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.
Gestational Age by Ultrasound
Estimate gestational age from fetal biometric measurements — CRL, BPD, HC, AC or FL — ideal when LMP is uncertain.
Why measuring the fetus can be more accurate than the last menstrual period date in some situations?
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.
Copy the values from your ultrasound report and select the measurement type.
- Select the biometric measurement type (CRL for 1st trimester; BPD, HC, AC or FL for 2nd/3rd trimester).
- Enter the value in millimeters exactly as shown on the ultrasound report.
- Enter the exam date to get the estimated EDD; otherwise only gestational age will be calculated.
Sources and references for this tool
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.
- INTERGROWTH-21st — Fetal Growth StandardsUniversity of Oxford / INTERGROWTH-21st — International fetal growth standards used for the CRL formula in this calculator.
- ACOG — Ultrasound in PregnancyACOG — ACOG guidelines for ultrasound-based dating and fetal biometry.
- Hadlock FP et al. — Sonographic Estimation of Fetal Age and WeightRadiology (RSNA) — Seminal Hadlock reference for fetal biometry tables (BPD, HC, AC, FL).
- ISUOG Practice Guidelines — Use of Doppler Ultrasonography in ObstetricsISUOG — ISUOG international guidelines for obstetric ultrasound use.