Two due dates — which one counts?
Many people get a due date from the first day of their last period and then a slightly different one at their dating scan. Both are estimates. Clinicians use a standard rule to decide which to keep: if the scan disagrees with the LMP date by more than a set number of days, the scan wins; otherwise the LMP date stays. The allowed difference depends on how far along you were at the scan, because scans become less precise for dating as pregnancy progresses.
ACOG redating thresholds
| Gestational age at the scan | Change to the scan date if it differs by more than |
|---|---|
| Up to 8w6d | 5 days |
| 9w0d – 13w6d | 7 days |
| 14w0d – 15w6d | 7 days |
| 16w0d – 21w6d | 10 days |
| 22w0d – 27w6d | 14 days |
| 28w0d and later | 21 days |
These figures come from the American College of Obstetricians and Gynecologists’ Committee Opinion on methods for estimating the due date. Other countries use similar rules.
How the calculator works
- It works out how far along you were on the scan day according to your last period.
- It compares that with the gestational age the scan measured.
- It checks the difference against the threshold for that stage and tells you which EDD would usually be kept.
Example: last period May 15, 2026; scan on July 20, 2026 measuring 8 weeks 0 days. By LMP you would be 9 weeks 3 days, so the scan is 10 days behind — more than the 5-day limit for an 8-week scan — and the scan’s EDD of March 1, 2027 would be used instead of February 19, 2027.
Important
This tool shows the general rule. Your provider makes the final decision and may consider several scans, IVF dates or other information. IVF pregnancies are usually dated from the transfer, not redated by scans. For a scan-only calculation, use the Ultrasound Due Date Calculator.