Estimate gestational age from fetal biometric measurements, CRL, BPD, HC, AC or FL, ideal when LMP is uncertain.
Naegele's Rule (LMP + 280 days) assumes a 28-day cycle with ovulation on day 14, which does not apply to all women. First-trimester ultrasound, especially CRL between 8 and 12 weeks, is considered the most reliable gold standard for estimating the due date, with a margin of error of only ± 3–5 days. In IVF pregnancies, irregular cycles or uncertain LMP, fetal biometry is the essential resource.
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<iframe
src="https://jkit.tools/embed/en-US/gestational-age-ultrasound-calculator"
width="100%"
height="600"
style="border:0"
loading="lazy"
title="Gestational Age by Ultrasound"
></iframe>These references help contextualize formulas, standards, APIs and limitations used on this page. They do not replace professional validation when a result has legal, financial, medical or operational impact.
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.
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.
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.
Values entered remain only in your browser. We do not store or transmit this data.