It's always better to be honest with your Dr.'s. Not exactly sure if you're concerned for yourself/spouse or a family member or friend.
Good luck to you (and this is just my opinion, but you might want to change your screenname - I'm sure you didn't mean it but it can be taken as offensive).
Also, I don't know if this really matters, but they dont test the baby's blood, they test the babys first poop, which will show any drug taken during the entire pregnancy.
They will test the baby that shows signs of NAS. Babies going through wd will exhibit signs of withdrawal and they need to be treated. It will look A LOT better if you come clean now then let them find out the hard way. Also much safer for the baby. Do the right thing.
From what I've been told by people on here they do. But my dr told me they normally don't for everyone. Better safe than sorry tho.
Is it you or your cousin you are concerned for? Either way the most important thing is telling the OB about the drugs so they have an understanding of the possible complications witht the delivery.
Most places are testing the baby at birth. Are you asking for yourself?