Crown-rump length

The embryo and fetus float in the amniotic fluid inside the uterus of the mother usually in a curved posture resembling the letter C. The measurement can actually vary slightly if the fetus is temporarily stretching (straightening) its body.

The measurement needs to be in the natural state with an unstretched body which is actually C shaped.

Different human fetuses grow at different rates and thus the gestational age is an approximation.

The length of the umbilical cord is approximately equal to the CRL throughout pregnancy.

Some authorities however casually interchange these terms [citation needed] and the reader is advised to be cautious.

A commonly used estimate of gestational age in weeks is (as described by Verburg et al.[2]):

An ultrasound showing an embryo measured to have a crown-rump length of 1.67 cm and estimated to have a gestational age of 8 weeks and 1 day
Diagram showing crown-rump length by gestational age. The blue line is the mean and the green area delimits the 3rd versus the 97th percentile . [ 1 ]
Ultrasound image of the foetus at 12 weeks of pregnancy in a sagittal scan. Measurements of fetal Crown Rump Length (CRL).