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Better-nourished mothers => chonk fetuses => thicker forearms => increased utilization of arterial supply in the immediate environment of the the fetal median artery => median artery is less likely to atrophy during a critical period of gestation.

No genetic mutations necessary.

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I wonder how many other structures exist during gestation that don’t survive past infancy in the common case. This seems similar to baby teeth in that we need them early in life but they are replaced with more permanent structures or in the case; needed during fetal development but not needed at all after birth.

The circulatory system of a fetus undergoes many changes at the time of birth. For example, in a fetus, the left and right atria of the heart are connected (allowing blood to bypass the lungs, which are not yet in use). Upon birth, that "hole inside the heart" (the foramen ovale) normally closes, redirecting blood flow to the lungs for oxygenation.

For another example, look up the ductus arteriosus.

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