Day 1?

Rachael • Married to my wonderful wife, mom to D, twins on the way. lots of other roles and titles.
I haven't startlove d my period yet but I can tell it's about to happen. My doctor will do a day 2 ultrasound. Should I count this as day 1 since it's going to happen sometime today? I either have to do my ultrasound tomorrow or Monday so day 2 or 4