Fig 1. Diagram showing the possible pathways through which parental genotypes can influence birth weight.

The black path represents direct fetal genetic effects on birth weight, and the red path represents maternal genetic factors which have an indirect effect on birth weight by modifying the intrauterine environment. This figure illustrates that, due to the correlation between maternal and fetal genotypes, genetic association analyses should model both maternal and fetal effects. Other environmental factors and gene-environment interactions that may influence birth weight are not shown.