How many weeks do you think I am

I had sex on the 7th of September and received my first positive on the 20th. I ovulated without a period ( last period was July 29th). I went to the doctor today because for the entire week I’ve been spotting and today I’m full on bleeding. They diagnosed as a threaten miscarriage since they couldn’t see anything on the ultrasound nor where the blood is coming from. My HCG came back at 50. I have to go back Monday to see if it will double or if I’m possibly having a miscarriage but it sucks not knowing how far along I really am. Based of the dates would this put me around 3 weeks?