Measurement of the mean gestational sac diameter is the first parameter that can be used to determine gestational age. • The rate of growth of the normal gestation sac is 1.1 mm/day. • Measurement of the crown–rump length (CRL) between 6 and 12 weeks is the most accurate dating parameter.

How do you measure crown rump length?

Crown Rump Length (CRL) on Ultrasounds The baby is measured, in centimeters, from the top of their head (crown) to the bottom of their buttocks (rump). 1 The limbs and yolk sac are not included in the measurement. The CRL can be measured starting around six or seven weeks of pregnancy up until 14 weeks.