Invited commentary: Interpreting associations between high birth weight and later health problems

High birth weight (>4.0 kg) has been associated with a wide range of health problems later in life. The interpretation of these statistical associations may be difficult, however. These difficulties are closely linked to methodological challenges in this research, such as filtering out confounding from family factors, disentangling associations with prenatal processes from associations with postnatal processes, and uncovering what birth weight actually represents. The well-conducted study by Kristensen et al. (Am J Epidemiol. 2014;180(9):876-884), presented in this issue of the Journal, offers an interesting example of how one can filter out confounding from family factors. In an elegant series of analyses, the authors show how an apparent inverse association between birth weight and later intelligence among those in the highest range of the birth weight scale became a positive association when proper adjustment for family factors was made. Sibling comparisons were important here.