Should I trust the accuracy on an ultrasound to date a pregnancy
The earlier in pregnancy the dating ultrasound is preformed the more accurate it is. During early pregnancy there is very little variation in fetal development. Since there is so little variation dating ultrasounds are usually very accurate.
Later in pregnancy, especially after 20 weeks, there is a wide variation or normal and dating ultrasounds can be off by a few weeks in either direction. It is best not to look at later ultrasounds for dating and to stick with the due date that was determined early in your pregnancy.