A Little Health Bad News for First Born Children

As family size has fallen in the world over the past few decades, the effects of birth order have come under the analytical microscope. One of the first to do so was Judith Blake, who in 1992 published Family Size and Achievement, which reviewed the evidence that first-born children have a lot of social advantages that work to their benefit over time. Today's Nature has a new set of results that are not such good news for the first-born.

There has been a steady reduction in birth rates throughout the world1. Therefore, average family size has decreased, with a consequent increase in the proportion of first-born children in many countries. Thus, any adverse health outcomes associated with being first-born would affect an increasing proportion of the world's population.

There is some evidence that birth order influences growth and metabolism, from infancy to early adulthood. First-born babies have lower birth weight, but more rapid growth and weight gain in infancy, such that in childhood they are taller than later-borns. Importantly, first-born children have reduced insulin sensitivity and higher daytime blood pressure. Although the height discrepancy is reduced by early adulthood, first-borns have greater adiposity. Further, first-borns have been shown to have a less favourable lipid profile in young adulthood, with higher LDL-C, total cholesterol, and triglyceride concentrations than later-borns. Thus, being first-born may be associated with persistent changes in metabolism and body composition, that may lead to greater risk of developing type 2 diabetes mellitus and cardiovascular disease.

This was a small study [26 first-borns and 24 second-borns], and although the results are statistically significant, the differences are not not substantively large. Still, it is something to keep an eye on, since this could influence health and death rates in ways that were otherwise unexpected.