Actually, the whole giygas as a fetus is overated and overused. The original creator has said, that the only reason giygas is the way he is, is to envoke the feeling of fear and what he thinks fear would be like. He used the memoried of fear that he obtained watching a Japanese horror movie when he wass a kid as inspiration. There is no mention of a fetus or otherwise. As far as the name Mother, that is because in the first game, mother, (whereas Earthbound is mother 2) the whole point of the game is to use Giegue's (also possible to be Giygas in 2) mother's song to make him realize what is going on and to stop him from attacking earth. His mother also happens to be either your mother or grandmother, I forget which.