Meeting friend tonight. What do I tell them when they notice I'm not drinking.

Emma • Was trying for 7. months. And on the 19th jan we had our baby boy.
Ok got a BFP on Wednesay. So excited. Meeting some friends tonight for dinner and they wanted to get some cocktails before. Can't use the antibiotic reason as we always have a joke about a friend doing this. So this will be a give away. I don't really want to tell these friend just yet as haven't told my mum yet as I want to make sure we get though first few weeks ok. These friends are friends with other people I don't want to know untill I know everything give ok. Any ideas what to say to why I'm not having a drink?