Crown Rump Length (CRL) on Ultrasounds

Crown-rump length is an ultrasound measurement that's used during pregnancy. The baby is measured, in centimeters, from the top of their head (crown) to the bottom of their buttocks (rump).

When measuring the baby, their limbs and the yolk sac aren't included. The crown-rump length can be measured starting around six or seven weeks of pregnancy up to 14 weeks. Here's what you need to know.

How Crown-Rump Length Is Used to Date Pregnancy

Between eight weeks and 14 weeks, your health care provider can use your baby's crown-rump length to date your pregnancy. Typically, pregnancy dating was done by adding nine months and seven days to the date of your last menstrual cycle.

But providers have realized that the crown-rump length can be more useful in predicting your due date. This information is particularly helpful in early pregnancy because, after 24 weeks, an ultrasound is less accurate in predicting your baby's gestational age.

What Does Abnormal Crown-Rump Length Mean?

Once the fetus's crown-rump length surpasses 7 millimeters, a heartbeat should be detected by ultrasound. If no heartbeat or cardiac activity is found, then the pregnancy is likely a missed miscarriage.

A missed, or silent, miscarriage typically occurs without the normal miscarriage symptoms. Outward signs of a miscarriage are often masked because the placenta may continue to supply hormones.

People with a mean gestational sac diameter (MSD) of less than 5 millimeters greater than the crown-rump length are more likely to experience a first-trimester miscarriage—even if the baby has a normal heart rate. Decreased crown-rump length is associated with chromosomal anomalies like trisomy 18 and other trisomies associated with growth restriction.

Common chromosomal abnormalities

Here are some potential chromosomal abnormalities that may be tied to shorter crown-rump length.

• Down Syndrome: When a baby has Down syndrome, they have an extra copy of chromosome 21. Medical professionals aren't sure exactly what causes Down syndrome, though age at the time of pregnancy may play a role.
  
• Turner Syndrome: This syndrome only impacts babies assigned female at birth. When a baby has Turner syndrome, one of their X chromosomes is missing or altered. Most people with Turner syndrome will not go through puberty unless they receive hormone therapy.
• Edwards Syndrome (Trisomy 18): A baby with Trisomy 18 will experience severe developmental delays due to an extra chromosome 18. Most babies born with Trisomy 18 will live only about four days, and about 5% to 10% will live until they are 1 year old.
• Patau Syndrome (Trisomy 13): Babies with Patau syndrome have an extra copy of chromosome 13, which can cause severe intellectual disability and physical abnormalities. Trisomy 13 occurs in occurs in about 1 in 16,000 babies. Because many babies with Patau syndrome have life-threatening complications, most will not live beyond a few days after birth.
  

Other Measurements During an Ultrasound

Early in the pregnancy, health care providers also use the gestational sac, yolk sac, and heart rate to verify pregnancies and to provide insight into the health of the pregnancy. Deviations from what's anticipated may warrant additional investigation.

After 14 weeks, a combination of head circumference, biparietal diameter, abdominal circumference, and femur length are measured to come up with a gestational age and/or weight.