I use the Golden Rule on this one: at what stage of deterioration would I need to be in - even if it could be healed - before I wouldn't want to be healed?
If I lost my arm or my eye or my legs - I'd want to be healed. But if my brain was destroyed (even if the rest of my body was still alive), I don't see keeping my body alive to be any use to me. In essence, my brain is 'me'.
So, until a fetus develops a brain (and there's a sticky area), it does not need to be alive. Any more than my skin cell needs to be alive. I don't cry if I intentionally kill a skin cell or some sperm, so there's nothing special about an embryo.
So, abortion should be allowed. Especially if the parent will be a bad parent, and the child will suffer unduely