Crown-Rump Length (CRL) Measurement Chart

Regular check-ups, prenatal screening, and scans are essential during pregnancy. Tests used in dating the pregnancy, estimating gestational age, and looking for chromosomal abnormalities might be prescribed by your doctor so your pregnancy can progress smoothly. Estimation of the crown-rump length (CRL) of the fetus is one such scan.  It is used in fetal ultrasound to estimate gestational age and monitor fetal growth, typically measured during the first trimester of pregnancy. It is also considered the most accurate measurement for determining gestational age in the first trimester of pregnancy, particularly between 7 and 13 weeks of gestation.

Also Read: Common Trimester Wise Tests during Pregnancy

What is CRL?

CRL is the length of the foetus, measured from the top of its head to its buttocks. This measurement is in centimetres and doesn’t consider the limbs or the yolk sac. Since CRL can be taken from about 6 to 7 weeks of pregnancy with an upper limit of 14 weeks, it is useful in calculating the foetus’s gestational age. The low biological variability at this stage of pregnancy makes it the most accurate estimation of your baby’s gestational age.

Once the gestational age is evident from the CRL, your doctor can provide an estimated delivery date. The earlier this scan is conducted, the more accurate it is. Note that the gestational age is different from the fertilisation age. Gestational age is calculated from the first day of your last menstrual period, whereas fertilisation age is typically two weeks less than the gestational age.

Uses of Crown Rump Length Measurement

Crown-rump length (CRL) has several important uses in obstetrics and gynaecology. These include:

1. Estimating gestational age

CRL is an accurate measure of gestational age during the first trimester of pregnancy, and is used to estimate the due date and monitor fetal development.

2. Monitoring fetal growth

CRL is also used to monitor fetal growth throughout pregnancy. Regular ultrasounds are performed to measure the crown-rump length and ensure that the fetus is growing properly.

3. Identifying abnormalities

CRL measurements can help identify abnormalities in fetal development. Deviations from the expected growth rate can be an early sign of a potential problem, prompting further testing and follow-up care.

4. Guiding delivery decisions

CRL measurements can also help guide decisions about delivery methods. For example, if the fetus is significantly larger or smaller than expected for its gestational age, a different delivery method (such as a C-section) may be recommended.

5. Evaluating multiple gestations

In cases of multiple gestations (such as twins or triplets), CRL measurements can help determine whether the fetuses are developing similarly. This information is used to monitor the health of the fetuses and plan delivery.

6. Determining fetal weight

CRL measurement can also be used to estimate fetal weight. Using mathematical formulas, the CRL measurement can predict fetal weight, which can be useful in determining appropriate delivery methods and planning for potential complications.

What Does Crown Rump Length Indicate About Your Baby’s Health?

The CRL scan assists your doctor in evaluating the well-being and development of your baby in the womb. An average baby measures about 51 cm tall and weighs around 3.5 kg at the time of birth. With this scan, it’s possible to find out your baby’s length and weight at different stages of your pregnancy. These are some issues that CRL helps to reveal:

• Presence of a heartbeat – If the CRL measurement is 7 mm or more, a transvaginal ultrasound can detect the foetus’ heartbeat. This type of ultrasound is conducted through the vagina instead of from outside which is how most ultrasounds are done.
• Miscarriage – A CRL can reveal the absence of a heartbeat and, in turn, a missed miscarriage. In such cases, the expecting mom doesn’t suffer from the usual symptoms of miscarriage like pain or bleeding. Also known as a silent miscarriage, the placenta keeps on producing the pregnancy hormone, leading the woman to believe that she is still pregnant.  
• If your mean sac diameter (MSD) is less than 5 mm greater than the CRL measurement, a first-trimester miscarriage may be impending. This can occur in spite of detecting a normal heartbeat in your baby.
• CRL measurements that are on the lower side may also be indicative of chromosomal abnormalities such as Edwards Syndrome (trisomy 18), triploidy, or other growth-related problems.
• According to a study, there is a direct correlation between CRL measured before the 10th gestational week and birth weight. This is a one-of-a-kind study as other studies focus on the correlation between both factors only after the 10th week of gestation. A total of 632 ultrasound scans were performed on pregnant women who didn’t have any complications. The results indicated that there is a positive association between CRL in the early first trimester and the birth weight of the foetus. This can help to predict low birth weight (LBW) and ensure parents-to-be are emotionally prepared for premature delivery and newborn care.

Does It Determine the Gender of Baby?

Crown-rump length (CRL) is used in fetal ultrasound to estimate gestational age and monitor fetal growth. However, it is not a reliable method for determining the gender of a baby. Research has shown no significant difference in CRL measurements between male and female fetuses. While there are some claims that CRL can be used to predict gender based on the size of the fetal genital tubercle, these claims have not been supported by scientific evidence.

Instead, fetal gender is typically determined by ultrasound during the second trimester of pregnancy (between 18-22 weeks), when the genitalia are more developed and can be visualised on the ultrasound image. Even then, there is a small margin of error in gender determination, and ultrasound technicians may not always be able to determine the gender with certainty. Therefore, while CRL is a useful measurement in estimating gestational age and monitoring fetal growth, it is not a reliable method for determining the gender of a baby.

Crown Rump Length Chart

The CRL diagram was first presented by Robinson in 1975 and is, still, the primary reference for pregnancy dating as well as evaluation. Below, we provide a CRL chart for your reference.

These are the approximate foetus measurements at different stages of pregnancy. Other factors, such as the mother’s age, smoking habits, and the amount of folic acid consumed, may influence crl in pregnancy.

Every baby is different and slight variations in growth and development are normal. Your doctor’s benchmark measurements may also differ from the CRL chart. After a CRL scan, make sure you have a chat with your gynaecologist about your baby’s progress and ask for a detailed report on the same.

FAQs

1. Can the CRL measurement be incorrect?

Like all measurements, CRL can be incorrect in some cases. Factors such as fetal position, maternal obesity, and technical errors during the ultrasound, can affect the accuracy of the measurement. However, CRL is considered a highly accurate method for estimating gestational age in the first trimester of pregnancy.

2. What if CRL is lower than expected?

A lower-than-expected CRL measurement may indicate a problem with fetal growth. Further testing may be recommended to identify potential issues in such cases.

3. Is CRL smaller for females?

Scientifically, there is no significant difference in CRL measurements between male and female fetuses. However, gender can be accurately determined by ultrasound around 18-20 weeks gestation, where the genitals become visible on the ultrasound.

4. What is the normal range of CRL at 8 weeks?

At 8 weeks gestation, a normal range for CRL in pregnancy is typically between 16-24 mm. It is important to note that the range may vary depending on the accuracy of the measurement and the individual characteristics of the fetus and mother.

Also Read: Giving Birth To a Baby With Down’s Syndrome