Fetal Measurement CRL (Crown-Rump Length): Your Complete Guide

The Crown-Rump Length, or CRL, is one of the most important measurements taken during early pregnancy. If you have had a dating scan between 11 and 14 weeks, you will see this figure on your scan report. This complete guide explains what the CRL is, how it is measured, why it matters, and how it is used to calculate your due date and confirm that your pregnancy is progressing as expected.

What Is the CRL?

The Crown-Rump Length is the length of your baby measured from the top of the head (the crown) to the bottom of the buttocks (the rump). In the first trimester, babies naturally curl up in a C-shape, so this single measurement captures most of the baby's body length. It is taken using ultrasound, with the sonographer drawing a line on the screen from the crown to the rump.

The CRL is measured in millimetres and increases steadily day by day. By 6 weeks it may be just a few millimetres, while by 12 weeks it is typically around 55 to 65 mm.

Why Is the CRL So Important?

In early pregnancy, babies grow at a remarkably consistent and predictable rate. This makes the CRL the most accurate way to date a pregnancy. Because ovulation and conception dates can be uncertain, the CRL gives your care team a reliable, objective way to calculate your gestational age and set your estimated due date.

Research has shown that CRL dating is more accurate than calculating from the first day of your last menstrual period, which is why the NHS uses CRL to confirm your due date at the dating scan.

When Is the CRL Measured?

The CRL is most useful between 11 weeks and 14 weeks, which is why the NHS dating scan is offered in this window. Before 11 weeks, the baby is too small for the measurement to date the pregnancy as precisely. After 14 weeks, babies start to stretch out and grow at more individual rates, so CRL becomes less reliable and other measurements such as head circumference and femur length take over.

How Accurate Is It?

When measured between 11 and 14 weeks, the CRL can date a pregnancy to within about five to seven days. This is why, if your CRL gives a due date more than seven days different from the one based on your last period, your care team will usually switch to the scan-based date.

What Does Your CRL Number Mean?

Your scan report will show the CRL in millimetres alongside the gestational age it corresponds to. For example, a CRL of 60 mm typically equates to around 12 weeks and 3 days.

If the CRL is notably smaller or larger than expected, your care team may discuss possible reasons, which can include natural variation, uncertain dates, or, less commonly, a concern about growth. A single measurement is rarely enough to draw firm conclusions, and a follow-up scan is often arranged.

What If the CRL Is Behind Dates?

A slightly small CRL can simply mean your dates were slightly out. It can also reflect the natural variation between babies.

Summary

The CRL is a simple, safe, and highly accurate measurement that plays a central role in dating your pregnancy and confirming healthy early growth. Understanding what it is and what your number means can help you feel more confident when you read your scan report.