when to correct due date by ultrasound LMP difference

When Does Ultrasound Correct the Estimated Due Date?

ACOG defines specific discordance thresholds between LMP and ultrasound for each trimester, above which the EDD should be revised. This page explains these criteria and why the established EDD should not change in subsequent exams.

Medical disclaimer: this calculation is educational and does not replace physician evaluation. Biometric measurements have inherent margin of error and must always be interpreted in clinical context.

Biometric measurement type

Crown-Rump Length (CRL)in mmUltrasound exam dateOptional — to calculate EDD

How to use this calculator

Select the biometric measurement type from your ultrasound report, enter the value in millimeters, and optionally enter the exam date to get the estimated EDD.

CRL1st trimester · 6–14 wks

BPD2nd/3rd trim. · 12–40 wks

HC2nd/3rd trim. · 12–40 wks

AC2nd/3rd trim. · 14–40 wks

FL2nd/3rd trim. · 14–40 wks

The margin of error for biometry-based estimates is ± 0.5 to 2.5 weeks, depending on the measurement and gestational age. CRL in the 1st trimester is the most precise parameter.

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ACOG criteria for EDD correction

Up to 13 weeks and 6 days: difference > 7 days justifies correction by fetal biometry. From 14 to 27 weeks 6 days: difference > 14 days. From 28 weeks onward: difference > 21 days. The earlier the ultrasound, the greater the precision.
Once established by ultrasound, the EDD should not be changed based on later exams — even if subsequent measurements suggest a different GA. Abnormal fetal growth does not change the EDD; it indicates a growth problem.

When to correct or maintain the EDD

5-day discordance in 1st trimester

Input: DPP pela DUM: 15/out · DPP pelo US de 10 semanas: 20/out
Expected output: Manter DPP pela DUM

5-day difference is below the ACOG threshold (7 days in 1st trimester).

10-day discordance in 1st trimester

Input: DPP pela DUM: 15/out · DPP pelo US de 10 semanas: 25/out
Expected output: Corrigir para DPP pelo ultrassom: 25/out

10-day difference exceeds the 7-day threshold — biometry takes precedence.

Full tool FAQ

Which measurement is most accurate for estimating GA?

CRL (Crown-Rump Length) in the 1st trimester (weeks 6–14) is the most precise parameter, with a margin of error of only ± 3–5 days. After week 14, composite biometry (BPD + HC + AC + FL) tends to be more accurate than any single measurement. Precision decreases progressively throughout gestation.

Can ultrasound 'correct' the due date calculated from LMP?

Why does the margin of error increase later in pregnancy?

Can this calculator be used for twin pregnancies?

What is the clinical significance of each measurement?

Which formulas does the calculator use?

Can I combine more than one measurement for better accuracy?

What should I do if the GA estimated here differs from my ultrasound report?

Frequently asked questions

Why does the EDD not change in 3rd-trimester ultrasounds?

In the 3rd trimester, normal biological variation between fetuses is so large that biometry alone cannot reliably distinguish between a small baby and an incorrect EDD. Therefore, the EDD established in the 1st trimester remains as the reference.