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
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Abstract |
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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 20002001. During the 78 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 78 year old children.
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.26 This hypothesis has also been criticized because of inconsistencies, residual confounding, and selection bias in some of the studies.710
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 78 year old children.
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Methodology |
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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.
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Results |
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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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Discussion |
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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 78 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
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Acknowledgments |
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References |
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