The effect of birth weight on hospitalizations and sickness absences: a longitudinal study of Swedish siblings

We examine the effect of birth weight on health throughout childhood, adolescence, and early adulthood, focusing on two health outcomes: all-cause and cause-specific hospitalizations and sickness absences. The outcomes are important, not only from a health perspective but also from a labor market perspective, as the inability to fully participate in the labor force due to impaired health is known to have important long-term consequences. Our analysis focuses on differences between siblings, using full-population Swedish register data on cohorts born between 1973 and 1994. The relationship between birth weight and health is strongest during infancy, after which it weakens throughout childhood and adolescence. In adulthood, a stronger relationship again appears, suggesting a U-shaped relationship over the examined part of the life course. During childhood and adolescence, birth weight influences all examined disease types with the exception of cancers, with nontrivial effect sizes in relative terms. During adulthood, morbidity due to mental diseases dominates, primarily through conditions with early-age origins. Consequently, we provide new evidence that birth weight matters for both short- and long-term health outcomes and that it is of a dynamic nature in terms of its magnitude and which disease types are affected. Lastly, our results remain robust to a range of sensitivity analyses, including nonlinear specifications of birth weight, and to estimations based on a sample of same-sex twin pairs, allowing us to further reduce the influence of genes.