How can conception date determined by early trans u/s done at 6 weeks 3days be accurate if you can still get pregnant w/in the 2 weeks of gestational age?

The size counts: 6 weeks is a historic term based on first day of last menses, with conception on the first day of week 2. Ultrasound measures size. Every pregnancy starts as a single cell. The closer to that point the image, the more accurate the estimate. At 6 wks 3 days the accuracy is about +/- 5 days. So if that is the ultrasound size the conception was 4 wks 3 days +/- 5 days prior.