What Crown Rump Length (CRL) Means on Ultrasound

Crown-rump length (CRL) is an ultrasound measurement that is used during pregnancy. The baby is measured, in centimeters, from the top of their head (crown) to the bottom of their buttocks (rump). The limbs and yolk sac are not included in the measurement. The CRL can be measured starting around six or seven weeks of pregnancy up until 14 weeks.

CRL may be useful in calculating gestational age. With this gestational age, doctors can estimate your potential due date. The earlier the first ultrasound is performed, the more accurate the baby's gestational age will be.

Once the fetus has developed past 14 weeks, head circumference, biparietal diameter, and femur length measurements are used to determine how the baby is progressing. The length of the umbilical cord is typically the same as the CRL throughout pregnancy. 

Once the fetus's CRL surpasses 7 mm, a heartbeat should be detected by transvaginal ultrasound –– an ultrasound that is performed through the vagina and not on top of the abdomen like a standard ultrasound.

If no heartbeat or cardiac activity is detected, then the pregnancy is likely a missed miscarriage. A missed, or silent, miscarriage typically occurs without the normal miscarriage symptoms. The placenta may continue to supply hormones, which can mask the outward signs of a miscarriage. 

Women with a mean sac diameter (MSD) of less than 5 mm greater than the CRL are more likely to experience a first-trimester miscarriage –– even if the baby has a normal heart rate. Decreased crown-rump length can also diagnose chromosomal anomalies like trisomy 18 (Edwards syndrome) and other trisomies associated with growth restriction.