Abstract
Background:Previous studies have found greater adiposity and cardiovascular risk in first born children. The causality of this association is not clear. Examining the association in diverse populations may lead to improved insight.Methods:We examine the association between birth order and body mass index (BMI), systolic and diastolic blood pressure (SBP/DBP) in the 2004 Pelotas cohort from southern Brazil and the Avon Longitudinal Study of Parents and Children (ALSPAC) from Bristol, south-west England, restricting analysis to families with two children in order to remove confounding by family size.Results:No consistent differences in BMI, SBP or DBP were observed comparing first and second born children. Within the Pelotas 2004 cohort, first born females were thinner, with lower SBP and DBP; for example, mean difference in SBP comparing first with second born was −0.979 (95% confidence interval −2.901 to 0.943). In ALSPAC, first born females had higher BMI, SBP and DBP. In both cohorts, associations tended to be in the opposite direction in males, although no statistical evidence for gender interactions was found.Conclusions:The findings do not support an association between birth order and BMI or blood pressure. Differences to previous studies may be explained by differences in populations and/or confounding by family size in previous studies.
Highlights
The association between smaller birth size and greater cardiovascular risk has been well documented.[1,2,3] as this association is observed across the whole range of birth weights, rather than being a threshold whereby only those of low birth weight are at increased risk, it has been suggested that a wide range of factors other than birth size may be important.[4]First born children tend to be of lower birth weight than subsequent children born to the same mother; for example, one Norwegian study found an average of 200 g increase in birth weight between first and second born children.[5]
As low birth weight is associated with adverse cardiovascular risk in later life,[1,2,3] it has been hypothesised that first born children may be at greater risk of adverse cardiovascular health than their later born siblings
Despite first born children being shorter and smaller at birth, we did not observe any consistent associations between birth order and height, weight, body mass index (BMI), systolic blood pressure (SBP) or diastolic blood pressure (DBP) in a cohort born in 2004 in Pelotas, southern Brazil or in the Avon Longitudinal Study of Parents and Children (ALSPAC) cohort, born in 1991/1992 in Bristol, United Kingdom
Summary
First born children tend to be of lower birth weight than subsequent children born to the same mother; for example, one Norwegian study found an average of 200 g increase in birth weight between first and second born children.[5] There is evidence from some studies that being a first born child is associated with more adverse body composition and cardiovascular risk. METHODS: We examine the association between birth order and body mass index (BMI), systolic and diastolic blood pressure (SBP/DBP) in the 2004 Pelotas cohort from southern Brazil and the Avon Longitudinal Study of Parents and Children (ALSPAC) from Bristol, south-west England, restricting analysis to families with two children in order to remove confounding by family size. In ALSPAC, first born females had higher BMI, SBP and DBP In both cohorts, associations tended to be in the opposite direction in males, no statistical evidence for gender interactions was found. Differences to previous studies may be explained by differences in populations and/or confounding by family size in previous studies
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