Edgar Cayce said that the spirit doesn't actually enter the fetus until shortly before birth. He said sometimes a spirit will decide that the most probably course the infants life will take won't serve its interests, so it ejects early. I read Cayce gave a reading near the end of his life, where a young girl came to him and it was actually the spirit of his sister that died in infancy. Something about his parents alcoholism or something like that made her decide not to go through with the life she would have faced with that kind of a problem.