Obviously Pregnant not drinking at my own party!

Hey all! So I am only 4/5 weeks pregnant with our first baby. 
We are very happy and can't wait to share the news with friends and family.
We are hosting Christmas this year and it's also my birthday party on 30th December.
Now for someone who enjoys a nice glass of wine or cocktail, I just know everyone is going to be super shocked and question why I am not drinking!!
We don't really want to tell everyone as it will still be too early?
Has anyone had this kind of situation? And what to do say? 
Thanks xx