Girls, has anyone had a discrepancy in pregnancy due dates based on me…

Girls, has anyone had a discrepancy in pregnancy due dates based on menstrual cycles and ultrasounds? Mine according to my period is already passed, but according to ultrasound I'm only 38 weeks. My gynecologist said we would count by ultrasound, but now at the maternity ward (I'm lying with a common cold) they say it's time to give birth. Share your experience if you have encountered this. I understand that everything is individual, but still want to figure things out.