How reliable is an ultrasound scan?

Pregnancy ultrasound dating scan.?

• Can a dating scan be wrong? Since becoming pregnant I have had two scans, An internal scan and a dating ultrasound scan. The internal scan made the dates match up but having the ultrasound they told me im 5 days futher then the internal scan said. If this is the case the dates do not match up but the internal scan is spot on. This has really confused me? Can they get the date wrong?

• Answer:

  Ultrasound due dates have a margin of error of roughly 1.2 weeks, so doctors will usually keep the original due date (from your last menstrual period date) if the ultrasound due date is within that margin of error. If you can't remember your last menstrual period date or have consistently irregular cycles, the doctor can use an early ultrasound to give you a fairly reliable due date.