Once you talk with your OB you will know more and hopefully some of your anxiety will ease up too.
You took 40 mgs of Percs in your 20th week, if I recall correctly. You've posted several questions about this. No one can tell you the exact answer you are looking for. Yes, it can show up in meconium. The minimum levels would depend on the reference values for whatever lab ran the tests.
You're obsessing over this, and I get it. Addiction makes us do things we KNOW ARE BAD FOR US but we do them anyways. You've said you're going to tell your doctor. That's good, but you also need to get some help over your drug usage. Saying it is a one-time thing is trying to use your own willpower over addiction. It never works. Please, I implore you...go to a meeting, or make an appointment with a therapist or both.
What sort of drug did you use? I am glad to hear you are going to tell your OB. That is very important to do. Let us know how it goes.
I know what you have posted. I KNOW that using can have long term, detrimental effects on a developing baby. I KNOW that being honest with your doctor is the best option for you and the baby.
I KNOW that getting angry at me or anyone else trying to help your baby will backfire.
I know that you need help, but no one can force you to accept it, and that's sad.
My grandmother used to say if someone says something that makes you angry, it's because its hitting too close to home.
I wish your baby the best.
Instead of being concerned how to cover it up, you may want to be more concerned with the effects using could have on a developing baby.
Obviously, the many questions you are asking imply this was more than a one time concern.
Talk honestly with your ob, and let her/him help the baby to be as healthy as it can be.