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Ultrasound Due Date Calculator

Calculate your due date and estimated last menstrual period from ultrasound measurements.

Sources: ACOG Committee Opinion 700 — Methods for Estimating Due Date · ISUOG — Practice Guidelines: ultrasound assessment · NHS — Ultrasound scans in pregnancy

How the Ultrasound Due Date Calculator Works

This calculator determines the estimated due date (EDD) from data reported on the ultrasound report. It is particularly useful when the last menstrual period date is uncertain or when the ultrasound shows different dating.

The Calculation Method

The calculation is straightforward and precise:

  1. Estimated LMP = Ultrasound date - (weeks x 7 + days)
  2. EDD = Estimated LMP + 280 days (40 weeks)
  3. Current weeks = days elapsed from estimated LMP to today

Ultrasound Dating: The Gold Standard

First-trimester ultrasound is considered the most accurate method for dating pregnancy. The CRL (Crown-Rump Length) measurement between weeks 7-13 offers accuracy of +/-5-7 days.

Accuracy by Trimester

PeriodParameterMargin of Error
1st trimester (to 13+6)CRL+/-5-7 days
2nd trimester (14-27+6)Fetal biometry+/-10-14 days
3rd trimester (28+)Fetal biometry+/-21 days

When Ultrasound Dating Overrides LMP

According to ACOG guidelines, the due date is recalculated based on ultrasound when the discrepancy exceeds:

  • 7 days in the first trimester (up to 8+6 weeks)
  • 10 days between 9+0 and 13+6 weeks
  • 14 days in the second trimester
  • 21 days in the third trimester

Frequently Asked Questions

How accurate is ultrasound dating?▼
Accuracy depends on the trimester. In the first trimester (up to 13+6 weeks), the CRL measurement is the most accurate with a margin of +/-5-7 days. In the second trimester, the margin rises to +/-10-14 days. In the third, +/-21 days.
What if the ultrasound date differs from the LMP calculation?▼
Per ACOG guidelines, if the difference exceeds 7 days in the first trimester (or 10 days in the second), the due date is recalculated based on ultrasound. This is because early ultrasound is more reliable, especially for women with irregular cycles.
What is the CRL measurement?▼
CRL (Crown-Rump Length) measures the embryo from head to bottom, performed in the first trimester between weeks 7-13. It's the most accurate dating parameter because embryonic growth at this stage is very uniform across all fetuses.