The age of viability is only 21 weeks now. Very low odds, but its there. Who is to say that it cant be lower?

An alternative ruling that may simultaneously piss everyone off and make everyone happy:

1) Abortion is banned except for the medical necessity of the mother.

2) Preterm delivery is available at anytime.

Want to be induced early in the 3rd trimester, go for it. Want to be "induced" at 5 weeks, sure. Yes the result is still likely the same.

This does create a problem of financial viability. I presume attempting to keep a 21 week premie alive is very expensive. Should the mother be required to expend all possible resources to save it? No.