Hi. You may find more direct answers from the pregnancy or nicotine forums. This one is open to many substances as well though. Have you talked with your OBGYN doctor about safe methods for quitting while pregnant? Wanting to quit is the first step, and doing it safely for you and your baby are the most important. When you do kick the habit, it'll be easy once you had the baby to go back....remember how hard it was to quit and don't pick up again. The second hand smoke is just as dangerous to your child.
You are increasing the risk of premature delivery through smoking just nicotine. Whereas one weed joint is almost like 30 cigarettes alone. It is going to your baby for whom it may be harder than you can imagine. Also Addiction may be transmitted through genes expressing their nature directly or indirectly. Don't put your ife as well as yourbabys life at risk. Stop for a while, not for ever but for a while and you may start again after your delivery. Its just temporary. Take the step and chose to give yourself as well as your baby a break from the drugs.