Or you could just say you don't feel like drinking. No real need to lie... my mum had her 40th celebrations when I was 6 weeks pregnant and I just said I didn't want to drink, and I can still have fun without alcohol (even better fun because you can be as silly and crazy as you like but theres no alcohol related embarrassment!). What kind of person is going to push you to drink if you've said no...? It's also not a lie, as you really don't want to drink while pregnant.

PS. If you ARE going to use antibiotics excuse, don't say you have a virus because you don't go an antibiotics when you have a virus