Due date by ultrasound
How is a due date worked out from an ultrasound?
At the scan, the sonographer measures the baby and gives the pregnancy age in weeks and days. The calculator adds the remaining days to reach 40 weeks: scan date + 280 days minus the age at the scan.
Early scans are the most accurate. In the first trimester the measurement used is the crown-rump length (CRL), and ACOG considers a first-trimester ultrasound the most accurate way to date a pregnancy.
When does the scan date replace the last-period date?
ACOG recommends keeping the due date from your last period unless the ultrasound date differs by more than these amounts:
- Scan before 9 weeks: more than 5 days
- 9 weeks to 15 weeks and 6 days: more than 7 days
- 16 weeks to 21 weeks and 6 days: more than 10 days
- 22 weeks to 27 weeks and 6 days: more than 14 days
- 28 weeks or later: more than 21 days
Once a due date is set from an early scan, it is not usually changed by later scans.
Frequently asked questions
Why did my due date change after my scan?
If the baby measured bigger or smaller than expected for your last-period date by more than the limits above, your doctor uses the scan date instead.
Which scan should I use if I had more than one?
Use the earliest scan. Measurements are most accurate in the first trimester, and later scans are not normally used to change the date.
Can a late scan give an accurate due date?
Less so. After about 22 weeks babies vary more in size, so a third-trimester scan can be off by two to three weeks.
Is my information saved?
No. The calculation runs in your browser and nothing is stored.