How do you count which week you’re in?

I know you can count how far along you are by your LMP and from ovulation (conception) but I ovulated a week later then Glow predicted. My LMP was Jan 29 and I opk’d and ovulated on Feb 19. I got a faint positive on Feb 28 which made me 8-9 DPO and went to my OB and she confirmed the bloodwork on Feb 28th. She calculated my estimated due date from my LMP and said I was 4wks and 3 days but I know when I ovulated so what week should I go by? I usually have 29-31 day cycles. I have an ultrasound on March 18 due to my miscarriage in August