First Trimester Dating by CRL
Crown-rump length (CRL) provides the most accurate gestational age estimate during the first trimester. Enter the CRL measurement from your ultrasound to calculate gestational age and projected dates.
Valid range: 0.5 cm (5 mm) to 8.4 cm (84 mm).
Optional. Add the scan date to project estimated conception and due date.
How to Use This Calculator
Obtain the CRL measurement
Use the crown-rump length recorded on the ultrasound report. Measure in millimeters (mm) when possible.
Enter the ultrasound date
Including the scan date enables the calculator to backdate the last menstrual period estimate and project due dates.
Review the gestational age estimate
Compare with clinical history. The earliest, most accurate ultrasound should guide the official estimated due date (EDD).
Formula
Gestational age (days) = 8.052 × √CRL (mm) + 23.73
Gestational age (weeks + days) = GA (days) converted to weeks and remainder days
Estimated due date (EDD) = Scan date − GA (days) + 280
Estimated conception date = Scan date − GA (days) + 14
Full Description
Crown-rump length is the gold standard for first-trimester dating because embryonic growth is consistent before the influence of maternal and genetic factors. Accurate dating reduces early induction or post-term complications and aligns prenatal screening windows. Beyond 13 weeks, other biometric parameters (biparietal diameter, femur length) are used, but they have larger margins of error compared to CRL in early gestation.
Frequently Asked Questions
What if the measured gestational age differs from LMP dating?
Use the ultrasound-based dating when the difference exceeds 5–7 days in the first trimester, per professional guidelines.
Can CRL be used after 13 weeks?
Accuracy declines. Switch to second-trimester biometric parameters after CRL exceeds 84 mm.
Do multiple pregnancies require different formulas?
CRL dating is still appropriate for twins. Use the earliest, best-quality measurement for each fetus.
How precise is CRL dating?
Within ±3–5 days when measured between 7 and 10 weeks. Accuracy lessens before 6 weeks or after 13 weeks.