Ultrasound Dating vs Last Period Dating

Guides. Updated October 11, 2026. Calculated with the standard pregnancy dating method used by ACOG and the NHS.

Your first due date usually comes from your last period. An early ultrasound can confirm or change it.

Why scans are more accurate

In the first trimester, babies grow at a very similar rate, so measuring the baby's length gives a precise estimate of age. Period-based dates assume ovulation on day 14, which isn't true for everyone.

When the date changes

If the scan and period dates differ by more than about a week in the first trimester, providers usually use the scan date. Later scans are less accurate for dating because babies' sizes vary more.

For conception calculations

If your due date was changed after a scan, use the updated date to estimate conception.

This article is general information, not medical advice. Talk to a doctor or midwife about your own pregnancy or fertility.