So dont tell him!! Just hand in your resignation, and he'll be none the wiser. Dont feel you have to have an excuse to resign.
But if you do want to tell, just say that you have some great news, you're pregnant, and for lifestyle reasons you think it would be best to finish up at Chrissie time, and do some work from home.
What is your notice requirements? I wouldn't be telling until I had to!!
Good luck,
Fi