Crown Rump Length (CRL) to Gestational Age

cm
Health

Crown Rump Length (CRL) Calculator estimates gestational age from fetal crown-rump measurement via ultrasound. CRL is the distance from the top of the fetal head (crown) to the base of the spine (rump), used primarily in the first and early second trimester for accurate dating.

Formula

GA (weeks) ≈ 8.4 + CRL(cm) × 1.4 [for 2.5-5 cm range] Margin of error: ±3-4 days (varies by trimester and measurement technique)
  • CRL is most accurate for dating in 8-13 week range (±3 days margin)
  • Accuracy decreases as pregnancy progresses; use other biometric parameters in second/third trimester
  • Measurements should be taken from outer edge of fetal skull to tip of sacrum
  • Different ultrasound societies (ACOG, ISUOG, AIUM) may use slightly different conversion curves
  • This calculator provides estimates only; formal dating requires qualified obstetric assessment

CRL Dating Example

Inputs
  • Crown Rump Length: 5 cm

A CRL of 5 cm suggests approximately 15 weeks + 1-2 days gestational age with ±3 days margin of error. Combine with maternal history (LMP) for precise dating.

Frequently asked questions

What is Crown Rump Length?
CRL is an ultrasound measurement from the fetal head crown (top) to the sacrum rump (base of spine), used to estimate gestational age in first and early second trimester.
When is CRL measurement most accurate?
CRL dating is most accurate between 8-13 weeks of gestation, with accuracy of ±3 days. Accuracy decreases with advancing gestation.
What is the margin of error?
First trimester CRL has ±3 days margin; second trimester measurements have ±4 days margin due to increasing biologic variability.
How do I use this result?
Combine the CRL-estimated gestational age with LMP (last menstrual period) and other biometric parameters for accurate pregnancy dating.
Can CRL be used after 13 weeks?
Beyond 13 weeks, use other measurements like biparietal diameter (BPD), femur length (FL), and head circumference (HC) for better accuracy.
Is this calculator a substitute for ultrasound?
No. This tool assists with understanding CRL-to-GA conversion. Professional obstetric ultrasound assessment is essential for accurate pregnancy dating and monitoring.