What excuse can I use to not drink?

We are visiting my parents this weekend and I know they're gonna offer us beer and whatnot. How can I tell them no without being too suspicious. Last week I told them I couldn't drink because I had a really bad headache. And my mom wanted to make ceviche (a form of raw fish) and I told her no cause I'm tired of it.. although it's literally my favorite dish. I dont know what excuse to tell them, cause I usually at least just have a beer with them. We aren't planning to tell anyone at least until the first ultrasound. I hope they aren't picking up too many clues and ruining the surprise lol