Not all laws have included such provisions (see for example BMJ 2003;327:1009 which was passed and signed by Bush), and those that do are so vague that it does things like leave doctors at great risk of getting their lives ruined for doing what they think is right for their patient or would make procedures that are medically necessary to save the life of the mother illegal any time it wasn't also an active and immediate emergency (such as in this bill: https://legiscan.com/MS/text/SB2116/id/1846191)

It turns out when lawmakers have even bothered at all to play lip service to protecting the lives of pregnant women they still haven't done a very good job actually making sure they are protected.