I've had a few happy hours with work or business colleagues recently (and obviously I have no intention of informing work yet, I'm in a high pressure, competitive field and want to avoid being "the pregnant woman" as long as possible). These are the various things I've done:
1) Get there early, find female bartender or waitress to confide in, ask for sprites or ginger ales with lime in short glasses. This works well for happy hour tables served by wait staff and the same waitress will continue to bring you "another" the rest of the evening.
2) If you arrive with everyone else or can't get there early, ask to see a menu of the beverages if you get there late, then not be ready when everyone else orders and ask them to come back. Then, after waiter/waitress leaves, go up to bar or follow waiter and place your "ginger ale and lime" or "sprite and lime" beverage.
3) Have a drink buddy, aka someone who gets you 'drinks' from the bar so it's not suspicious with your group if you are always going up to the bar and ordering alone. This means you have to to tell someone, obviously, and they must have acting skills. My husband and my one work colleague who knows have both done well with this.
4) I was training for a half marathon (that I ran in Jan) before I got pregnant, and I had to stop a lot of the drinks anyway before running because it was too dehydrating before longer runs on Saturdays. Say you are training for a run or other athletic event and watching your beverages because it dehydrates you. May not work depending on circumstances.
I've never said the antiobiotics excuse -sorry, but it's too obvious, and will get people talking here. I'm already a prime "target" for the gossip (right point in career time-wise, married for over 5 years, right age, etc.) at the office.
P.S. I heard you aren't supposed to drink Tonic water because the quinine?