Technically some states...and all of Canada...have no such limits.
And no such abortions of healthy, viable fetuses take place. Obviously no laws are necessary. Not only that, no doctor is forced to do such an abortion.
Beyond that, it's an extremely difficult procedure at that point and there's only 3-4 places in the US that can do them...and they're only doing ones where the fetuses are severely defective.
@minnie616 has the sources for this. It's more dangerous and painful for the woman than labor, since (for one reason) her body does not prepare for labor, like dilating the cervix.
It's the last bastion of those with no real argument against abortion...pretending that this happens. Yet they can never show that any take place. And why would they? At that point, labor is safer and less painful and she can make a cool $20,000 or more in a private adoption.