I Went Through Ultrasound At The Radiologist, Got Checked On

I went through Ultrasound at the radiologist, got checked on 3/23/2016. First day of my last menstrual cycle was 2/4/2016. Ultra sound results show that the GA is 6W6D which would date back to 2/4/2016, this confuses me, how could the GA date back to the 1st day of my period. I currently cant undergo another ultrasound, pending appointment on April 25. Im just concerned that the radiologist might have been inexperienced.