I think the best reason for the legality of abortion is the well being of the people who get them.
It's not a binary thing, just because it's banned doesn't mean it would stop.
The thing is Abortion isn't just a decision made on a whim, it is one of the hardest decisions for a person to make. If someone gets an abortion they must be very determined to not have the baby and a lot of people would be just as determined even if it was banned.
This means that abortions would be forced underground. No longer will it be a case of a professional medical procedure taken place by someone who's qualified in a sterile environment with the proper equipment. If it was forced underground, then it would become a dangerous procedure, those desperate to get an abortion because they are not ready would also be risking their own life.