I'm About 7 Months Pregnant I Believe. The Doctors Are

I'm about 7 months pregnant I believe. The doctors are still questioning how far along i am. I've been using Meth most of my pregnancy. I wanted to know about how long does it take to get out of the babies system before I go into labor. I know I shouldn't have been using during my pregnancy but it's hard to stay away from it more than ever now. Just don't want my child coming out dirty for meth an getting taken away from me. Please help.