Answering a facebook question

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Here is the short answer. It was not always so. Even in Exodus 21:22-23, feticide is not necessarily a capital crime and Exodus is full of capital offenses like murder.

Following tradition, including contemporary Biblical interpretation, English common law came up with the "born alive rule".

If a woman be quick with child [five months pregnant], and by a Potion or otherwise killeth it in her womb; or if a man beat her, whereby the child dieth in her body, and she is delivered of a dead child, this is a great [misdemeanor], and no murder: but if the child be born alive, and dieth of the Potion, Battery or other cause, this is murder: for in Law it is accounted a reasonable creature, in rerum natura [in existence], when it is born alive.

Common Law thought that pregnancy was a dichotomy; the vessel started growing months before the soul was implanted. There was no punishment for killing a souless vessel, but if there was a soul, the crime was a misdemeanor unless the child was born outside the womb alive.

In 1984, the Massachusetts Supreme Court argued that medical science had outdated the "born alive rule" and "“infliction of prenatal injuries resulting in the death of a viable fetus, before or after it is born, is homicide.” Since then at least 38 states have updated their laws accordingly.[1]

PS - 1984 is the same year that New York overturned the common law tradition that said it was not rape if a husband forced his wife to have sex even under threat of death.