Due date by ultrasound
Due date by ultrasound calculator
Enter your scan date and the age or crown–rump length from your report to see your due date, and compare it with your period date.
Updated
In short
How is a due date worked out from an ultrasound?
In the first trimester, the scan measures the baby's crown–rump length, which gives the age to within about 5 to 7 days. The due date is the scan date plus the days left to reach 40 weeks. If your last period gave a different date, ACOG suggests using the scan date when the two differ by more than a set number of days.
Your estimate
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This is an estimate, not a diagnosis or birth control.
General information, not medical advice. Your doctor or midwife can tell you what your own results mean.
How it's calculated
- Age from CRL (Robinson and Fleming, with the BMUS factor): days = 8.052 × √(1.037 × CRL in mm) + 23.73.
- Due date = scan date + (280 days − age at the scan).
- ACOG uses the scan date if it differs from the period date by more than 5 days (to 8w6d), 7 days (9w0d–15w6d), 10 days (16w0d–21w6d), 14 days (22w0d–27w6d) or 21 days (from 28w0d).
- Once set by the first accurate scan, a due date usually isn't changed by later scans.
Gyne learns your real cycle and adjusts these estimates as you log.
Frequently asked questions
Which is more accurate, my period or the scan?
An early scan, up to 13 weeks 6 days, is the most accurate way to date a pregnancy that wasn't conceived with IVF. Your doctor compares it with your period date.
Why didn't my due date change at the 20-week scan?
Babies grow at different rates later in pregnancy, so later scans are less reliable for dating. Doctors usually keep the date from the first accurate scan.
What is crown–rump length?
The length of the embryo or fetus from the top of the head to the bottom. It's the measurement used to date a pregnancy in the first trimester.
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