How To Measure Crown-Rump Length (CRL) In Pregnancy?

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Crown-rump length or CRL is the measurement of the fetus from top to bottom. In this post, we explain how to measure crown-rump length in pregnancy.

During pregnancy, your doctor may prescribe several prenatal tests, scans, or procedures to ensure the proper growth and safety of the developing fetus.

The doctor suggests these tests based on genetics, medical history, and other pregnancy-related complications. However, CRL is a vital procedure that helps evaluate the baby’s growth and development. Read on to learn more about CRL, what it tells about the baby, and how it is measured.

What Is CRL?

CRL or crown-rump length is the measurement of the fetus or C-shaped embryo from the top to bottom or the crown to rump, excluding the limbs. Thisultrasound scanis one of the vital measures necessary for antenatal care during thefirst trimesterand is done between six andthirteen weeksapproximately.

The crown to rump length, also known as the greatest length (GL), is an accurate measurement in centimeters to determine the fetal age.It also helps to assess the accurate gestational age, which helps doctors to know more about the baby’s health and also estimate the date of delivery(1)(2).

What Does CRL Tell You About A Baby’s Health?

The CRL scan will let the doctor learn about the early developmental stages of your baby. Some of them include:

• Tracking heart rate:The embryonic heart rate from 6-6.2 weeks is around 100bpm or beats per minute, while it increases to 120-160bpm between 6.3 and 7 weeks(3). Your doctor will keep track of this through the crown-rump length and gestation age to determine how healthy your baby is.

• Identifying complications:The scan helps detect complexities that may arise during the earlystages of pregnancy. For instance, if MSD (mean gestational sac diameter) – CRL = less than 5mm, the risk of pregnancy loss is around 8%.If the difference between MSD and CRL is between 6 and 10mm, the risk is 3-4%.And if it is greater than 10mm, the loss would be less than 1%(4). Identifying these signs in time will help you avoid additional complications.

• Monitoring fetal growth:As the CRL measures the fetal development, any delay or discrepancy in the fetal growth could be a sign of impending loss(5). That is why doctors keep monitoring the length and mass throughout the pregnancy.

• Recognizing abnormalities:Comparing CRL and mean sac diameter helps to detect fetal size and growth retardation. Hence, the ultrasonographic screening during the first trimester also helps the doctor figure out if there are any chromosomal anomalies and risks such asPatau syndromeⁱXChromosomal condition causing severe physical and intellectual disabilities,Down syndromeⁱXGenetic condition resulting in distinctive physical and intellectual limitations, andEdwards syndromeⁱXSevere chromosomal condition affecting fetal growth and causing developmental delays(6).

After the first trimester, CRL might not be an accurate measurement because the baby grows in size and becomes active.And so, some other obstetric ultrasound sonography scans are used to keep a watch on aspects of fetal weight and fetal anatomy such as femur length, abdominal circumference, and fetal biparietaldiameterⁱXDistance between bones forming the top and sides of the skull, used to estimate the size of the fetus(7).

How Is CRL Measured?

Doctors use various formulations to evaluate the CRL, gestational age, and other aspects. One of them is a simple math equation to find the gestation age using the CRL. According to that, the gestational age equals to six weeks plus crown-rump length multiplied by a number of days [GA = 6 weeks + (CRL x days)] (2).

此外,医生使用crown-rump勒ngth chart as a reference to perform the primary evaluation. An example of such, the Monash chart, is given below(8).

CRL (mm)	GA (wks, days)	GA (days)
1.0 mm	6W1D	42.9
1.5 mm	6W1D	43.3
2.0 mm	6W2D	43.7
2.5 mm	6W2D	44.1
3.0 mm	6W3D	44.5
3.5 mm	6W3D	45.0
4.0 mm	6W3D	45.4
4.5 mm	6W4D	45.8
5.0 mm	6W4D	46.2
5.5 mm	6W5D	46.6
6.0 mm	6W5D	47.0
6.5 mm	6W5D	47.4
7.0 mm	6W6D	47.8
7.5 mm	6W6D	48.2
8.0 mm	7W0D	48.6
8.5 mm	7W0D	49.1
9.0 mm	7W0D	49.5
9.5 mm	7W1D	49.9
10.0 mm	7W1D	50.3
10.5 mm	7W2D	50.7
11.0 mm	7W2D	51.1
11.5 mm	7W3D	51.5
12.0 mm	7W3D	51.9
12.5 mm	7W3D	52.3
13.0 mm	7W4D	52.7
13.5 mm	7W4D	53.2
14.0 mm	7W5D	53.6
14.5 mm	7W5D	54.0
15.0 mm	7W5D	54.4
15.5 mm	7W6D	54.8
16.0 mm	7W6D	55.2
16.5 mm	8W0D	55.6
17.0 mm	8W0D	56.0
17.5 mm	8W0D	56.4
18.0 mm	8W1D	56.8
18.5 mm	8W1D	57.3
19.0 mm	8W2D	57.7
19.5 mm	8W2D	58.1
20.0 mm	8W2D	58.5
20.5 mm	8W3D	58.9
21.0 mm	8W3D	59.3
21.5 mm	8W4D	59.7
22.0 mm	8W4D	60.1
22.5 mm	8W5D	60.5
23.0 mm	8W5D	60.9
23.5 mm	8W5D	61.4
24.0 mm	8W6D	61.8
24.5 mm	8W6D	62.2
25.0 mm	9W0D	62.6
25.5 mm	9W0D	63.0
26.0 mm	9W0D	63.4

GA:gestational age, W: weeks, and D: days.

The chart is considered accurate for 6 to 9 weeks of the gestation period. However, you should not rely on it completely as it is only an approximation. As every pregnancy is different, you can come across multiple dissimilarities. Therefore, it is always better to consult your doctor and get regular reports regarding the fetal well-being rather than rely on the charts alone.

Did you try estimating your baby’s health using CRL measurements? Let us know about your experiences.

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