Accuracy of ultrasound dating
With today's modern equipment, we can obtain very reliable images and measurements of even very early pregnancies, sometimes even seeing a heartbeat as early as 5-6 weeks!
If you do know the conception date, this is counted as day 14, not day one.
The dating scan isn't designed to look for more subtle abnormalities, though.
That will happen at your next routine scan at about 20 weeks However, some major problems can be seen at your dating scan, such as issues with the umbilical cord or with your baby's skull or abdominal wall.
However, in the first trimester there is very little variation in fetal size, and so it turns out that an ultrasound done between 7 and 13 weeks is the most accurate. Compare the maximum error that each ultrasound can potentially have, and you'll see what I mean: This means that a pregnancy for which no period dates are available, and who did not get an exam or ultrasound until the third trimester, can have an EDC which could conceivably fall within a range as broad as 42 days! The main situation where this can cause a bit of friction between provider and patient is when the patient has uncertain dates, and/or a late ultrasound, and feels like she's close enough to full term to ask for her labor to be induced, yet she has a cervix which is unripe, or unfavorable for induction.
This can sometimes lead, if one isn't careful, to unnecessary induction of labor, which can result in unnecessary cesarean section, or delivery of a premature baby, who then requires transfer to a special care nursery.