genomic-relatedness-matrix restricted maximum likelihood (GREML) analysis, a method that uses genome-wide data from unrelated individuals, in answering a number of unsolved questions in the familial aggregation of childhood BP.

The relationship between body mass index and uric acid: a study on Japanese adult twins

OBJECTIVES

The present study aimed to investigate the association between body mass index (BMI) and uric acid (UA) using the twin study methodology to adjust for genetic factors.

METHODS

The association between BMI and UA was investigated in a cross-sectional study using data from both monozygotic and dizygotic twins registered at the Osaka University Center for Twin Research and the Osaka University Graduate School of Medicine. From January 2011 to March 2014, 422 individuals participated in the health examination. We measured height, weight, age, BMI, lifestyle habits (Breslow's Health Practice Index), serum UA, and serum creatinine. To investigate the association between UA and BMI with adjustment for the clustering of a twin within a pair, individual-level analyses were performed using generalized linear mixed models (GLMMs). To investigate an association with adjustment for genetic and family environmental factors, twin-pair difference values analyses were performed.

RESULTS

In all analysis, BMI was associated with UA in men and women. Using the GLMMs, standardized regression coefficients were 0.194 (95 % confidence interval: 0.016-0.373) in men and 0.186 (95 % confidence interval: 0.071-0.302) in women. Considering twin-pair difference value analyses, standardized regression coefficients were 0.333 (95 % confidence interval: 0.072-0.594) in men and 0.314 (95 % confidence interval: 0.151-0.477) in women.

CONCLUSIONS

The present study shows that BMI was significantly associated with UA, after adjusting for both genetic and familial environment factors in both men and women.

Examination of the Relationship between Oral Health and Arterial Sclerosis without Genetic Confounding through the Study of Older Japanese Twins

OBJECTIVE

Although researchers have recently demonstrated a relationship between oral health and arterial sclerosis, the genetic contribution to this relationship has been ignored even though genetic factors are expected to have some effect on various diseases. The aim of this study was to evaluate oral health as a significant risk factor related to arterial sclerosis after eliminating genetic confounding through study of older Japanese twins.

SUBJECTS AND METHODS

Medical and dental surveys were conducted individually for 106 Japanese twin pairs over the age of 50 years. Maximal carotid intima-media thickness (IMT-Cmax) was measured as a surrogate marker of arterial sclerosis. IMT-Cmax > 1.0 mm was diagnosed as arterial sclerosis. All of the twins were examined for the number of remaining teeth, masticatory performance, and periodontal status. We evaluated each measurement related with IMT-Cmax and arterial sclerosis using generalized estimating equations analysis adjusted for potential risk factors. For non-smoking monozygotic twins, a regression analysis using a "between within" model was conducted to evaluate the relationship between IMT-Cmax and the number of teeth as the environmental factor controlling genetic and familial confounding.

RESULTS

We examined 91 monozygotic and 15 dizygotic twin pairs (males: 42, females: 64) with a mean (± standard deviation) age of 67.4 ± 10.0 years. Out of all of the oral health-related measurements collected, only the number of teeth was significantly related to arterial sclerosis (odds ratio: 0.72, 95% confidence interval: 0.52-0.99 per five teeth). Regression analysis showed a significant association between the IMT-Cmax and the number of teeth as an environmental factor (p = 0.037).

CONCLUSIONS

Analysis of monozygotic twins older than 50 years of age showed that having fewer teeth could be a significant environmental factor related to arterial sclerosis, even after controlling for genetic and familial confounding.

Cardiovascular Mortality in Twins and the Fetal Origins Hypothesis

The intrauterine growth patterns for twins are characterized by normal development during the first two trimesters and reduced growth during the third trimester. According to the fetal origins hypothesis this growth pattern is associated with risk factors for cardiovascular morbidity and mortality. We studied cause-specific mortality of 19,986 Danish twin individuals from the birth cohorts 1870–1930 followed from 1952 through 1993. Despite the large sample size and follow-up period we were not able to detect any difference between twins and the general population with regard to all-cause mortality or cardiovascular mortality. Hence, the intrauterine growth retardation experienced by twins does not result in any “fetal programming” of cardiovascular diseases. There is still an important role for twins (and other sibs) to play in the testing of the fetal origins hypothesis, namely in studies of intra-pair differences, which can assess the role of genetic confounding in the association between fetal growth and later health outcome.

Association of Birth Weight and Current Body Size to Blood Pressure in Female Twins

It has been proposed that low birth weight is associated with high levels of blood pressure in later life. The aim of this study was to assess the relationship of blood pressure to birth weight and current body size during growth and adulthood. A total of 711 female multiple births, with one group of 244 in their growth phase mean age 12.0 (2.3)(SD) years and the other of 467 adults (mean age 35.2 (12.6) years), had height, weight and both systolic (SBP) and diastolic (DBP) blood pressures measured, and self-reported their birth weight. Regression analyses were performed to assess the cross-sectional and within-pair associations of blood pressure to birth weight, with and without adjustments for current body size. Within-pair analysis was based on 296 twin pairs. Cross-sectionally, a reduction in birth weight of 1 kg was associated with 2 to 3 mm Hg higher age-adjusted SBP, which was of marginal significance and explained about 2% of the population variance. Adjustment for body mass index did not significantly change this association. Within-pair analyses found no association between birth weight and SBP or DBP, even after adjusting for current body size. After age, current body size was the strongest predictor of systolic BP. The weak association of blood pressure to birth weight cross-sectionally is of interest, but any within-pair effect of birth weight on blood pressure must be minimal compared with the effect of current body size.

Early-life origins of adult disease: national longitudinal population-based study of the United States

OBJECTIVES

We examined the relation between low birth weight and childhood family and neighborhood socioeconomic disadvantage and disease onset in adulthood.

METHODS

Using US nationally representative longitudinal data, we estimated hazard models of the onset of asthma, hypertension, diabetes, and stroke, heart attack, or heart disease. The sample contained 4387 children who were members of the Panel Study of Income Dynamics in 1968; they were followed up to 2007, when they were aged 39 to 56 years. Our research design included sibling comparisons of disease onset among siblings with different birth weights.

RESULTS

The odds ratios of having asthma, hypertension, diabetes, and stroke, heart attack, or heart disease by age 50 years for low-birth weight babies vs others were 1.64 (P < .01), 1.51 (P < .01), 2.09 (P < .01), and 2.16 (P < .01), respectively. Adult disease prevalence differed substantially by childhood socioeconomic status (SES). After accounting for childhood socioeconomic factors, we found a substantial hazard ratio of disease onset associated with low birth weight, which persisted for sibling comparisons.

CONCLUSIONS

Childhood SES is strongly associated with the onset of chronic disease in adulthood. Low birth weight plays an important role in disease onset; this relation persists after an array of childhood socioeconomic factors is accounted for.

Leukocyte telomere length and physical ability among Danish twins age 70+

Leukocyte telomere length (LTL) shortens with age and is potentially a biomarker of human aging. We examined the relation of LTL with physical ability and cognitive function in 548 same-sex twins from the Longitudinal Study of Aging Danish Twins. LTL was measured by Southern blots of the terminal restriction fragments (TRF). Physical ability was evaluated using a self reported scale of 11 questions, while cognitive function was scored by MMSE and a cognitive composite score sensitive to age-related decline. A random intercept model revealed a positive, significant association between LTL and physical ability. For every unit increase in physical ability score, LTL increased by 0.066 kb (p = 0.01), equal to approximately three years of age-dependent LTL shortening. A matched case-co-twin design showed that the group consisting of the twins from each pair with the longer LTL also displayed better physical ability (p < 0.01). Moreover, the intra-pair difference in LTL was associated with intra-pair difference in physical ability (p < 0.01), confirming the association. However, we found no association between cognitive function and LTL. The LTL-physical ability association in the elderly provides further support to the premise that LTL is an index of somatic fitness in the narrow context of human physical health.

Birth weight effects on children's mental, motor, and physical development: evidence from twins data

OBJECTIVE

To determine the effect of very low birth weight (VLBW; <1500 g) and moderately low birth weight (MLBW; 1500-2499 g) on children's mental and motor development and physical growth during the first 2 years of life and whether VLBW and MLBW babies catch up to normal birth weight (NBW; > or =2500 g) children by age 2.

METHODS

We use data on dizygotic (DZ) and monozygotic (MZ) twins and singleton births from the first two waves of the Early Childhood Longitudinal Study-Birth Cohort (ECLS-B), a nationally representative dataset of children born in the US in 2001. We estimate the effects of VLBW and MLBW on children's mental and motor development scores, weight-for-age, weight-for-length, weight-for-height, and length-for-age z-scores at 9 months and 2 years. We examine whether differences in outcomes within twin pairs are related to differences in their birth weights. The within-twins analysis is conducted on samples of DZ and MZ twins. For comparison, we also estimate birth weight effects on child outcomes from multivariate linear regression models using the full singleton and twins' sample. We also estimate the effect of being small-for-gestational age (SGA; birth weight <10th percentile for gestation) using the same set of models in order to separate out the effects of fetal growth restriction from prematurity.

RESULTS

Evaluation of all births showed that VLBW and MLBW have large negative effects on mental development, motor development, and growth at 9 months and 2 years of age. However, results from within-twin models with DZ twins that control for shared maternal and environmental factors showed much less effect of birth weight on mental or motor development, but continued large effects on growth for the VLBW group. Within-twin models with MZ twins that control for shared maternal, environmental, and genetic factors showed statistically insignificant effects of birth weight on mental and motor development, but continued effects on growth. Similar patterns were found when examining the effects of SGA.

CONCLUSIONS

After controlling for the influence of maternal, environmental, and genetic factors, low birth weight has at most a small negative effect on children's mental and motor development in their first 2 years of life. However, low birth weight is a major risk factor for children's physical growth in the early years and there is no evidence of catch-up by age 2.

Undernutrition and obesity associated with high blood pressure in children and adolescents from João Pessoa, Paraíba, Brazil

The objective of this study was to describe the prevalence, correlation, and association of undernutrition and obesity with high blood pressure (HBP). One thousand five hundred seventy (1570) students (808 boys and 762 girls), aged 7-12 years, from João Pessoa, Paraíba (Northeastern Brazil) participated. Measurements of stature, body weight, skinfolds [triceps (TS) and subscapular (SS)], upper-arm circumference (UAC), upper-arm fat area (UAFE), total upper-arm area (TUAA), and BP were taken. Four criteria were adopted to classify undernutrition and obesity. HBP was defined as systolic BP (SBP) and/or diastolic BP (DBP) values >or=90th percentile. Analysis of covariance, Spearman's correlation, logistic regression, and multiple linear regression were used. In the logistic regression model, undernutrition was not associated with HBP; however, the chances of HBP increased when two or more obesity indicators were present [boys: odds ratio (OR) = 2.08, 95% confidence interval (CI 95%) = 1.26-3.41; girls: OR = 2.26, CI 95% = 1.44-3.55]. In the multiple regression, the BMI, SS, and UAFE explained 15% of the variance of high SBP (r (2) = 0.153) in boys, whereas the body mass index (BMI) and TUAA accounted for 16% of the variance of the SBP (r (2) = 0.166) in girls. The DBP was influenced by the SS in boys (r (2) = 0.022) and the TUAA (r ( 2 ) = 0.054) in girls. There was an association between obesity and HBP. The BMI, SS, and UAFE in boys and the BMI and TUAA in girls explained approximately 16% of elevated SBP.

Familial (shared environmental and genetic) factors and the foetal origins of cardiovascular diseases and type 2 diabetes: a review of the literature

Several researchers have argued that observed associations between birth weight and cardiovascular diseases, and type 2 diabetes, may be confounded by familial (shared environmental and genetic) factors. However, most studies have found that shared environmental factors, including socio-economic factors, do not influence the foetal origins of adult diseases. Results from two twin studies suggest that genetic factors may be of importance for the association between birth weight and risks of coronary heart disease, but findings from intergenerational studies are not consistent with genetic confounding. More studies have assessed the importance of genetic factors with respect to risk factors of coronary heart, including raised blood pressure and lipid levels. Recent findings suggest that the association between birth weight and hypertension is independent of genetic factors. In contrast, recent twin and intergenerational studies favour the hypothesis that the association between birth weight and risk of type 2 diabetes is confounded by genetic factors.

Is birth weight a risk factor for ischemic heart disease in later life?

BACKGROUND

An inverse association between birth weight and ischemic heart disease (IHD) has been seen in observational studies.

OBJECTIVE

We wanted to determine the strength and consistency of the association between birth weight and subsequent IHD.

DESIGN

We conducted a systematic review of observational studies.

RESULTS

Seventeen published studies of birth weight and subsequent IHD were identified that included a total of 144,794 singletons. Relative risk estimates for the association between birth weight and IHD were available from 16 of these studies. Additional data from 2 unpublished studies of 3801 persons were also included. In total, the analyses included data from 18 studies on 4210 nonfatal and 3308 fatal IHD events in 147,009 persons. The mean weighted estimate for the association between birth weight and the combined outcome of nonfatal and fatal IHD was 0.84 (95% CI: 0.81, 0.88) per kilogram of birth weight (P<0.0001). No significant heterogeneity was observed between estimates in different studies (P=0.09), nor was there evidence of publication bias (P=0.3, Begg test). Neither restricting the analysis to fatal IHD events nor adjusting for socioeconomic status had any appreciable effect on the findings.

CONCLUSIONS

These findings are consistent with a 1 kg higher birth weight being associated with a 10-20% lower risk of subsequent IHD. However, even if causal, interventions to increase birth weight are unlikely to reduce the incidence of IHD materially. Further studies are needed to determine whether the observed association reflects a stronger underlying association with a related exposure or is due (at least in part) to residual confounding.

Birth weight, puberty, and systolic blood pressure in children and adolescents: a longitudinal analysis

We examined the association between birth weight and systolic blood pressure (SBP) from pre-puberty to late puberty in a cohort of American children. Ninety-eight children aged 4-12 years at baseline were followed annually for 2-6 years with at least two Tanner stages. Annual measures included SBP, age, gender, race, birth weight, Tanner stage, and body composition using dual-energy X-ray absorptiometry and computed tomography. Birth weight was inversely correlated with SBP in pre-pubertal children (r=-0.23, P<0.05), especially in white children. SBP persisted at a higher level from pre-puberty through late puberty among children with low birth weight (<2500 g). However, SBP significantly increased from pre-puberty to early or late puberty among children with high birth weight (>or=4000 g). After adjusting for visceral fat, one unit change of birth weight category was associated with a 2.6 mm Hg reduction in SBP (P<0.05), but this association was attenuated as puberty progressed. The changes in SBP across puberty followed different trajectories in children with low vs high birth weight. Attenuation in the association between birth weight and SBP from pre-puberty to late puberty may be influenced by sexual maturation.

Hypotheses on the Fetal Origins of Adult Diseases: Contributions of Epidemiological Studies

Epidemiological studies have demonstrated associations between low birth weight and cardiovascular disease, type 2 diabetes and their risk factors in adult life. These findings have led to sharp debates in the literature concerning potential methodological study flaws and the effect size and causality of the associations. More recent studies seem to have overcome most methodological flaws and suggest a small effect size of low birth weight on adult diseases for the individual. However, the effect size may still be important on a population level. Various causal pathways have been hypothesized as mechanisms underlying these associations. These hypotheses have proposed central roles for (1) fetal undernutrition, (2) increased cortisol exposure, (3) genetic susceptibility and (4) accelerated post-natal growth. These hypotheses have been studied in various epidemiological study designs. Thus far, it is still not known which mechanisms underlie the associations between low birth weight and diseases in adult life. The causal pathways linking low birth weight to diseases in later life seem to be complex and may include combined environmental and genetic mechanisms in various periods of life. Well-designed epidemiological studies are necessary to estimate the population effect size and to identify the underlying mechanisms. This knowledge is needed to develop strategies for identifying groups at risk and prevention focused on early life.

Discordant twin growth in utero and differences in blood pressure and endothelial function at 8 years of age

OBJECTIVE

Low birth weight is associated with increased prevalence of hypertension and cardiovascular disease in adults. The aim of this study was to evaluate genetic and intrauterine environmental contributions to blood pressure (BP) and vascular functions in twins with discordant growth in utero.

SUBJECTS

We studied 31 twin pairs (21 monozygous and nine dizygous), mean age 8 years) with large within-pair differences in birth weight. Among the monozygous pairs, nine had suffered from twin-to-twin-transfusion syndrome (TTTS).

METHODS

Apart from BP, we determined diameters and elasticity of the carotid artery and abdominal aorta with ultrasonography, and endothelial function in skin vessels with a laser Doppler technique, before and after transdermal delivery of acetylcholine and nitroglycerin.

RESULTS

Eight of 62 twin subjects had a systolic BP above the 90th percentile in a North-American reference population. Among these, seven/eight were monozygous with a history of poor fetal growth and/or TTTS. In monozygous twin pairs without TTTS, systolic BP and pulse pressure were higher and vascular endothelial function was impaired in the lower birth weight twin. In the TTTS group, the lighter twin had a narrower carotid artery but there was no within-pair difference in arterial elasticity. Pre-eclampsia during the index pregnancy enhanced within-pair differences in BP but abolished within-pair differences in endothelial function.

CONCLUSIONS

Severe fetal growth retardation contributes to higher BP, arterial narrowing and endothelial dysfunction in childhood. Pre-eclampsia may act both as an effect modifier and confounder of these associations.

Fetal, Developmental, and Parental Influences on Childhood Systolic Blood Pressure in 600 Sib Pairs

BACKGROUND

Little is known about the contribution of maternal and paternal factors to the inverse association between birth weight and later blood pressure in human offspring. A study of within- and between-family associations of birth weight with blood pressure, which collected data on both parents, would address this gap in our knowledge.

METHODS AND RESULTS

The study examined families composed of mother, father, and 2 full sibs delivered between 38 and 41 weeks' gestation within 36 months of each other. A total of 1967 families meeting our inclusion criteria were contacted and 602 were examined (children 5 to 14 years old, 1998 to 2000). Birth weight and gestational age were available from obstetric records. Systolic blood pressure in childhood was inversely associated with birth weight within families (-2.3 mm Hg/kg, 95% CI -4.4 to -0.3) after adjustment for gestational age, sex, height, and weight at examination. The between-family effect (-1.5 mm Hg/kg, -3.1 to 0.0) was strengthened on adjustment for maternal and paternal height and weight, whereas adjustment for paternal and maternal systolic blood pressure at examination independently attenuated the effect.

CONCLUSIONS

The existence of an inverse association of birth weight with systolic blood pressure within families (adjusted for height and weight at examination) demonstrates that factors that vary between pregnancies in the same woman (including fetal genotype) can influence the later blood pressure of offspring. We conclude that this apparent fetal programming effect on blood pressure will not be eliminated solely by interventions aimed at modifying growth and cumulative nutritional status from conception through childhood or other fixed characteristics of future mothers.

Developmental determinants of blood pressure in adults

Over the past 20 years a large and varied body of research has attempted to make the case for the developmental origins of elevated adult blood pressure (BP). Experimental animal research has identified plausible biological mechanisms through which fetal nutritional insufficiency may affect adult BP. The majority of human epidemiologic studies demonstrate an inverse association of birth weight (the most commonly used marker of fetal nutrition) with adult BP and higher risk of hypertension among individuals with lower weight at birth. The most adverse BP outcomes occur among individuals who were small at birth but relatively large as adults, a finding that suggests a role for postnatal growth. We critically review the literature on proposed mechanisms and epidemiologic evidence for developmental origins of adult BP and hypertension, considering associations with birth weight, maternal nutrition during pregnancy, child growth patterns, and infant feeding.

Birth Characteristics and Risk of High Systolic Blood Pressure in Early Adulthood

BACKGROUND

Although an inverse association between size at birth and blood pressure has been found in several studies, few studies have adjusted for the influence of socioeconomic and familial effects.

METHODS

We investigated whether the association between birth weight and systolic blood pressure in young men is confounded by socioeconomic factors in adolescence or familial factors (ie, common genes and shared environment). Our population-based cohort study comprised 330,768 Swedish men born between 1973 and 1981, and conscripted for military service between 1991 and 2000. The analyses of family effects were restricted to 89,856 siblings from the initial cohort. A high systolic blood pressure at conscription was defined as a systolic blood pressure >/=140 mm Hg. Birth weight for gestational age <-2 standard deviation scores was considered "light for gestational age."

RESULTS

Compared with men who had normal birth weight for gestational age, men who had been born light for gestational age were at increased risk of high systolic blood pressure (odds ratio = 1.14; 95% confidence interval = 1.07-1.22), even after adjustment for important confounders such as socioeconomic status. The increase in risk of high systolic blood pressure related to 1 standard deviation score decrease in birth weight for gestational age was similar within families (1.08; 1.04-1.12) and between families (1.05; 1.03-1.08).

CONCLUSIONS

This study suggests that low birth weight for gestational age slightly increases the risk of high systolic blood pressure, and that the association appears not to be confounded by socioeconomic or familial effects.

Regression models for twin studies: a critical review

Twin studies have long been recognized for their value in learning about the aetiology of disease and specifically for their potential for separating genetic effects from environmental effects. The recent upsurge of interest in life-course epidemiology and the study of developmental influences on later health has provided a new impetus to study twins as a source of unique insights. Twins are of special interest because they provide naturally matched pairs where the confounding effects of a large number of potentially causal factors (such as maternal nutrition or gestation length) may be removed by comparisons between twins who share them. The traditional tool of epidemiological 'risk factor analysis' is the regression model, but it is not straightforward to transfer standard regression methods to twin data, because the analysis needs to reflect the paired structure of the data, which induces correlation between twins. This paper reviews the use of more specialized regression methods for twin data, based on generalized least squares or linear mixed models, and explains the relationship between these methods and the commonly used approach of analysing within-twin-pair difference values. Methods and issues of interpretation are illustrated using an example from a recent study of the association between birth weight and cord blood erythropoietin. We focus on the analysis of continuous outcome measures but review additional complexities that arise with binary outcomes. We recommend the use of a general model that includes separate regression coefficients for within-twin-pair and between-pair effects, and provide guidelines for the interpretation of estimates obtained under this model.

Embryonic and fetal programming of physiological disorders in adulthood

In the past decade, data from numerous epidemiological studies have indicated strong inverse associations between birth weight and risk of coronary heart disease, hypertension, type 2-diabetes, and other diseases in adulthood. The "Barker hypothesis" thus postulates that a number of organ structures and functions undergo programming during embryonic and fetal life. This developmental programming determines the set points of physiological and metabolic responses in adult life. Alterations of nutrient availability during gestation may lead to developmental adaptations, via hormonal maneuvers by the embryo and fetus that readjust these set points. These adaptive measures have short-term benefits to the embryo and fetus, so that the newborn will be better prepared for the adverse environment (e.g., undernutrition). However, adequate nutritional support during postnatal life that enables catch-up growth may create metabolic conflicts that predispose the adult to aberrant physiological functions and, ultimately, increased risk of disease. It is plausible that other adverse in utero conditions, including exposure to developmental toxicants, may similarly alter adult disease susceptibility. This article provides an overview of the Barker hypothesis, its supporting evidence, the current advances in understanding the biological mechanisms underlying this phenomenon, and its implications for developmental toxicology.

Studies of twins: what can they tell us about the fetal origins of adult disease?

There has been much interest in evidence that people with lower birthweight have higher risk of adult cardiovascular disease, but the causal pathways underlying such observations are uncertain. Study of twins offers an opportunity to shed light on the underlying causal pathways, in particular by investigating the role of 'shared' factors vs. factors affecting each individual fetus. This involves comparing results of within-cohort vs. within-pair analyses. Twins share many factors during gestation but birthweight discordance (difference in birthweight within a twin pair) cannot be determined by these shared factors and must relate to factors affecting growth of each individual fetus. If associations seen in a cohort of twins remain in within-pair analyses, then factors specific to each individual must be involved in the underlying causal pathways. Conversely, if the relationships disappear or substantially diminish in within-pair analyses, then factors common to the pair must be involved. Comparison of findings in monozygotic vs. dizygotic twins may provide insights into the role of genetic factors, although issues related to chorionicity need to be taken into account. We tabulate published data and conclude that differences in methodology and analyses preclude informative meta-analysis, and that analysis of pooled data would provide more useful information.

A discussion of some statistical methods for separating within-pair associations from associations among all twins in research on fetal origins of disease

Twin data can be used to gain insights into the origin of associations between factors arising in fetal life and the risk of later disease. This is because twin data afford an opportunity to conduct paired analyses that take the influence of shared factors into account. When an association that is present in an unpaired analysis is present also in a paired analysis, there is evidence that the causal pathway linking the fetal factor and the disease may have a fetal origin. If the association disappears in the paired analysis, there is evidence that it may have has arisen from a shared source such as the mother. The relevant factors include diet and socio-economic status. There are several statistical approaches to this. The simplest involves comparing, say, a coefficient from a regression of an outcome on a fetal factor for all subjects in a twin sample, with the coefficient obtained from regressing the within-pair difference in the outcome on the within-pair difference in the fetal factor. Alternative approaches involve simultaneously estimating regression parameters for between- and within-pair components. These approaches permit similar inferences about whether the association is due to individual (fetal) or shared (maternal) factors, and are valid in the circumstances that non-shared factors missing from the regression model do not influence the regression estimates.

Separating within and between effects in family studies: an application to the study of blood pressure in children

In epidemiology the analyses of family or twin studies do not always fully exploit the data, as information on differences between siblings is often used while between-families effect are not considered. We show how cross-sectional time-series linear regression analysis can be easily implemented to estimate within- and between-families effects simultaneously and how these effects can then be compared using the Hausman test. We illustrate this approach with data from the Uppsala family study on blood pressure in children with age ranging from 5.5 to 12.3 years for the younger and from 7.5 to 13.8 years for the older siblings. Comparing the effect of differences in birth weight on blood pressure within-family (in full siblings) and between-families (in unrelated children) allows us to study the contributions of fixed and pregnancy-specific maternal effects on birth weight and consequently on blood pressure. Our data showed a 0.88 mmHg decrease (95 per cent confidence interval: -1.7 to -0.03 mmHg) in systolic blood pressure for one standard deviation increase in birth weight between siblings within a family and 0.88 mmHg (95 per cent confidence interval: -1.6 to -0.2 mmHg) decrease in systolic blood pressure for one standard deviation increase in birth weight between unrelated children. These estimates were controlled for sex, age, pubertal stage, body size and pulse rate of the children at examination and for maternal body size and systolic blood pressure. The within- and between-families effects were not significantly different, p = 0.19, suggesting that fixed and pregnancy-specific factors have similar effects on childhood systolic blood pressure.

Fetal adaptation to stress: Part II. Evolutionary aspects; stress-induced hippocampal damage; long-term effects on behavior; consequences on adult health

Humans share adaptative capacities to stress with other species, as demonstrated on amphibians: the physiological response to experimental water volume and food deprivation results in the activation of the endocrine axes that drive metamorphosis, in particular the neuroendocrine stress system. Unfavorable effects may, however, occur, probably due to inappropriate timing and/or duration of stress: recent experiments are converging to show a profound impairment of hippocampal functioning in the offspring of mothers exposed to prenatal stress. Moreover, fetal changes are likely one of the risk factors for a number of diseases in adulthood.

The association between birth weight and plasma fibrinogen is abolished after the elimination of genetic influences

Low birth weight is associated with an increased risk of atherothrombosis, which may be related in part to the association between low birth weight and high plasma fibrinogen. The association between birth weight and fibrinogen may be explained by intrauterine, socio-economic or genetic factors. We examined birth weight and fibrinogen in 52 dizygotic and 56 adolescent monozygotic (genetically identical) twin pairs. The dizygotic but not the monozygotic twins with the lowest birth weight from each pair had a fibrinogen level that was higher compared with their co-twins with the highest birth weight [dizygotic twins: 2.62 +/- 0.46 g L(-1) vs. 2.50 +/- 0.41 g L(-1) (P = 0.04); monozygotic twins: 2.42 +/- 0.45 g L(-1) vs. 2.49 +/- 0.39 g L(-1) (P = 0.2)]. These findings suggest that the association between birth weight and plasma fibrinogen is abolished after the elimination of genetic influences and therefore that this association has genetic causes. Improvement of intrauterine nutrition may not lower fibrinogen levels in later life.

Unravelling the fetal origins hypothesis: is there really an inverse association between birthweight and subsequent blood pressure?

BACKGROUND

The association between birthweight and subsequent blood pressure levels has been considered to provide some of the strongest, and most consistent, support for the "fetal origins" hypothesis of adult disease. It had been estimated that a 1 kg higher birthweight is typically associated with a 2-4 mm Hg lower systolic blood pressure.

METHODS

55 studies that had reported regression coefficients of systolic blood pressure on birthweight (with 48 further studies that reported only the direction of this association), and seven such studies within twin pairs, were identified. Each study was weighted according to the inverse of the variance of the regression coefficient (ie, "statistical size"), and combined using a "fixed effects" approach.

FINDINGS

Among the 55 studies that reported regression coefficients, there was a clear trend (p<0.0001) towards weaker associations in the larger studies: -1.9 mm Hg/kg in those with less than about 1000 participants; -1.5 mm Hg/kg with about 1000-3000 participants; and -0.6 mm Hg/kg with more than 3000 participants. By contrast with the inverse associations reported in 52 of these 55 studies, only 25 of the 48 studies that did not report regression coefficients found an inverse association (p<0.0001 for heterogeneity). Almost all of these regression coefficients had been adjusted for current weight (whereas few were adjusted for potential confounding factors), and removal of this adjustment in the larger studies reduced the estimated association to -0.4 mm Hg/kg. For studies within monozygotic twin pairs, the combined estimate was -0.6 mm Hg/kg with adjustment for current weight, and was also reduced without this adjustment.

INTERPRETATION

Claims of a strong inverse association between birthweight and subsequent blood pressure may chiefly reflect the impact of random error, selective emphasis of particular results, and inappropriate adjustment for current weight and for confounding factors. These findings suggest that birthweight is of little relevance to blood pressure levels in later life.