Relation between birthweight and blood pressure among 7–8 year old rural children in India

Rajesh Kumar1, Sutapa Bandyopadhyay1, Arun Kumar Aggarwal1 and Madhu Khullar2

1 Departments of Community Medicine, and 2 Experimental Medicine and Biotechnology, Post Graduate Institute of Medical Education and Research, Chandigarh-160012, India

Correspondence: Prof. Rajesh Kumar, Head, Department of Community Medicine, Post Graduate Institute of Medical Education and Research, Chandigarh-160012, India. E-mail: rajeshkum{at}sancharnet.in


    Abstract
 Top
 Abstract
 Methodology
 Results
 Discussion
 References
 
Background Fetal malnutrition has been proposed as a cause of higher blood pressure. However, some studies have shown no or little effect of birthweight on blood pressure. Confounding and selection bias have been a problem in some of the studies. The ideal approach for avoiding selection bias is to conduct a prospective cohort study with minimal loss during follow-up. Therefore, the relationship of birthweight with blood pressure was examined in a cohort born during 1992–1993.

Methods A cohort of 214 babies, born to usual residents in 10 villages of Haryana state in India on weekdays from September 1992 to November 1993 whose birthweight had been recorded by a trained field worker within 24 hours of delivery, were followed up during 2000–2001. During the 7–8 year period, 17 children had died and 12 had migrated. A trained public health nurse examined the remaining 185 children and measured their weight, height, and blood pressure.

Results Mean birthweight was 2.7 kg and 21.1% had low birthweight (<2500 g). Current weight, height, and body mass index (BMI) were associated with birthweight (P < 0.05). Systolic blood pressure (SBP) increased significantly with rise in current weight and height. Relationship of SBP with birthweight seems to be a U-shaped distribution. Compared with the middle birthweight tertile group, in the lowest and highest birthweight groups boys have higher SBP in the higher BMI category (>=13.5 kg/m2) and girls have higher SBP in both lower and higher BMI categories. ANOVA analysis showed that SBP and diastolic blood pressure (DBP) were not significantly different in birthweight tertile groups among boys, but for SBP the difference was statistically significant among girls (P = 0.03). However, multivariate linear regression analysis that included socioeconomic status and current anthropometeric measures (weight, height, BMI) revealed that birthweight is not associated with SBP or DBP, and the interaction between birthweight and gender was also not significant for SBP and DBP (P = 0.09).

Conclusion The findings of this population-representative cohort from rural India with 94% follow-up suggest that birthweight is not associated with blood pressure among 7–8 year old children.


Keywords Birthweight, blood pressure, children, malnutrition

Accepted 14 May 2003

The fetal origins of adult chronic diseases hypothesis states that a baby's undernourishment before birth and during infancy, as manifested in patterns of fetal and infant growth, ‘programmes’ the development of raised blood pressure, fibrinogen concentration, and glucose intolerance.1 However, some studies have shown no or little effect of birthweight on blood pressure.2–6 This hypothesis has also been criticized because of inconsistencies, residual confounding, and selection bias in some of the studies.7–10

Small size at birth and low rate of fetal growth are common in developing countries.11 In India, maternal nutrition is poor and prevalence of low birthweight (<2500 g) is 30%.12 The fetal origin of chronic disease hypothesis has profound public health implications for India and other developing countries. A retrospective study from Southern India has also reported a significant association between small size at birth and coronary artery disease among adults.13 However, this study was also marked by selection bias. The ideal approach for avoiding selection bias is to study prospectively a cohort with minimal loss during follow-up. This study was, therefore, undertaken to investigate the association of birthweight with blood pressure among a cohort of 7–8 year old children.


    Methodology
 Top
 Abstract
 Methodology
 Results
 Discussion
 References
 
This study was carried out in 10 villages of Panchkula district in Haryana state of India. The study villages had a population of 8800. During a 15-month period from September 1992 to November 1993, there were 339 live births in these villages. Children born to usual residents were followed up in 2000–2001. A field worker had recorded the birthweight of 269 children who were born on weekdays, except Sunday, within 24 hours of birth using a spring balance. Mothers who had come to their parental home for childbirth from different districts had delivered 55 babies. Hence, for this study, 214 children were eligible. A house-to-house survey was conducted and parents were interviewed to follow their children. Seventeen babies had died and 12 children had migrated, therefore, 185 children were enrolled in the study. After parental consent, height was measured in the Frankfurt plane to the nearest mm using an anthropometer. Weight was measured in light clothing without shoes to the nearest 0.1 kg using a well-calibrated weighing balance. Blood pressure was measured on the left arm at the level of the heart after rest for 10 minutes. A mercury sphygmomanometer with appropriate size cuff according to mid arm circumference was used to record the blood pressure. Diastolic blood pressure (DBP) was recorded as disappearance of Korotkoff phase V sounds. Two readings were taken at 5-minute intervals in a sitting position and average of both the readings was taken as the blood pressure. One public health nurse using the same mercury sphygmomanometer took all the blood pressure measurements. She was unaware of the birthweight of the child. The SPSS computer package was used for data analysis. Mean birthweight, age, current weight, height, body mass index (BMI), and socioeconomic score were calculated for birthweight categories (tertiles). Socioeconomic score was computed by combining scores for each year of education (15 levels), occupation (5 levels), housing condition (3 levels), and caste (3 levels) of the father by assigning equal score to each of the variables. Linear regression was done to test the significance of association of systolic blood pressure (SBP) and DBP with quantitative variables (socioeconomic score, age, current weight, height, BMI, and birthweight). Analysis of variance was used to test the significance of association of SBP and DBP with categorical variables (sex, birthweight tertile groups, BMI categories (< and >= median). A scatter plot of birthweight and blood pressure (SBP and DBP) was examined and a quadratic regression line was fitted.

Multivariate analysis was done to find out association of blood pressure with birthweight after taking into account effect of socioeconomic status (SES), current weight, height, and BMI, and interaction between birthweight and gender was also evaluated.


    Results
 Top
 Abstract
 Methodology
 Results
 Discussion
 References
 
Of the 185 children that were followed up, 21.1% had low birthweight (<2500 g). A comparison of the demographic and anthropometric parameters in various birthweight tertile groups shows that current weight, height, and BMI are significantly associated with birthweight, whereas age, sex, and socioeconomic score distribution were statistically similar in the three birthweight tertile groups (Table 1).


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Table 1 Distribution of birthweight, demographic, and anthropometric characteristics of 7–8 year old rural children in India, 2000–2001

 
Mean SBP and DBP at age 7–8 years were 113.5 and 74.7 mmHg respectively. Correlation between SBP and DBP was statistically significant (r = 0.435 P = 0.01). SBP was not statistically significantly different between boys (114.2) and girls (112.8) but DBP was significantly higher among boys (75.5) compared with the girls (73.6) (P = 0.03).

Linear regression analysis revealed that SBP increased significantly with increase in current weight and height, but did not have significant variation in relation to SES score and current BMI. DBP did not vary according to any of these variables (Table 2). SBP and DBP seem to have a U-shaped distribution according to birthweight (Figure 1). However, the P-value for the quadratic term in the regression model revealed that this relationship is not statistically significant (P = 0.4).


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Table 2 Predictors of blood pressure among 7–8 year old rural children in India, 2000–2001 (N = 185)

 


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Figure 1 Scatter plot, quadratic regression line, and mean blood pressure lines of birthweight with systolic (+———) and diastolic (x – – – – – –) blood pressure respectively among 7–8 year old children in rural India, 2000–2001

 
Table 3 presents mean SBP and DBP among boys and girls for various BMI and birthweight groups. Compared with the middle birthweight tertile category, the lowest and highest birthweight tertile group children have higher SBP in the higher BMI category (P = 0.2). Among girls, the SBP and DBP are higher in both BMI groups in the lowest and highest tertiles compared with the middle birthweight tertile group (P = 0.03). However, this pattern occurs among boys in the higher BMI group only (P = 0.8).


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Table 3 Mean blood pressure according to birthweight, body mass index (BMI), and sex among 7–8 year old rural children in India, 2000–2001

 
The multivariate linear regression model shown in Table 4 revealed that birthweight is not associated with SBP or DBP. Inclusion of SES in the model did not have a significant effect on the regression coefficient. The association of birthweight and blood pressure remained non-significant but became stronger after adjustment was made for current weight, height, or BMI. The interaction between birthweight and current weight was significant (P = 0.004). The linear regression model did not show a significant differential association of birthweight with blood pressure among boys and girls (P-value for interaction term between birthweight and sex was 0.09).


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Table 4 Multiple regression coefficients for systolic and diastolic blood pressure by birthweight, with socioeconomic status (SES) and current weight, height, and body mass index (BMI) among 7–8 year old rural children in India, 2000–2001 (N = 185)

 

    Discussion
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 Abstract
 Methodology
 Results
 Discussion
 References
 
The fetal origin of chronic adult disease hypothesis, generated by ecologic studies, has been tested in case-control, cohort, and longitudinal studies in European, African, Asian, and American populations.13–24 The relationship between blood pressure and birthweight is described by the regression coefficient for the linear regression of SBP on birthweight, without adjustment, or after adjustment for current size (weight, height, BMI). While some studies show that an inverse relationship exists, others do not support it. Most of the studies describing the linear regression of blood pressure on birthweight showed negative regression coefficients. Recently, a U-shaped relation between birthweight and blood pressure was reported, with higher SBP corresponding to low and high birthweights.25 The findings of our population-representative cohort from rural India with 94% follow-up suggest that birthweight is not associated with blood pressure among 7–8 year old children.

Confounding by SES and obesity is a problem in studying the relationship of birthweight with blood pressure.10 SES in our study was not associated with birthweight and blood pressure. Thus there is very little scope for confounding by SES in this study. Stratified analysis using BMI categories, among boys and girls separately, revealed among girls a significant association between birthweight groups and SBP (P = 0.03) and it remained significant among girls even after adjustments were made for current BMI (Table 3). Among boys this relationship was not significant. Linear regression analysis did not show a significant differential association between birthweight and blood pressure in boys and girls (P = 0.09 for interaction term of birthweight with sex). One study of adolescents showed a negative coefficient for boys and a positive coefficient for girls.3 However, a recent meta analysis found no gender differences.26 The birthweight and blood pressure association remained non-significant but became stronger after adjustment for current weight was made. There was significant interaction of birthweight with current weight in relation to SBP (P = 0.004). It seems postnatal growth is more important for blood pressure than birthweight as suggested by Lucas.27

Systematic review supports an inverse relationship between blood pressure and birthweight in children and adults.8,9 However, claims of a strong inverse relationship may chiefly reflect the impact of random error and inappropriate adjustment for current weight and other confounding factors.10 Most of the studies included in the systematic reviews are from developed countries. Two studies have been reported from India.2,13 One of these studies,13 which supports the relationship, was a retrospective study where follow-up was only 41%. Another study, which does not support the relationship, was among babies born in hospital.2 However in India about 70% babies are born at home. Selection bias was avoided in our study by recruitment of babies in the community and by following most of them. Our cohort children are representative of the wider population in India in terms of birthweight. The loss to follow-up due to migration was very small (6%). There was no statistically significant difference in the birthweight of children who were born to residents of the villages (2.7 kg) and those who migrated (2.9 kg), however, those who died had significantly lower birthweight (2.3 kg). Another potential source of bias is differential ascertainment. This was overcome because one public health nurse, who recorded the blood pressure of all the children, did not know birthweights of the children. Measurement of blood pressure is subject to error and bias.8 Observer digit preference and choice of cuff size are important concerns. An appropriate size cuff was used in our study but a random zero sphygmomanometer or an automated device was not available.

The findings of our population-representative cohort with 94% follow-up suggest that birthweight is not associated with blood pressure among 7–8 year old children in rural India. However, this study did not have sufficient power to detect a less than 3 mmHg rise in SBP among low birthweight compared with normal birthweight babies. Intervention studies are required in developing countries before finally confirming or refuting the fetal origin of chronic adult disease hypothesis. In our rural population28 prevalence of hypertension among adults (>20 years) is only 4%, and very few people are obese. Rural children in our population are growing up in low socioeconomic conditions and most of them are likely to remain normotensive despite high levels of low birthweight, as it is known that the roots of high blood pressure are sown in childhood. However, it will be interesting to study prospectively whether any differences in biological markers of chronic diseases emerge between various birthweight groups during their adult life, particularly if some of them indulge in risky lifestyles.


KEY MESSAGES

  • This population-based cohort study from rural India with very little loss in follow-up suggests that birthweight is not associated with blood pressure among 7–8 year old children.
  • Association of birthweight and blood pressure needs further investigation in different population groups especially from developing countries where low birthweight prevalence has been quite high.

 


    Acknowledgments
 
We sincerely thank Ms Anita Tiwari for her help in data collection and data entry. Our institution provided funds for this project under the Institute Research Scheme.


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