Sometimes when a person believes enough in what their doing, their body changes to meet the persons belief. There are some cases in women, who generally want to be pregnant so much that their body starts showing signs or pregnancy, when they actually aren't pregnant, it's called Pseudocyesis. It is the belief that you are expecting a baby when you are not really carrying a child. People with pseudocyesis have many, if not all, symptoms of pregnancy -- with the exception of an actual fetus.
This also applies to other things too, thats why people generally have the symptoms of vampirism, or mermaids or dragons, fairies, etc. You cannot become another non-existent being with magick, or become a one at all because it is not possible and they do not exist.