I had an ultrasound done 1/19/15 the tech said I

Patient: I had an ultrasound done 1/19/15 the tech said I was 9 weeks 5 days, went for an early gender reveal on 2/17 to find out I was only 12 weeks 3 days when I should have been 13 weeks 6 days. How is my sonogram so far off on such an early scan? CONFUSED AND WORRIED

Doctor: