Thought I was 8 weeks but I'm actually 5!!

Christy

How does this happen?? The first day of my last period was November 15th. So I was thinking I was about 7.5 weeks. When they did my ultrasound all they saw was the gestational sac, the yolk sac, and a fetal pole, but no heartbeat. He said I'm only measuring 5w2d. How could my ovulation be so off?? My periods were pretty regular before getting pregnant.