I'm not sure I understand your question. Are you asking when does Mulder find out Scully is pregnant? Well, it's when he returns from being abducted. He returns in episode " Dead Alive" Also, for the rest of season 8, he is there three words, Vienen,which is when he is thrown from his job in the x-files. He knows about Scully por then. lol.. Then he is gone in the beginning of the 9th season due to the super soldier he was said he killed.
