I would make up some simple story about a recent ER trip or at least a trip to your doctor...where you had an aweful stomach pain (liver pain) and the doc did all the tests, put you on medication and said to avoid alchohol for the next few months. (I actually had something similar to that happen to me and that's what my doctor said). I think that it is better to tell this story before everyone orders drinks, so it looks like you are upset that you can't join them.
And for my friends and family, when I was in my first trimester, this is what I did. When I come to a party/restaurant, I ask for a glass of wine/cocktail, etc right away. So, everything looks "normal" to everyone. Then occasionally I pretend to take a sip and then either give it to my husband or quietly get rid of it. No one ever asked any questions or suspected anything. In fact, after I told them the news at 19 weeks, many kept saying: "but you were drinking!!!"...and were surprised by my "no, I wasn't"
P.S. I also posted this answer on January DDC.