Hi,
I was about 6 weeks pregnant at New Year and keeping it secret until the scan. I managed to bluff my way through it by drinking schloer while at home, then only drinking drinks ordered by myself and my husband while in the pub - lots of "I'll have a gin & tonic please" before he returned with a glass of tonic water. I thought I would be sussed, especially by my sober mate who had announced she was pregnant a few days earlier, but I don't think people watch you as closely as you imagine. Bottle beers were also useful... you can look as if you are swigging from them without actually having anything (or allowing yourself the occasional small sip) - my other half liked this method because it meant he kept picking my bottle up and drinking from that so that was going down as well.
Good luck at keeping the secret if that's what you want to do! and enjoy your party!