If a woman gives birth to a child with a serious defect, and the defect is such that an animal with a similar defect would not be viable, that is, would be a terefah, then this is not considered a viable child, and the woman does not have the birth-impurity. There is, however, a point of view that an animal that is terefah actually can survive. According to this view, the child is viable, and the mother has the impurity of childbirth.
The sons of Rabbi Chiya went to inspect their fields. When they came back, their father asked them if they were presented with any questions. They said, “They asked us about a child with unformed face, and we ruled this a viable child and assigned to his mother birth-impurity.” Rabbi Chiya said, “Go back and retract your ruling. You thought to rule stringently, just in case, but you cannot do this here, because this stringency leads to a leniency in the end, when she is given a long period of purity following childbirth.”
If a woman aborts a child having the semblance of Lilith, the mother of demons, who have a human face and possess wings, then the mother has the impurity of childbirth, because it is a human child, only it has wings.
Art: Charles Sprague Pearce - Women in the Fields