I would think the child's pain would be considerable. Most of the screwed up, abused people I've known were constantly trying to ingratiate themselves with the abuser. When you're entirely abandoned, you wouldn't even have that outlet.
Pregnancy should be by choice, with a mother and father committed to the child. Even if their own relationship goes south, the committment to the child should remain as strong. The sacrifices increase in that situation, but it goes with the territory.
I agree - abandoning my child would be unthinkable, regardless of the actions of the mother.