A dating scan is more accurate. If the scan date differs from your last period dates, the scan date will be used. The scan will usually show that your pregnancy is less far along than suggested by your LMP.

How accurate are baby dating scans?

The earlier the ultrasound is done, the more accurate it is at estimating the babys due date. Ultrasounds performed during the first 12 weeks of pregnancy are generally within 3 - 5 days of accuracy. The most accurate time is between 8 and 11 weeks gestation.

Can ultrasounds be wrong about how far along you are?

An ultrasound is actually the most accurate way to date a pregnancy because all fetuses grow at a consistent rate during the first trimester and early second. In other words, if your baby measures 9 weeks 2 days when you have your ultrasound, thats how far along you are, no matter when your last period was.