How do I know how far along I am?

S.A.M • Mommy to twin girls!!! 🎀🎀

I know I need to just wait until an appointment and/or blood test but just wondering how to figure this out since I don’t have regular periods. I thought you counted from the first day of our last period? Well I did fertility meds and then trigger shot but didn’t have a period before I did the letrozole. So I triggered on 12/16 so I am just counting from there? So I would only be 3 weeks this Saturday?