How to avoid alcohol with friends you haven't told?

Caroline
I just found out I'm pregnant! But my hcg is low and there's a chance it's ectopic or not viable. I'm waiting to have an ultrasound on Monday. 
​My problem is I have plans for margaritas with some girlfriends this weekend - which I'm totally excited about. But I'm not ready to tell any of them I'm pregnant - especially since I'm not even sure if it's gonna last. But I also want to avoid alcohol just in case. 
​How do I not drink without being totally obvious?!