I would check your states laws. It could be illegal for them to test your baby without your consent. Weed leaves your system after 30 days, but for a baby in utero I'm not sure. I would assume the same amount of time. You could ask your doctor but say your friend didn't know she was pregnant and smoked weed and is worried about the baby testing positive for weed?? Just an idea.
Weed stays in your system for like 2 weeks I think. But even if they do, I don't think they will call you out on it unless it's a recurring thing. Congrats and good due date, thays my son's bday he will be 4 this year :)