I'm not sure about all abnormal births, but I've written a couple of previous answers about twins:

How did people explain identical twins before we knew how conception works?

How did medieval European monarchs handle twins?

Very briefly, not much happened with twins. Some people thought they were a sign of the mother's, er...immorality, but most people realized twins were a natural possibility. Twins (and their mother) had a higher risk of dying in childbirth, but there are a few cases of royal/noble twin births and there must have been a lot of surviving twins among the regular population who we just don't know about.