Prenatal appointments

So I’m really early on in my pregnancy and haven’t had my first ultrasound, I was just recently told that the take a urine sample from you every time you go in. I’m wondering if this is for drugs? Because before I found out I was an everyday marijuana smoker and I know it still has to be in my system. I’m nervous that I will test positive, and if I do what happens after that?? I haven’t smoked since I found out I was expecting but I was a heavy smoker before. Help!