John Calvin Commentary Genesis 25:24

"And when her days to be delivered were fulfilled, behold, there were twins in her womb." — Genesis 25:24 (ASV)

And when her days to be delivered were fulfilled. Moses shows that the internal struggle in her womb continued until the time of delivery; for it was not by mere chance that Jacob seized his brother by the heel and tried to come out before him. The Lord indicated by this sign that the result of His election is not immediately apparent, but rather that the intervening path was strewn with troubles and conflicts. Therefore, Esau’s name was assigned to him because of his roughness, which even from his earliest infancy took on a manly form; but the name Jacob signifies that this giant, striving in vain with his boasted strength, was still overcome.