If you tell them they will be looking for a reason and might even take baby weather it has drugs in system or not because you admitted to using while pregnant and that is basis enough unfortunately. So please don't do anymore...I know it's hard but only reason u should have relapsed is because of withdraw and if ur not having wd then really not much excuse as to why u relapsed to your Dr's. The good thing is it does not stay in ur system long...bad news the test babies meconium (first poop) for drugs and that goes back all the way to 20 weeks of your pregnancy. I smoked weed all the way until 2 weeks before my daughter was born, and I did not show in her system, and the did test here because I tested positive for weed the whole time and positive for cocaine once on my very first vist to my on/gyn when I first found out I was pregnant. You have not tested positive for drugs at your on/gyn during your prenatal visit that is a good thing...occasionally the don't test the meconium (first poop) and only babies blood or urine and unless u used 36hrs prior to birth it will not show in urine but can be detected in blood two weeks prior.
From what I've heard you should come clean with your doctor. If you tell him prior that your having an addiction problem , they will probably try and help you. If they test the baby and find out or suspect the baby is having withdrawals they might take it. So it's probably best to be honest. Hopefully others will respond that know for sure.