How can anyone be sure that a mother who chooses to murder some of her children through abortion, won't decide she made a mistake allowing any to live and just murder the others? Get that!
How can we be sure a mother who wants an abortion but is prevented from getting one won't murder the kids later?
We can't be sure of s---.
I'm of the opinion that our society could criminalize abortions without a huge cost to women if we included a few protections for pregnant women:
1) they would always be allowed to give up their child upon birth, no matter what
2) they could not be penalized by their school/work/the gov't for taking time off to have the child (we would have to recognize the hardship inherent in being pregnant, and possibly even pay women to carry their children, if we're going to prevent them from having abortions and make them criminally liable for harming the fetuses, for example by alcohol abuse)
3) it might make sense to allow abortions in limited circumstances (such as for drug addicts who would not be clean during the pregnancy), but then we could demand those women go on birth control to prevent future pregnancies until they clean themselves up. It would just be too much work to try to prevent an alcoholic woman from drinking during her pregnancy if she didn't even want to be pregnant.
4) no exceptions for rape/incest. Allowing that would only encourage women to make false claims. And the children in question are still human; it's not their fault who their parents are or how they were conceived.
We would still have to recognize the possibility that women would deliberately miscarry. This is the biggest threat. If women are going to have abortions anyway, perhaps that is the best reason to keep abortion legal. Society is not going to be perfect no matter what, and it's not government's job to make it that way.
There is also the issue of whether women should have the right to decide whether any of their children will come into the world. People feel strong responsibilities to their children. Not having any control over a child would probably make a woman feel very bad (this could be balanced if she was happy it was adopted away to a loving family, but it's not always going to be this way)
There is also the argument that legal abortion is good for society. Crime goes down when abortion goes up. When women are allowed to choose when to have their children, they're less likely to raise criminal bastards. I don't think this is a moral argument for allowing the killing of children ("sorry, jr., but you're 25% more likely to become a felon, so you have to die"), but it certainly is a practical reason.