In scientific jargon, these babies have lethal mutations.
I would not condemn someone for aborting a terminally ill baby. However, I think they are emotionally and psychologically better off if they go ahead and have the baby. With abortion, there is always some doubt—what if the tests were wrong, mixed up in the lab or something? And the mother has the knowledge that she had her child killed. When the child dies naturally, there is no doubt, and the mother does not have to live with knowing she killed her child.
There are hospices for women pregnant with terminally ill infants. They seem to be very compassionate and understanding.
I have no experience with this situation, but I agree with you, and Sarah's decision.