Measuring small at 7wk7day

Angel

Went for my first ultrasound today....positive about my period date but measuring only 6 wks. Heart rate was 100 which the doctor said is normal for 6 wks. Go back in a week for another ultrasound to make sure baby is growing. I jut don’t understand how baby’s is supposedly only 6 weeks when I’m positive about my dates!! This happen to anyone else? What was your outcome?