Summary. Fetal crown-rump length (CRL) was determined by ultrasound in 105 healthy pregnant women. Each fetus was measured two to four times between the 7th and 14th week, giving a total of 300 observations. The descriptive curve of CRL as a function of age was very similar to those published by others. The longitudinally collected observations showed that CRL in female fetuses was significantly smaller compared with that in male fetuses (P<0·01). There was no demonstrable effect of maternal age, height or parity on fetal CRL. Measurement of CRL enables assessment of gestational age to within ±4–6 days (95% confidence interval), with greatest precision at CRL measurements of 31–40 mm. The discrepancy between ultrasound age as estimated from a CRL measurement and menstrual age may be a variable of prognostic significance in threatened abortion and maternal diabetes.