A transvaginal or transabdominal ultrasound scan helps record various growth parameters of the baby, including the heart rate. One among the different growth measurements recorded is CRL- the prime measurement defining the actual growth or length of the baby.
During pregnancy, the baby floats in the amniotic fluid and is usually in the shape of a ‘C’ where the baby’s limbs are bent, touching its head. It is not possible to take a full-length measurement of the baby- from head to toe. The baby’s height, specifically from week 6 of pregnancy, is measured from its head to its buttocks or rump in an ultrasound scan. Hence the name CRL or Crown Rump Length.
This article discusses how CRL is measured in an ultrasound, the use of CRL in calculating the gestational age, and what a low CRL means?
In this Article
What is CRL measurement in Ultrasound Scan Report?
CRL stands for Crown Rump Length of the fetus measured during an ultrasound. The length from the head to the rump, buttocks, or lower back of the baby is viewed and noted in millimeters, and the limbs and yolk sac measurements are not included. The CRL measurements of the baby are usually taken from week 6 or week 7 of pregnancy to week 14 of pregnancy.
The above image is a sagittal section of the fetus where the head is in line with the body. The Orientation for accurate measurement is when the tip of the nose is a level of or above the horizontal abdominal wall. The image magnification is ideal when CRL occupies more than 60% of image space, and the entire crown-rump length is visible. CRL can be taken even if the limbs are not visible in the image.
The CRL measurements can differ when the baby stretches during the ultrasound. Ideally, the baby needs to be in a relaxed C position. The caliper’s (the unit of ultrasound that enables easy measurements) positioning during ultrasound also can give varying CRL measurements. Therefore, CRL is measured three times, and the mean of three acceptable measurements is used.
How is CRL useful?
CRL is an important parameter considered to calculate gestational age. Babies grow at different rates, and the calculation of gestational age based on CRL is just an approximation.
Here, mothers often get confused between fetal age and gestational age.
The gestational age is different from fertilization or fetal age. Gestation age is counted from the first day of the last menstrual period. It technically includes the period before conception or fertilization of gametes.
Fetal age or fertilization age is the age of the fetus calculated since fertilization of gametes which is approximately 14 days after gestational age.
For instance, 6 weeks of gestational age means the fetal age is 4 weeks. And so, CRL can help mothers confirm their gestational and fetal age.
Based on the studies conducted by Verburg et al., The gestational age in relation to the CRL measurements is tabulated as below with a graphical representation of the same.
|CRL (mm)||Gestational age (weeks + days)|
|5th centile||Median||95th centile|
|5||5 + 6||6 + 2||6 + 6|
|10||7 + 0||7 + 4||8 + 1|
|15||7 + 5||8 + 2||9 + 0|
|20||8 + 2||9 + 0||9 + 5|
|25||8 + 6||9 + 4||10 + 2|
|30||9 + 2||10 + 0||10 + 6|
|35||9 + 5||10 + 3||11 + 2|
|40||10 + 1||10 + 6||11 + 5|
|45||10 + 3||11 + 2||12 + 1|
|50||10 + 6||11 + 5||12 + 4|
|55||11 + 1||12 + 0||13 + 0|
|60||11 + 3||12 + 3||13 + 3|
|65||11 + 6||12 + 5||13 + 5|
|70||12 + 1||13 + 0||14 + 0|
|75||12 + 3||13 + 3||14 + 3|
|80||12 + 5||13 + 5||14 + 5|
|85||13 + 0||14 + 0||15 + 1|
|90||13 + 2||14 + 2||15 + 3|
|95||13 + 4||14 + 4||15 + 5|
|100||13 + 6||15 + 0||16 + 1|
The average fetal length (CRL) between 5 to 95 percentile helps estimate gestational age.
CRL of the baby falling between 5 to 95 percentile suggests normal fetal growth. However, Decreasing CRL for more than two months can indicate chromosomal anomalies like trisomy 18 and others that can cause stunted growth. These possibilities can further be eliminated by genetic screening of the fetus.
Once the CRL of the baby exceeds 7mm, it can detect the baby’s heartbeats. There is a chance of early pregnancy loss in case of no fetal heartbeat detection or nil cardiac activity.
If the difference between the mean sac diameter (MSD) and CRL <= 5 mm, there is a high possibility of a first-trimester pregnancy loss.
What are the factors influencing the CRL growth of the fetus?
The growth of the baby and the crown to rump length of the baby are greatly influenced by maternal factors such as
- Medications, and folic acid intake
- Lifestyle factors- Smoking, Alcohol, Caffeine
CRL or Crown Rump Length is an essential growth determining parameters of the fetus calculated between week 6 to week 14 of pregnancy. The length of the fetus from head to buttocks or rump can help estimate and reconfirm the gestational age. Decreased CRL can indicate chromosomal anomalies that can be confirmed further via genetic tests like NIPT. For other information on CRL and to better understand the CRL measurements of your ultrasound report, connect with your doctor.