Crown Rump Length Calculator
Convert crown-rump length measurements to gestational age using the Robinson and Fleming equation for early pregnancy dating.
What Is Crown-Rump Length?
Crown-rump length (CRL) is an ultrasound measurement from the top of the fetal head to the bottom of the buttocks. It is the most accurate biometric marker for dating pregnancy in the first trimester, typically between about 6 and 13 weeks. See also the Clomid Ovulation Calculator for fertility timing tools.
Robinson and Fleming Formula
Gestational age in days equals 8.052 times the square root of CRL in millimeters multiplied by 1.037, plus 23.73. This equation is widely used for early pregnancy dating when CRL is measured by transvaginal or transabdominal ultrasound.
Using CRL Measurements
CRL dating is most reliable before 14 weeks. After that, biparietal diameter and other second-trimester markers are preferred. Always confirm dates with clinical history and repeat ultrasound when measurements are uncertain.
Frequently Asked Questions
Should CRL be entered in mm or cm?
Select millimeters or centimeters in the calculator. Centimeter values are converted to millimeters before applying the formula.
What gestational age range is valid for CRL?
The Robinson and Fleming formula is intended for CRL from about 2 mm to 84 mm, roughly 6 to 13 weeks gestation.
How are weeks and days calculated?
Total gestational days are rounded to the nearest whole day. Weeks are total days divided by 7, and remaining days are the modulus.
Is CRL more accurate than last menstrual period dating?
When ultrasound is performed in the first trimester, CRL-based dating is generally more accurate than menstrual history alone, especially with irregular cycles.
Can twins affect CRL dating?
Use the CRL of each fetus separately in multiple gestations. Discordant sizes may require individualized follow-up.
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