As pregnancy progresses, the accuracy of an ultrasound for predicting due dates decreases. Between 18 and 28 weeks of gestation, the margin of error increases to plus or minus two weeks. After 28 weeks, the ultrasound may be off by three weeks or more in predicting a due date.

Can a dating scan be wrong by 2 weeks?

It is very common when scanning an early pregnancy to find that the due date does not match the menstrual history. Sometimes the dates can be more than a week off and sometimes even as much as 4 weeks.

Which date is more accurate ultrasound or LMP?

Because few women know the exact day they ovulated or conceived, an ultrasound done in the first trimester of pregnancy has been shown to the be the most accurate way to date a pregnancy. If an ultrasound date in the first trimester differs from your LMP date by seven days or more, we would go with the ultrasound.

How do you know if your baby is big for gestational age?

Babies may be called large for gestational age if they weigh more than 9 in 10 babies or 97 of 100 babies of the same gestational age. In the U.S., this means babies born at 40 weeks gestation who weigh more than 8 pounds 13 ounces (4,000 grams) or 9 pounds, 11 ounces (4,400 grams) at birth.