It's never good to have weird chemicals in your system during pregnancy. I don't know of any study showing that weed is harmful or beneficial to a young child, but I've also never seen any study listing -any- outside substance as beneficial to a fetus. My advice would be to take a break for a while, and find out more from a health care professional.

After all, if you have to stay sober for 9 months, it's not the end of the world.