Due Date from Ultrasound Calculator
Calculate an ultrasound-derived due date from scan age, compare it with LMP dating, and see the ACOG discrepancy threshold for clinical review.
Convert the gestational age recorded on an ultrasound report into an estimated due date. An optional last-period date adds a separate comparison with ACOG’s redating thresholds. The main result always remains the date calculated from the scan; deciding the official pregnancy date requires the clinical history.
How to use this tool
- Enter the scan date.
- Enter the reported whole weeks and additional days, from zero to six.
- Optionally enter the last menstrual period (LMP) date.
- Compare both due dates and discuss discrepancies with your maternity clinician.
Formula
Ultrasound due date = scan date + 280 − (7 × weeks + days).
ACOG’s threshold table uses gestational age based on LMP, not the ultrasound age. Its discrepancy thresholds are more than 5, 7, 10, 14 or 21 days, depending on that age. Exactly the threshold does not exceed it. A late scan alone must not automatically change an established due date.
Example
A scan on March 2, 2026 reporting 8 weeks 0 days gives March 2 + 224 days = October 12, 2026. With LMP January 1, LMP dating gives October 8 and an age of 8 weeks 4 days at the scan. The 4-day discrepancy does not exceed the 5-day threshold. Both estimates remain visible.
Frequently asked questions
Which scan dates pregnancy most accurately?
First-trimester crown–rump length measurement is the most accurate ultrasound approach. Later size differences may reflect growth rather than a different pregnancy age.
Will this tell me to change my due date?
No. Thresholds support clinical decisions; LMP certainty, prior scans and assisted-reproduction dates also matter. Once an early due date is established, changes are uncommon.
Why is third-trimester comparison flagged?
Third-trimester dating is least reliable. Redating a small fetus could conceal growth restriction, so ACOG calls for consideration of the complete clinical picture and follow-up.
Sources
This tool gives an estimate for general information only. It is not financial, tax, legal or medical advice — check with a qualified professional before you rely on the result.
Last updated: 2026-10-30