The Serpent was walking on two and he had semblance slightly similar to that of a man. You should know that those who are able to turn a man into a wolf, a cat or a donkey - his eyeball does not change and so the serpent who underwent a change - his eyes did not change. Therefore one who miscarried and the fetus had a form of a snake - the mother is ritually impure as one who gave birth because his eyeballs are similar to those of a man.