Hello, I would be happy to help you with your question.
So you have never done any drugs before in your entire life, but now, at 20 weeks of pregnancy
, you smoked weed. This does not make total sense, but we all make decisions that we regret. Fortunately, this one episode should not affect your baby. Also, at most routine OB visits, a drug screen is not done. If this is one isolated episode in your entire life, I would not think it necessary to say anything about it, and I would strongly have you consider not doing this again during the pregnancy.
I hope that this helps and good luck!