Is there a reason you were taking pills? The best and safest way is to be truthful with your OB as if there are surprises at birth CPS will step in.
Yup it will show up for sure when they test his/her first stool. You need to be honest, and if you're taking such a strong pain killer well you're pregnant and you know it's wrong and they aren't prescribed then you may need to seek some help. I remember when I first came here when I quit using oxy it was a huge help. BUT it's wasn't enough, you need outside help as well. I've been off oxy for a year and a half and still go to AA meetings 2-6 times a week. Seek help for you and your babe. I'm not judging you, honest I've been exactly where you are. Them couple will always turn into a couple more and a couple more and so on....