Birth weight
To some extent, birth weight is heritable: If their parents had a low birth weight the babies may also have one. (This is the same for a high birth weight).  The heritability of birth weight ranges from 25-40 %. There is a complex relationship between a baby's genes and the maternal environment that the child is developing in. Foetal genes influence how the fetus grows in the womb, and the genes of the mother influence how the environment affects the growing fetus.