A sonogram is 100% accurate, insomuch as it's basically a picture. Interpretations of that picture may vary, and may or may not be accurate, but the sonogram itself is a sounding of whatever is being examined, incapable of being "inaccurate."
When figuring the due date by a sonogram, the doctor will measure the crown-rump length (CRL). The length of the embryo on the longest axis constitutes the crown-rump length. This is actually...