To date, a large number of researchers have documented associations between lower birth weight and a range of health problems later in life. However, for a substantial amount of existing studies, it is unclear whether these associations reflect causality. Aiming at providing causal evidence in this field of research, Jonas Helgertz (University of Minnesota and Lund University) and Anton Nilsson (Lund University) analysed full-population Swedish register data on siblings and twins born between 1973 and 1994, and followed until 2011. The study considers both inpatient visits and absences due to illness by type of disease, and examines how the effect of low birth weight changes from birth until adulthood. This is one of the first studies to look at the effect of birth weight on sick days in adulthood. Due to its importance to both the individual’s ability to successfully pursue a career in the labour market and to a country’s public finances (in particular for generous welfare states, like Sweden), this outcome represents an important complement to hospital stays.
Overall, their results suggest a U-shaped pattern: Low birth weight has sizeable effects during the first years of life. Its influence diminishes in importance during later childhood and adolescence, and past the age of 18, the influence becomes stronger again, with broadly similar conclusions regarding both hospitalisations and sickness absences. During childhood and adolescence, the health penalty linked to a lower birth weight is represented by a range of disease types, suggesting generally worse health rather than one single disease mechanism. In early adulthood, a different pattern is found, with a strong emphasis on mental health. The effect on this outcome is quite substantial, and mental illness almost entirely drives the effect of birth weight on the number of days spent hospitalised in adulthood. Albeit not quite as dramatic, a similar story emerges for absences caused by sickness.
Having documented the role of birth weight, this study reinforces the importance of programmes such as WIC (the Special Supplemental Nutrition Program for Women, Infants, and Children) in the USA, which provide nutritional counselling and supplemental food to pregnant women. Programmes like this are likely to reduce costs not only due to physical health problems among children, but also due to mental health problems in the longer run.