Crown Rump Length
Estimate gestational age from fetal crown-rump length (CRL) ultrasound. Uses the corrected Robinson & Fleming equation for pregnancy dating. Free CRL calculator with charts.
About This Calculator
The Crown Rump Length (CRL) Calculator estimates fetal gestational age from ultrasound measurements using the validated Robinson & Fleming formula. This tool is designed for expectant parents and healthcare professionals who need to convert a CRL measurement — taken from the top of the fetal head (crown) to the bottom of the buttocks (rump) — into an accurate pregnancy dating result.
The calculator uses the corrected Robinson & Fleming equation: Gestational Age (days) = 8.052 × √(CRL × 1.037) + 23.73, where CRL is in millimeters. This formula was ranked among the four most accurate fetal assessment equations by the INTERGROWTH-21st Project. CRL measured between 9+0 and 13+6 weeks is the most reliable method for determining gestational age, with an accuracy of ±5-7 days.
Enter your CRL value in millimeters or centimeters and get the estimated gestational age in weeks and days, total days of pregnancy, and the current trimester. A CRL of 55 mm corresponds to approximately 12 weeks and 1 day of gestation.
Regional Notes
India: The standard first-trimester dating scan is recommended between 11 and 14 weeks. Most Indian ultrasound machines use the Hadlock or Robinson & Fleming formulas for CRL calculation.
US: ACOG recommends first-trimester ultrasound dating between 11+0 and 13+6 weeks. CRL is the preferred biometric parameter for gestational age assessment in the first trimester.
UK: NICE guidelines recommend offering an early ultrasound scan between 10 and 14 weeks to determine gestational age and detect multiple pregnancies. CRL measurement is the standard method for pregnancy dating.
Frequently Asked Questions
How do I calculate gestational age from crown-rump length?
To calculate gestational age from CRL using the corrected Robinson & Fleming formula: multiply CRL in mm by 1.037, take the square root, multiply by 8.052, then add 23.73. The result is gestational age in days. For example, a CRL of 55 mm gives about 12 weeks and 1 day.
What is crown-rump length (CRL) in pregnancy?
Crown-rump length (CRL) is the measurement from the top of the fetal head (crown) to the bottom of the buttocks (rump), taken via ultrasound. It is the most accurate way to determine gestational age during the first trimester (up to 14 weeks).
What is a normal CRL measurement at 12 weeks?
At 12 weeks of pregnancy, the average crown-rump length is approximately 55-60 mm (about 5.5-6 cm). CRL grows roughly 1 mm per day in the first trimester. Use the CRL calculator to convert any CRL measurement to gestational age in weeks and days.
How accurate is CRL for dating pregnancy?
CRL measurement between 9+0 and 13+6 weeks is the most accurate method for dating pregnancy, with an accuracy of ±5-7 days. According to ACOG, AIUM, and SMFM, first-trimester ultrasound is the preferred method to confirm or determine gestational age.
What is the difference between CRL and BPD in ultrasound?
CRL (crown-rump length) measures the whole embryo/fetus length and is used in the first trimester. BPD (biparietal diameter) measures the width of the fetal head and is used in the second and third trimesters for growth assessment. CRL is preferred for early pregnancy dating, while BPD is better for later growth monitoring.
Can I use CRL calculator after 14 weeks?
CRL measurement is most reliable between 7 and 14 weeks. After 14 weeks, the fetus becomes more curved and CRL is less accurate for dating. For pregnancies over 14 weeks, doctors use other measurements like BPD, head circumference, femur length, and the fetal weight percentile calculator.
What does the Robinson & Fleming formula for CRL look like?
The corrected Robinson & Fleming formula for calculating gestational age from CRL is: Gestational Age (days) = 8.052 × √(CRL × 1.037) + 23.73, where CRL is in millimeters. This formula was ranked among the four most accurate fetal assessment equations by the INTERGROWTH-21st Project.
Is the crown-rump length calculator free to use?
Yes, the CRL calculator is completely free to use with no registration required. You can also share your results via a unique URL that saves all input values for easy reference.