Exploring Genetic Influences on Birth Weight

A person’s birth weight is a robust predictor of their economic, educational, and health outcomes across the life course. But is birth weight simply a proxy for environmental conditions in-utero, or do biological processes related to birth weight themselves play an important role in healthy development during the prenatal period? To examine this question, we utilize an underexplored source variation in birth weight that is largely orthogonal to prenatal environmental conditions: an individual’s genes. We leverage the recent identification of specific genetic variants associated with birth weight to construct polygenic scores in two longitudinal studies and empirically validate and unpack the molecular genetic correlates of birth weight. A 1 SD increase in the birth weight polygenic score is associated with a ~100 gram increase in birth weight; sibling comparisons show this association mostly represents the causal effect of genes. The magnitude of the relationship between polygenic score and birth weight is increased for children who spend longer in the womb and children born to mothers with high maternal body mass index. Finally, the birth weight polygenic score causally affects downstream social and cognitive outcomes, providing support for the hypothesis that birth weight itself is intimately related to healthy prenatal development.