To answer this question, I always look to history and why it was legalized in the first place.
To paraphrase what I remember, women were getting pregnant and not all of them wanted their babies, whether it was to stay in the social norm (the "out-of-wedlock = bad" thing) or because they couldn't afford to keep the baby. The government told the doctors not to (or something), so the doctors did not abort the pregnancy. The women who did not want to have their babies (for their own reasons) decided they were not going to have their babies.
They went to people who did not know enough to safely abort the pregnancy or they went to the coat closet and got a wire hanger. Basically, women were dying in great percentages, and the People were like, "Government, this is your fault, fix it or GTFO." The government made it legal; doctors aborted pregnancies; death percentage dropped.
I think abortion is necessary, because no one is going to stop someone who has a parasite inside of them. No one.