Crown rump length
Crown rump length (CRL) is the length of the embryo or fetus from the top of its head to bottom of torso. It is the most accurate estimation of gestational age in early pregnancy, because there is little biological variability at that time.
Measurement
CRL is measured as the largest dimension of embryo, excluding the yolk sac and extremities. It is used as a primary measure of gestational age between 6-13 weeks. After 13 weeks, head circumference, biparietal diameter, and femur length measurements become more useful measurements for assessing fetal growth.
Practical points
The earlier in pregnancy a scan is performed, the more accurate the age assignment from crown rump length 4. If the original CRL measurement was adequate, the measurement is considered the baseline for all subsequent age measurements.
Overall, the accuracy of sonographic dating in the first trimester is +/-5 days (95% confidence range).
Cardiac activity should be present in an embryo with a CRL ≥7 mm 3. If it not detected at this size on transvaginal scanning performed by an experienced operator, it is an indicator of failed early pregnancy (missed miscarriage).
It has been reported that patients in whom MSD (mean sac diameter) is less than 5 mm greater than crown rump length (i.e. MSD - CRL = <5 mm) are prone to first trimester miscarriage, despite a normal heart rate.
Chromosomal anomalies, particularly trisomy 18 and triploidy are markedly associated with growth restriction, i.e. decreased crown rump length.
Related Radiopaedia articles
First trimester of pregnancy
-
first trimester
- ultrasound findings in early pregnancy
- confirming intrauterine gestation
- pregnancy of unknown location (PUL)
- first trimester vaginal bleeding
- early structural scan
- aneuploidy testing
Ultrasound - obstetric
- ultrasound (introduction)
- obstetric ultrasound
- first trimester and early pregnancy
- ectopic pregnancy
- multiple gestations
- subchorionic hematoma
- failed early pregnancy
-
second trimester
- fetal biometry
- fetal morphology assessment
- fetal echocardiography views
- non-visualization of the fetal stomach
-
soft markers
- nuchal fold thickness
- ventriculomegaly
- absent nasal bone
- choroid plexus cysts
- enlarged cisterna magna
- shortened fetal long bones
- echogenic intracardiac focus (EIF)
- echogenic fetal bowel
- aberrant right subclavian artery
- fetal pyelectasis / fetal renal pelvic dilatation
- single umbilical artery
- sandal gap toes
- amnioreduction
- Doppler ultrasound
- nuchal translucency
- 11-13 weeks antenatal scan
- chorionic villus sampling (CVS) and amniocentesis
- placenta
- other