Thanks guys! You have some excellent ideas! I really like the antibiotic idea (even though my coworkers will think I have cooties), and the designated driver idea.
So I was thinking of a couple ideas:
1) My coworkers know I'm really into fitness and ordinarily work out after work. So I might say that I'm going to a fitness event/class afterward, and I don't want to drink beforehand. The happy hour starts kind of early, so I think this might work.... AND might give me an excuse to leave early.
2) Another option is, later in the evening, I might just get a bottle of water and if people ask why I'm not drinking, I could say I've already had a couple drinks and have switched to water because I'm driving. I recently started driving to work (instead of using Metro), so this might be very believable!
I'm kind of hesitant for someone to actually SEE me with a drink they know has alcohol in it. Everyone knows I'm usually a heavy drinker and if I'm going to make the announcement a week later, I don't want people saying "She's pregnant?! But I saw her drinking a rum and coke at the happy hour!" I have some gossipy coworkers, and that is the LAST rumor I would want spread about me.