Biological evidence suggests that both mother and fetus are involved in triggering a normal delivery. A tendency of a child to have a gestational age at birth similar to the father’s could represent the effect of genes passed from the father to the fetus. Similar tendencies between mother and child could represent maternal genes passed to the fetus, as well as genes to the mother received from the grandmother that affect a woman’s capacity to carry a pregnancy.
The Medical Birth Registry of Norway contains data on all births in Norway from 1967 onward. We identified 77,452 pairs of boys and girls born at term who later became parents and linked their birth data to the birth records for their first child.
Gestational age of the child at birth increased on average 0.58 days for each additional week in the father’s gestational age (95% confidence interval 0.48–0.67) and 1.22 days for each additional week in the mother’s gestational age (1.21–1.32). Gestational age was, however, 0.65 days reduced for each additional kilogram in the father’s birth weight, presumably due to more rapid growth of the fetus triggering delivery.
Initiation of delivery has a fetal component that is heritable (passed from father and mother to child) and an additional maternal component that is also heritable. In addition, a more rapid rate of fetal growth appears to trigger delivery at earlier gestation.
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