Adam Clarke on Exodus 21:22

And hurt a woman with child - As a posterity among the Jews was among the peculiar promises of their covenant, and as every man had some reason to think that the Messiah should spring from his family, therefore any injury done to a woman with child, by which the fruit of her womb might be destroyed, was considered a very heavy offense; and as the crime was committed principally against the husband, the degree of punishment was left to his discretion. But if mischief followed, that is, if the child had been fully formed, and was killed by this means, or the woman lost her life in consequence, then the punishment was as in other cases of murder - the person was put to death; Exo 21:23.