Which measurement is most accurate for dating?

Crown-Rump Length (CRL) measured between 6 and 13 weeks is the most accurate method, with an error margin of about ±3–5 days. After 13 weeks, BPD and HC provide the best estimates, though accuracy decreases as pregnancy advances.

What formulas does this calculator use?

CRL uses the Hadlock 1992 formula: GA = 8.052 × sqrt(CRL_cm) + 23.73. BPD uses Hadlock 1984: GA = 9.54 + 1.482×BPD + 0.1676×BPD². FL uses Hadlock 1984: GA = 10.35 + 2.460×FL + 0.170×FL². HC uses Hadlock 1984: GA = 8.96 + 0.540×HC (cm). All are established, peer-reviewed formulas widely used in obstetrics.

What are the normal ranges for each measurement?

CRL is used from 6 to 13 weeks (approx 2–84 mm). BPD is used from 12 to 40 weeks (approx 20–100 mm). FL is used from 12 to 40 weeks (approx 8–80 mm). HC is used from 12 to 40 weeks (approx 70–380 mm). Entering a value outside these ranges may reduce accuracy.

Is the estimated due date the same as the delivery date?

No. The estimated due date (EDD) is a statistical estimate; only about 5% of babies are born exactly on their EDD. Normal delivery occurs between 37 and 42 weeks. Always confirm your EDD with your healthcare provider.

Can I use multiple measurements together?

This tool calculates each measurement independently. In clinical practice, composite biometric indices average multiple parameters for a more robust estimate. You can compare the results from each measurement type to see how consistent they are.

Does this replace medical advice?

No. This tool is for educational and informational purposes only. Gestational age should always be confirmed by a licensed healthcare provider who can interpret the full clinical context.

Gestational Age by Ultrasound

Name: Gestational Age by Ultrasound
Availability: InStock
Author: Nham Vu

Enter one or more ultrasound biometry measurements to instantly estimate gestational age in weeks and days, plus your estimated due date.

Ultrasound Measurement

Measurement Type

Crown-Rump Length — best for 6–13 weeks (2–84 mm)

Measurement Value (mm)

Ultrasound Exam Date

Enter a measurement on the left to see the gestational age estimate.

Summary

Enter one or more ultrasound biometry measurements to instantly estimate gestational age in weeks and days, plus your estimated due date.

How it works

Select the ultrasound measurement type: CRL (Crown-Rump Length), BPD (Biparietal Diameter), FL (Femur Length), or HC (Head Circumference).
Enter the measurement value in millimeters.
The calculator instantly displays the estimated gestational age in weeks and days.
The estimated due date is computed by adding the remaining weeks to today.
Switch between measurement types to compare estimates from different biometry parameters.
Note the trimester range to confirm the selected measurement type is appropriate.

Use cases

Confirm gestational age when last menstrual period is uncertain.
Cross-check LMP-based dating against ultrasound biometry.
Estimate due date from first-trimester CRL measurements.
Calculate gestational age from second or third trimester BPD, FL, or HC.
Support clinical documentation for obstetric care records.
Educate patients about what their ultrasound measurements mean.
Use as a quick reference tool in prenatal appointments.

Frequently Asked Questions

Last updated: 2026-07-01 · Reviewed by Nham Vu