Irregular period/post BC pregnancy

Hi everyone. It’s my first pregnancy and I’m wondering if anyone had a similar story. I was on BC for a long while. I stopped because husband and I wanted to try for a baby. First real cycle was 51 days, second cycle was 41 days. Third time period didn’t come and I found out I was pregnant 🥳

My question is how did you know how many weeks you were approximately? Doctor wants to make an appointment at 8-9 weeks which if my cycle was normal would probably be now but I don’t think it was? What was your experience?? Thanks in advance!