I know a couple who had/have issues like this - So because it became such an issue, he just started sneaking around at parties and doing it behind his missus' back! That was even worse.... but because she got so angry, and would pretty much put a end to the party because she'd go off her head, and he still wanted to very occasionally partake, he resulted to doing it behind her back at the parties. It was painful to watch. I ended up telling her what he was doing, as i couldn't handle her being treated like she was so clueless. Her partner saw it as her trying to control him. I think she has finally got through to him now though because he doesn't go out at all now.
Anyways i personally feel you should let it go if he claims it was his last hurrah before your baby comes. So once off meaning this was the first time he ever did it? If that's the case, then i'd let it go... But if you feel he is just plain disrespectful to you and this was the icing on the cake, then there is a lot more than just the once of drug use to address. I wouldn't make it just about the drug use if i were to address this with him, i would make this about his complete disregard for how you feel. As soon as you mention the line he did he will probably go into "Stop trying to control me" mode. and not even listen to what you're saying. But if you make it more about the disregard for your feelings, and your concerns about his self control once your baby is here,that may hit more to home? Bring it up calmly.... but id be stern as well. He needs to know when your baby is here you are not taking any more ****. You'll be a mumma bear then there will be no place for him in your cave if he is going to be selfish.