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 by then. lol.. Then he is gone in the beginning of the 9th season due to the super soldier he was said he killed.

In addiction to previous answer - We get to know about her condition (to pregnant) in the last scene in season 7 (episode "Requiem").