In plain words
Crown-rump length, or CRL, is a measurement taken on ultrasound in the first trimester. It runs from the top of the head to the bottom of the body, not including the legs. Because early growth is steady, this length is a good way to estimate how far along a pregnancy is. The American College of Obstetricians and Gynecologists treats it as the most accurate way to date a pregnancy in the first trimester, within about five to seven days. It is used when the due date from the last period is unclear or when it conflicts with an early scan. A mother may hear this number at an early ultrasound and be told a new due date. A CRL is not used after the first trimester, because growth varies more widely later.
The deeper version
Crown-rump length is measured in the sagittal plane with the fetus in neutral position, typically between about 45 and 84 mm, corresponding to roughly 9 to 14 weeks. Robinson and Fleming (British Journal of Obstetrics and Gynaecology, 1975) established the linear relationship with gestational age that remains the basis for early dating. Hadlock and colleagues (Radiology, 1992) refined the CRL with high-resolution real-time ultrasound. ACOG Committee Opinion 700 (2017) sets the rule: a first-trimester CRL should override menstrual dating when the discrepancy exceeds a set number of days, and the estimate carries a stated error of about five to seven days. Controversy persists about how to handle discrepancies when a patient is uncertain of the last menstrual period and about the reliability of CRL in very early or very late embryos.
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Sources
- American College of Obstetricians and Gynecologists — Committee Opinion 700: Methods for Estimating the Due Date, 2017
- Robinson HP, Fleming JE — A critical evaluation of sonar crown-rump length measurements. British Journal of Obstetrics and Gynaecology, 1975