Help! How pregnant am I?

Serafime

I’m reading that pregnancy counts 40 weeks from first day of last period. So, how pregnant am I? I ovulated late like cycle day 17 or 18 so when I did an early ultrasound the embryo measured younger than expected.

According to the ultrasound’s measurement: 9 weeks.

According to last period: 10 weeks.

So what is it??? 9 or 10 weeks?