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 an ultrasound be off by a few days?

Ultrasound dating of conception is not reliable for determining paternity because the test can be off by at least 5-7 days in early pregnancy.

How accurate are first trimester ultrasounds?

First-trimester ultrasound has an accuracy of +/- 5 to 7 days[1]. Last menstrual cycle, if known, should be used to estimate the gestational age before an ultrasound[1].

Can ultrasound tell how many weeks pregnant?

Calculate using a dating ultrasound A dating ultrasound, which measures the length of the fetus from crown to rump, is an accurate way to estimate your due date or how far along you are if completed within the first trimester (the first 13 weeks of pregnancy).