Accuracy of ultrasounds for dating
It may show a very minor problem, or something which may get better on its own. As with all screening tests, there can be false positive and false negative results. Then the circumference of the head becomes the best way to measure your baby Nuchal translucency can be measured from 11 to 14 weeks to estimate the risk of Down's syndrome and other chromosomal abnormalities. Some hospitals offer a nuchal translucency ultrasound to all women at 11 to 14 weeks, but this isn't available everywhere. [Epub ahead of print, accessed April 2009] Gray JAM, Ward P. National Down's Syndrome Screening Programme for England: a handbook for staff.
If you do know the conception date, this is classed as day 14 and not day one.
I had one at 6 4 which measured at 3 days behind my dates. I am fairly sure that my date is correct to within a day either side as I was using ovulation tests.
So I'd say that very early on it is hard for them to measure something so small correctly, but probably between 9-12 weeks they can be fairly accurate.
If you have an irregular cycle, or if you've recently been taking the contraceptive pill, working out the date from your LMP isn't a very accurate method.
Ultrasounds are more accurate than using the LMP and, if the ultrasound date differs from your period dates, the ultrasound date will be used.