Birth size and risk of insulin-dependent diabetes mellitus (IDDM)

Is socio-economic status associated with risk of childhood type 1 diabetes? Literature review

AIMS

Studies of social inequality and risk of developing type 1 diabetes are inconsistent. The present review aimed to comprehensively review relevant literature and describe what has been reported on socio-economic status or parental occupation and risk of type 1 diabetes in children.

METHODS

We searched for publications between 1 January 1970 and 30 November 2021. We focused on the most recent and/or informative publication in cases of multiple publications from the same data source and referred to these as primary studies.

RESULTS

Our search identified 69 publications with relevant data. We identified eight primary cohort studies with individual-level data, which we considered the highest quality of evidence. Furthermore, we identified 13 primary case-control studies and 14 semi-ecological studies with area-level socio-economic status variables which provided a weaker quality of evidence. Four of eight primary cohort studies contained data on maternal education, showing non-linear associations with type 1 diabetes that were not consistent across studies. There was no consistent pattern on the association of parental occupation and childhood-onset type 1 diabetes.

CONCLUSIONS

There is a need for more high-quality studies, but the existing literature does not suggest a major and consistent role of socio-economic status in the risk of type 1 diabetes.

Age at diagnosis, anthropometry and birthweight in two ethnically different cohorts of children with type 1 diabetes living in Northwest England or Singapore

AIM

To compare children with type 1 diabetes (T1D) living in the Northwest England, United Kingdom (UK) or Singapore, and to correlate age at diagnosis with birthweight and anthropometry at T1D diagnosis.

METHODS

We included 166 T1D children of white ethnicity in England (UK-White) and 185 T1D children of East-Asian ethnicity origin in Singapore (SG-Asian) who were born between 2002 and 2020.

RESULTS

The cohorts from UK-White and SG-Asian children differed significantly in FH of T1D (p < 0.001), FH of T2D (p < 0.001) and pubertal status at diagnosis (p = 0.01). Median interquartile range (IQR) for age at diagnosis was similar in the two groups. UK-White children had significantly higher birthweight SDS, height SDS, weight SDS and BMI SDS (all p < 0.001). Among the subgroup of 174 children who were prepubertal and diagnosed after age 5 years, the UK-White children were 11 months older than the SG-Asian children (p = 0.02) indicating that SG-Asian children at the time of T1D diagnosis were more likely to be in puberty compared with UK-White children (30% vs. 18%).

CONCLUSION

These two cohorts have substantially different genetic and environmental backgrounds, yet age at the diagnosis of T1D was similar except for the prepubertal children who were diagnosed after 5 years old. Timing of puberty and other factors may influence how early T1D presents during childhood.

Early Life Influences on Hearing in Adulthood: a Systematic Review and Two-Step Individual Patient Data Meta-Analysis

Objectives:

Adverse prenatal and early childhood development may increase susceptibility of hearing loss in adulthood. The objective was to assess whether indices of early development are associated with adult-onset hearing loss in adults ≥18 years.

Design:

In a systematic review and meta-analysis, four electronic databases were searched for studies reporting associations between indices of early development (birth weight and adult height) and adult-onset hearing loss in adults ≥18 years. We screened studies, extracted data, and assessed risk of bias. Authors were contacted to provide adjusted odds ratios from a logistic regression model for relationships between birth weight/adult height and normal/impaired hearing enabling a two-step individual patient data random-effects meta-analysis to be carried out. The study is registered with PROSPERO, CRD42020152214.

Results:

Four studies of birth weight and seven of adult height were identified. Three studies reported smaller birth weight associated with poorer adult hearing. Six studies reported shorter height associated with poorer hearing. Risk of bias was low to moderate. Four studies provided data for two-step individual patient data random-effects meta-analysis. Odds of hearing impairment were 13.5% lower for every 1 kg increase in birth weight [OR: 0.865 (95% confidence interval: 0.824 to 0.909)] in adulthood over two studies (N=81,289). Every 1 cm increase in height was associated with a 3% reduction in the odds of hearing impairment [OR: 0.970 (95% confidence interval: 0.968 to 0.971)] over four studies (N=156,740).

Conclusions:

Emerging evidence suggests that adverse early development increases the likelihood of hearing impairment in adulthood. Research and public health attention should focus on the potential for prevention of hearing impairment by optimizing development in early life.

The Association Between High Birth Weight and Long-Term Outcomes-Implications for Assisted Reproductive Technologies: A Systematic Review and Meta-Analysis

Background: Studies have shown that the prevalence of children born with high birth weight or large for gestational age (LGA) is increasing. This is true for spontaneous pregnancies; however, children born after frozen embryo transfer (FET) as part of assisted reproductive technology (ART) also have an elevated risk. In recent years, the practice of FET has increased rapidly and while the perinatal and obstetric risks are well-studied, less is known about the long-term health consequences. Objective: The aim of this systematic review was to describe the association between high birth weight and LGA on long-term child outcomes. Data Sources: PubMed, Scopus, and Web of Science were searched up to January 2021. Exposure included high birth weight and LGA. Long-term outcome variables included malignancies, psychiatric disorders, cardiovascular disease, and diabetes. Study Selection: Original studies published in English or Scandinavian languages were included. Studies with a control group were included while studies published as abstracts and case reports were excluded. Data Extraction: The methodological quality, in terms of risk of bias, was assessed by pairs of reviewers. Robins-I (www.methods.cochrane.org) was used for risk of bias assessment in original articles. For systematic reviews, AMSTAR (www.amstar.ca) was used. For certainty of evidence, we used the GRADE system. The systematic review followed PRISMA guidelines. When possible, meta-analyses were performed. Results: The search included 11,767 articles out of which 173 met the inclusion criteria and were included in the qualitative analysis, while 63 were included in quantitative synthesis (meta-analyses). High birth weight and/or LGA was associated with low to moderately elevated risks for certain malignancies in childhood, breast cancer, several psychiatric disorders, hypertension in childhood, and type 1 and 2 diabetes. Conclusions: Although the increased risks for adverse outcome in offspring associated with high birth weight and LGA represent serious health effects in childhood and in adulthood, the size of these effects seems moderate. The identified risk association should, however, be taken into account in decisions concerning fresh and frozen ART cycles and is of general importance in view of the increasing prevalence in high birthweight babies.

The Effect of Prenatal and Childhood Development on Hearing, Vision and Cognition in Adulthood

It is unclear what the contribution of prenatal versus childhood development is for adult cognitive and sensory function and age-related decline in function. We examined hearing, vision and cognitive function in adulthood according to self-reported birth weight (an index of prenatal development) and adult height (an index of early childhood development). Subsets (N = 37,505 to 433,390) of the UK Biobank resource were analysed according to visual and hearing acuity, reaction time and fluid IQ. Sensory and cognitive performance was reassessed after ~4 years (N = 2,438 to 17,659). In statistical modelling including age, sex, socioeconomic status, educational level, smoking, maternal smoking and comorbid disease, adult height was positively associated with sensory and cognitive function (partial correlations; pr 0.05 to 0.12, p < 0.001). Within the normal range of birth weight (10^th to 90^th percentile), there was a positive association between birth weight and sensory and cognitive function (pr 0.06 to 0.14, p < 0.001). Neither adult height nor birth weight was associated with change in sensory or cognitive function. These results suggest that adverse prenatal and childhood experiences are a risk for poorer sensory and cognitive function and earlier development of sensory and cognitive impairment in adulthood. This finding could have significant implications for preventing sensory and cognitive impairment in older age.

Birthweight and the risk of childhood-onset type 1 diabetes: a meta-analysis of observational studies using individual patient data

AIMS/HYPOTHESIS

We investigated whether children who are heavier at birth have an increased risk of type 1 diabetes.

METHODS

Relevant studies published before February 2009 were identified from literature searches using MEDLINE, Web of Science and EMBASE. Authors of all studies containing relevant data were contacted and asked to provide individual patient data or conduct pre-specified analyses. Risk estimates of type 1 diabetes by category of birthweight were calculated for each study, before and after adjustment for potential confounders.Meta-analysis techniques were then used to derive combined ORs and investigate heterogeneity between studies.

RESULTS

Data were available for 29 predominantly European studies (five cohort, 24 case-control studies), including 12,807 cases of type 1 diabetes. Overall, studies consistently demonstrated that children with birthweight from 3.5 to 4 kg had an increased risk of diabetes of 6% (OR 1.06 [95% CI 1.01-1.11]; p=0.02) and children with birthweight over 4 kg had an increased risk of 10% (OR 1.10 [95% CI 1.04-1.19]; p=0.003), compared with children weighing 3.0 to 3.5 kg at birth. This corresponded to a linear increase in diabetes risk of 3% per 500 g increase in birthweight (OR 1.03 [95% CI 1.00-1.06]; p=0.03). Adjustments for potential confounders such as gestational age, maternal age, birth order, Caesarean section, breastfeeding and maternal diabetes had little effect on these findings.

CONCLUSIONS/INTERPRETATION

Children who are heavier at birth have a significant and consistent, but relatively small increase in risk of type 1 diabetes.

Maternal age at birth and childhood type 1 diabetes: a pooled analysis of 30 observational studies

OBJECTIVE

The aim if the study was to investigate whether children born to older mothers have an increased risk of type 1 diabetes by performing a pooled analysis of previous studies using individual patient data to adjust for recognized confounders.

RESEARCH DESIGN AND METHODS

Relevant studies published before June 2009 were identified from MEDLINE, Web of Science, and EMBASE. Authors of studies were contacted and asked to provide individual patient data or conduct prespecified analyses. Risk estimates of type 1 diabetes by maternal age were calculated for each study, before and after adjustment for potential confounders. Meta-analysis techniques were used to derive combined odds ratios and to investigate heterogeneity among studies.

RESULTS

Data were available for 5 cohort and 25 case-control studies, including 14,724 cases of type 1 diabetes. Overall, there was, on average, a 5% (95% CI 2-9) increase in childhood type 1 diabetes odds per 5-year increase in maternal age (P = 0.006), but there was heterogeneity among studies (heterogeneity I(2) = 70%). In studies with a low risk of bias, there was a more marked increase in diabetes odds of 10% per 5-year increase in maternal age. Adjustments for potential confounders little altered these estimates.

CONCLUSIONS

There was evidence of a weak but significant linear increase in the risk of childhood type 1 diabetes across the range of maternal ages, but the magnitude of association varied between studies. A very small percentage of the increase in the incidence of childhood type 1 diabetes in recent years could be explained by increases in maternal age.

Birth weight, early weight gain, and subsequent risk of type 1 diabetes: systematic review and meta-analysis

Previous studies suggest that birth weight and weight gain during the first year of life are related to later risk of type 1 diabetes. The authors performed a systematic review and meta-analysis on these associations. Twelve studies involving 2,398,150 persons of whom 7,491 had type 1 diabetes provided odds ratios and 95% confidence intervals of type 1 diabetes associated with birth weight. Four studies provided data on weight and/or weight gain during the first year of life. High birth weight (>4,000 g) was associated with increased risk of type 1 diabetes (odds ratio = 1.17, 95% confidence interval (CI): 1.09, 1.26). According to sensitivity analysis, this result was not influenced by particular study characteristics. The pooled confounder-adjusted estimate was 1.43 (95% CI: 1.11, 1.85). No heterogeneity was found (I(2) = 0%) and no publication bias. Low birth weight (<2,500 g) was associated with a nonsignificantly decreased risk of type 1 diabetes (odds ratio = 0.82, 95% CI: 0.54, 1.23). Each 1,000-g increase in birth weight was associated with a 7% increase in type 1 diabetes risk. In all studies, patients with type 1 diabetes showed increased weight gain during the first year of life, compared with controls. This meta-analysis indicates that high birth weight and increased early weight gain are risk factors for type 1 diabetes.

Perinatal factors associated with subsequent diabetes mellitus in the child: record linkage study

AIMS

To report on associations between perinatal factors and the subsequent development of diabetes mellitus under the age of 30 years in the offspring.

METHODS

Analysis of linked hospital statistical records, comparing perinatal factors relating to the birth of 518 people admitted to hospital for diabetes with the same factors in 292 845 others, in a defined population in southern England from 1963 to 1999.

RESULTS

Diabetes mellitus was much more common in children of mothers with diabetes than in others (odds ratio 6.42; 95% confidence interval 4.18-9.86). There was no significant association with birthweight or gestational age separately. Diabetes was more common in those in the highest quintile of 'birthweight for gestational age' compared with the lowest four quintiles combined (odds ratio 1.33; 95% confidence interval 1.08-1.64), but there was no consistent gradient of increasing frequency of diabetes across the lowest four quintiles. There were no significant associations between diabetes and mothers' age, parity, social class, or smoking during pregnancy, or between babies' mode of delivery or any other perinatal factors investigated. All results were similar when the analysis was confined to diabetes in people aged < 15 years.

CONCLUSIONS

We found a strong association between diabetes in the child-mainly, if not entirely Type 1 diabetes-and maternal diabetes. Diabetes was slightly more common in the heaviest quintile of birthweight for gestational age than in other quintiles. There were no significant associations between diabetes and the other perinatal factors studied.

In utero undernutrition reduces diabetes incidence in non-obese diabetic mice

AIMS/HYPOTHESIS

Observational studies in humans suggest that low birthweight may decrease the risk of type 1 diabetes, but the mechanism is unknown. We hypothesised that antenatal undernutrition would decrease the incidence of type 1 diabetes in non-obese diabetic (NOD) mice.

MATERIALS AND METHODS

A 40% restriction of energy intake was applied to pregnant NOD dams from day 12.5 to day 18.5 of gestation, resulting in intrauterine growth retardation of offspring. All mice were fed a standard diet after weaning. Control and undernourished female offspring were followed to assess diabetes incidence. Male NOD mice were treated with cyclophosphamide to accelerate development of diabetes. Glucose homeostasis, body composition and pancreatic histology were compared in control and undernourished offspring.

RESULTS

Mean birthweight was lower in undernourished than in control mice (p = 0.00003). At 24 weeks of age, the cumulative incidence of spontaneous diabetes in female mice was 73% in control and 48% in undernourished mice (p = 0.003). In cyclophosphamide-treated male mice, antenatal undernutrition also tended to reduce the development of diabetes (p = 0.058). Maternal leptin levels were lower in undernourished dams on day 18.5 of pregnancy (p = 0.039), while postnatal leptin levels were significantly higher in undernourished offspring at 4, 20 and 27 weeks of life (p < 0.05). Beta cell mass was similar in both groups (control = 0.4 mg; undernourished = 0.54 mg; p = 0.24). Histological evidence of apoptosis at 20 weeks was greater in control than in undernourished mice (control = 6.3 +/- 1.4%; undernourished = 4.2 +/- 0.3%, p = 0.05).

CONCLUSIONS/INTERPRETATION

Antenatal undernutrition reduces the incidence of diabetes in NOD mice, perhaps via alterations in apoptosis.

Birth weight and type 1 diabetes among schoolchildren in Taiwan--A population-based case-controlled study

To explore the relationship between birth weight and type 1 diabetes, data from national birth registry and national surveillance of diabetes in Taiwanese schoolchildren were analyzed. From 1992 to 1997, all schoolchildren aged 6-18 years were screened for diabetes by a mass urine survey program in Taiwan Province. This cohort consisted of 1966 children with diabetes and 1780 of randomly selected subjects with normal fasting glycemia. Questionnaires were designed for telephone interviews with students' parents or physicians to classify subjects' types of diabetes. The birth history of each participant was obtained from the Taiwan's Birth Registry. After merging the data, there were 835 subjects, including 277 of type 1 diabetes and 533 of normal fasting glycemia available for the present analyses. The odds ratio (95% CI) for type 1 diabetes, after adjusting age, sex, socioeconomic status, family history of diabetes, birth order, breast-feeding, BMI, and gestational diabetes mellitus was 2.24 (1.11-4.50) for children with low birth weight (<5th percentile, i.e., < or =2600 g) when compared with the referent group of a birth weight of 3000-3542 g (equivalent to the 25-75th percentile). In conclusion, low birth weight was associated with increased risk of type 1 diabetes in Taiwanese schoolchildren.

The Belgrade childhood diabetes study: a multivariate analysis of risk determinants for diabetes

BACKGROUND

The aim of this study was to evaluate some hypotheses about factors related to the development of type 1 diabetes mellitus.

METHODS

A case-control study was conducted in Belgrade during the period 1994-1997. A total of 105 recently onset diabetic and 210 control children, individually matched by age (+/-1 year), sex and place of residence, were included in the study.

RESULTS

According to multivariate regression analysis, the following factors were related to type 1 diabetes: stressful events and symptoms of psychological dysfunction during the 12 months preceding the onset of the disease [odds ratio (OR) 3.48, 95% confidence interval (CI) 2.15-5.65; and OR 2.15, 95% CI 1.33-3.48], irregular vaccination (OR 16.98, 95% CI 1.38-208.92), infection during 6 months preceding the onset of the disease (OR 4.23, 95% CI 1.95-9.17), higher education of father (OR 1.50, 95% CI 1.05-2.14), mother's consumption of nitrosoamines-rich food during pregnancy (OR 4.33, 95% CI 1.95-9.61), alcohol consumption by father (OR 3.80, 95% CI 1.64-8.78), insulin-dependent and non-insulin-dependent diabetes mellitus in three generations of children's relatives (OR 20.04, 95% CI 4.73-84.81; and OR 5.52, 95% CI 2.45-12.46), and use of ultrasound diagnostic techniques during pregnancy (OR 0.42, 95% CI 0.17-1.00).

CONCLUSIONS

Among non-genetic factors, those affecting the child during pregnancy are especially important because of their preventability.

A new model for understanding the role of environmental factors in the origins of chronic illness: a case study of type 1 diabetes mellitus

There is a need for a new pathophysiological model explaining and linking the role of numerous non-genetic factors believed to contribute to origins of many chronic physical diseases. This article presents a theoretical model for explaining the confusing and often contradictory findings regarding the role of environmental influences in type 1 diabetes, a disease that has been widely studied, for which clear diagnostic criteria exist, and for which development of effective prevention strategies represents significant challenges. The model is formulated from the large database of research regarding increasing understanding of the interaction between environmental factors, physiology, and autonomic regulatory function. Data is integrated from research in the fields of the experience-dependent maturation of the nervous system and the neurophysiology of traumatic stress to demonstrate how disruptions in early bonding and attachment, including adverse events such as traumatic stress, are capable of causing: (1) long-term imbalances in autonomic regulatory function and (2) relative dominance of sympathetic or parasympathetic activity. The proposed model of autonomic dysfunction suggests that ongoing mechanisms promoting high glucose in the context of decreasing insulin production in type 1 diabetes represent a state of relative sympathetic dominance influenced by environmental factors affecting autonomic, immune and endocrine systems during critical period programming. The model further identifies a link between the many seemingly unrelated non-genetic risk factors, and appears capable of explaining contradictions and enigmas in epidemiological and clinical studies regarding non-genetic origins of type 1 diabetes, including the role of stress, variation in age of onset, and duration of the preclinical phase.

Parental age at delivery, birth order, birth weight and gestational age are associated with the risk of childhood Type 1 diabetes: a UK regional retrospective cohort study

AIMS

To investigate perinatal risk factors for childhood Type 1 diabetes in a UK population cohort.

METHODS

Perinatal data have been routinely recorded in Northern Ireland for all births in the period 1971-86 (n = 447 663). Diabetes status at the age of 15 years was ascertained in this cohort by identifying 991 children from 1079 registered with Type 1 diabetes diagnosed from 1971 to 2001 and date of birth in the period 1971-86.

RESULTS

Increased Type 1 diabetes risk was associated with higher maternal age, paternal age, birth weight and birth weight for gestational and lower gestational age. After adjustment for maternal age, the association between Type 1 diabetes and paternal age remained significant [relative risk (RR) = 1.52 (1.10, 2.09) comparing father's age 35 years or more to less than 25 years] but not vice versa [RR = 1.11 (0.80, 1.54) comparing mother's age 35 years or more to less than 25 years]. Increased birth order was associated with a significant decrease in the risk of Type 1 diabetes [adjusted RR = 0.75 (0.62, 0.90) comparing birth order three or more with firstborn], but this only became apparent when adjustment was made for maternal age. Furthermore this association with birth order was significant only for diabetes diagnosed under the age of 5 years.

CONCLUSIONS

Our analysis demonstrates, for the first time in a UK regional cohort setting, that maternal age and paternal age at delivery, birth order, birth weight and gestational age are significantly associated with Type 1 diabetes risk.

The Belgrade childhood diabetes study: prenatal and social associations for type 1 diabetes

The aim of this study was to investigate whether prenatal and social factors are associated with the development of type 1 diabetes. A case-control study was conducted in Belgrade during the period 1994-97. A total of 105 children </= 16 years old with recently onset of diabetes and 210 control children, individually matched by age (+/- 1 year), sex and place of residence, were included in the study. In the analysis using multivariable logistic regression, the following factors were significantly related to type 1 diabetes: poor socio-economic status (Odds ratio = 4.18, 95% confidence interval [2.01, 8.72]); higher education of father (1.53, [1.16, 2.03]); complications during pregnancy (3.39, [1.64, 7.00]); ultrasound scan during pregnancy (0.40, [0.20, 0.79]); gestational age > 41 weeks (11.11, [1.80, 68.52]); mother's consumption of foods containing nitrosamines during pregnancy (3.14, [1.68, 5.87]); and consumption of alcohol by father (4.54, [2.34, 8.79]).

[Characteristics and habits of parents of children with insulin-dependent diabetes mellitus]

The aim of this case-control study conducted in Belgrade during 1994-1997 was to investigate whether parental demographic characteristics and habits are associated with insulin-dependent diabetes mellitus (IDDM). Case group comprised 105 children up to 16 years old with IDDM and control group comprised 210 children with skin diseases. Cases and controls were individually matched by age (+/- one year), sex and place of residence (Belgrade). According to chi 2 test results, children with IDDM significantly had five or more family members and they also significantly more frequently had poor socio-economic status than their controls. Higher education of fathers was significantly more frequently reported in diabetic children, in comparison with their controls. Parents of diabetic children were significantly more frequently occupationally exposed to radiation, petroleum, and its derivates, organic solvents, dyes and lacquers. During pregnancy mothers of diabetic children significantly more frequently smoked cigarettes and consumed coffee, coca-cola, alcohol and foods containing nitrosamines. Fathers of diabetic children more frequently consumed alcohol.

Birth weight and childhood onset type 1 diabetes: population based cohort study

OBJECTIVE

To assess the associations between birth weight or gestational age and risk of type 1 diabetes.

DESIGN

Population based cohort study by record linkage of the medical birth registry and the National Childhood Diabetes Registry.

SETTING

Two national registries in Norway.

PARTICIPANTS

All live births in Norway between 1974 and 1998 (1 382 602 individuals) contributed a maximum of 15 years of observation, a total of 8 184 994 person years of observation in the period 1989 to 1998. 1824 children with type 1 diabetes were diagnosed between 1989 and 1998.

MAIN OUTCOME MEASURES

Estimates of rate ratios with 95% confidence intervals for type 1 diabetes from Poisson regression analyses.

RESULTS

The incidence rate of type 1 diabetes increased almost linearly with birth weight. The rate ratio for children with birth weights 4500 g or more compared with those with birth weights less than 2000 g was 2.21 (95% confidence interval 1.24 to 3.94), test for trend P=0.0001. There was no significant association between gestational age and type 1 diabetes. The results persisted after adjustment for maternal diabetes and other potential confounders.

CONCLUSION

There is a relatively weak but significant association between birth weight and increased risk of type 1 diabetes consistent over a wide range of birth weights.

No association between birth weight and Type 1 diabetes mellitus--a twin-control study

AIMS

To investigate, by means of a twin-control study, whether there is a relationship between birth weight and Type 1 diabetes mellitus.

METHODS

In the youngest part (n = 20 888 pairs) of the Danish Twin Register, 95 twin pairs (26 monozygotic and 69 dizygotic) with Type 1 diabetes were identified. Information about birth weight and birth length, prematurity, maternal age and parity was obtained from midwives' records and from the Danish Medical Birth Registry and analysed using Student's two-sided t-test, Chi-square test and logistic regression.

RESULTS

Birth data were available for 83 diabetic and 51 healthy twin individuals, altogether 67 twin pairs (20 monozygotic and 47 dizygotic). Mean birth weight was 2537 g and 2565 g (NS) in monozygotic and dizygotic pairs and the corresponding length at birth 47.6 cm and 48.2 cm (NS). In pairs discordant for diabetes, the mean birth weight was 2538 g in twins with diabetes and 2549 g in twins without diabetes (NS). Logistic regression in discordant pairs showed no relationship between diabetes status and birth weight, birth length, birth order, sex or prematurity. Logistic regression in concordant vs. discordant pairs showed no effect of mean birth weight and birth length, maternal age, parity or weeks of prematurity.

CONCLUSIONS

There is no association between Type 1 diabetes mellitus and birth weight in this study. Furthermore, variables related to birth weight and length cannot explain why some pairs are concordant while other remain discordant.