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 different 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. This ultrasound scan is one of the vital measures necessary for antenatal care during the first trimester and is done between six and thirteen weeks approximately. The crown to rump length, also known as the greatest length (GL), is an accurate measurement in centimeters to determine the age of the baby. 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 heart rate of an embryo 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 early stages 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 development of a baby, any delay in the fetal growth measurement 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 growth retardation of the fetus. Hence, the ultrasonographic screening during the first trimester also helps the doctor figure out if there are any chromosomal anomalies and risks such as Patau syndrome, Down syndrome, and Edwards syndrome (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 scans are used to keep a watch on aspects such as femur length, abdominal circumference, and fetal biparietal diameter (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). In addition to this, doctors use the crown-rump length chart as a reference to perform the primary evaluation. An example of such, the Monash chart, is given below (8). 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 your baby’s health rather than rely on the charts alone. Did you try estimating your baby’s health using CRL measurements? Let us know about your experiences.
Accurate magnification of the image on the scan The neutrality of the fetus The horizontal position of the fetus Precise placement of calipers Presence of an amniotic fluid pocket under the fetus’s chin
