- The CRL is a reproducible and accurate method for measuring and dating a fetus.
- Early ultrasonographers used this term (CRL) because early fetuses also adopted the sitting in the chair posture in early pregnancy. After 12 weeks, the accuracy of CRL in predicting gestational age diminishes and is supplemented by measurement of the fetal biparietal diameter.
In at least some respects, the term “crown rump length” is misleading:
There is no fetal crown and no fetal rump to measure for most of the first trimester.
Until 53 days from the LMP, the most caudad portion of the fetal cell mass is the caudal neurospone, followed by the tail. Only after 53 days is the fetal rump the most caudal portion of the fetus.
Until 60 days from the LMP, the most cephalad portion of the fetal cell mass is initially the rostral neuropore, and later the cervical flexure. After 60 days, the fetal head becomes the most cephalad portion of the fetal cell mass.
What is really measured during this early development of the fetus is the longest fetal diameter.
From 6 weeks to 9 1/2 weeks gestational age, the fetal CRL grows at a rate of about 1 mm per day.