First, they probably won't know you smoke marijuana
unless you tell the doctors or nurses.
If you don't tell they are unlikely to test you. While laws differ from state to state, as best as I can tell CPS would not take your baby away even if they found out but they might investigate. See http://community.babycenter.com/post/a28714043/il_law-_drug_testing_newborns...then_what
for a discussion.