Ultrasound Dating: Understanding CRL and Your Due Date

The Accuracy of Ultrasound

While calculating a due date from your last period (LMP) is the traditional method, an early ultrasound is widely considered the most accurate way to date a pregnancy. This is often referred to as a "dating scan."

What is CRL?

CRL stands for Crown-Rump Length. It is the measurement of the embryo or fetus from the top of the head (crown) to the bottom of the buttocks (rump). This measurement does not include the limbs or the yolk sac.

Why is CRL So Accurate?

In the first trimester (specifically between 7 and 14 weeks), all human embryos grow at a very similar and predictable rate. Because there is very little biological variation at this early stage, the size of the baby is a direct reflection of its gestational age.

By measuring the CRL, a sonographer can determine the age of the pregnancy within a margin of error of just 3 to 5 days.

When is CRL Used?

Doctors typically use CRL dating if:

• You have irregular menstrual cycles.
• You are unsure of the date of your last period.
• Conception occurred while using hormonal birth control.
• The ultrasound date differs from the LMP date by more than 7 days.

How to Use Our CRL Calculator

If you have your ultrasound report, look for the CRL value. It is usually measured in millimeters (mm). Select the "Ultrasound" tab in our calculator, enter the CRL value, and choose the correct unit (mm or cm). We will instantly calculate your estimated due date and current gestational age.