The ultrasonic measurement of fetal crown-rump length as a method of assessing gestational age.

The fetal crown-rump length (CRL) was measured by means of pulsed ultrasound, and the normal values between 47 and 101 days from the onset of the last menstrual period were determined from cross-sectional data in 253 patients. For any given CRL, the range was found to be within three days of the menstrual age with a maximum variation for a given occasion of three days. Detailed statistical analysis showed that the acceleration in the rate of change in the CRL was a constant and that the intrapatient variability, where measured, was low. A close correlation between the measurement and the date of ovulation was demonstrated in a further 40 patients.