Is it normal for babies to measure a week behind gestational age?

Hi there ladies! I’ve recently had my first ultrasound appointment, my gestational age based on the date of my last period should have been 9 weeks 1 day but in the ultrasound my babies were measuring exactly eight weeks. Doctors said everything looked good they both had heartbeats of 181 bpm.

This pregnancy is following a loss in February so I’m definitely worrisome this time around. I should have asked the question during the appointment but it didn’t occur to me until after. Is this a regular occurrence?