How accurate are ultrasounds for dating pregnancy?

Hi, and thanks for your medical question on eHealth forum!

Gestational age calculated by the longest length of the baby in first 12 weeks usually matches the last menstrual period (LMP) method plus minus a week. Hence this difference could come up between the two methods. In the second trimester this difference can be of plus minus 8-9 days. In third trimester the difference can be up to plus minus 2 weeks. A differnec of 10 days between the two methods at 13 weeks is normal. If in doubt, please consult your gynecologist. Hope this helps. Take care!