I like the "until it can sustain itself" argument but the issue is that's a moving bar. Every time technology improves that time gets a little bit shorter. A hundred years ago a five month fetus had little chance of surviving. Now that survival rate is 85%. If we had the ability to instantly and painlessly teleport a week-old fetus into an artificial womb, would that be cause to ban all abortion?

The only fully consistent argument I've heard is that a human mind is the definition of personhood and that parents should be able to terminate up until several months after birth, but that's pretty out there.