I'm no expert but the umbilical cord doesnt develop until week 5, the likelihood your baby gets any thing you ingest before that time is low. I mean dont use drugs but one time you should be ok. I would really reconsider what kind of lifestyle you're living though and consider making changes now that you do know pregnant.
The fetus should be fine. I would talk to your ob just in case, especially if you have a history of using...( wether out be random or not). The thing with children is that they are first, if that's not something you are ready to do, then you may not be ready to be a patent.