Crown-Rump Length Measurement in Pregnancy

Definition and Measurement Technique

The American College of Radiology defines crown-rump length (CRL) as the greatest linear dimension of the embryo or fetus measured from the top of the head to the bottom of the buttocks, excluding the limbs and yolk sac, and serves as the most accurate method for determining gestational age in the first trimester 1
The CRL represents the maximum straight-line measurement of the embryo/fetus from crown to rump, measured in millimeters, and becomes visible on transvaginal ultrasound at approximately 6 weeks gestational age when the embryonic pole first appears 1, 2, 3, 4
The measurement should be reported in millimeters, not centimeters, to ensure accuracy 2, 3

The American College of Obstetricians and Gynecologists recommends CRL measurement as the most accurate estimation of gestational age in early pregnancy, with precision within ±5-7 days when measured between 8-13 weeks gestation 5
When there is a discrepancy between LMP-based dating and CRL-based dating in the first trimester, the American College of Obstetricians and Gynecologists recommends adopting the gestational age determined by CRL measurement 5
CRL measurements between 39-84 mm correspond to the optimal screening window of 11+0 to 13+6 weeks gestation, though no universally accepted equation exists for CRL-to-gestational age conversion 2, 3

The American Institute of Ultrasound in Medicine recommends that an embryo with CRL ≥7 mm must demonstrate cardiac activity; absence of cardiac activity at this size is diagnostic of early pregnancy loss (embryonic demise) 1, 7, 8
In twin or higher-order multiple pregnancies, individual CRL measurements should be obtained for each embryo/fetus to assess for growth discordance, which may indicate early twin-to-twin transfusion syndrome, according to the Society for Maternal-Fetal Medicine 9

The American Institute of Ultrasound in Medicine recommends that sonographers performing CRL measurements for prenatal screening should provide paired NT/CRL measurements from at least 30 pregnancies to validate appropriate reference data before clinical interpretations 6, 2
Laboratories should require documentation of specialized training and successful submission of NT images from qualified sonographers through programs such as the NT Quality Review (NTQR) Program or Fetal Medicine Foundation (FMF), as recommended by the National Society of Genetic Counselors 6, 2, 3