What is crown to rump length (CRL)?
The crown rump length is a universally recognized term, very useful for measuring early pregnancies. The CRL (see image above between the two white + marked 1) is highly reproducible and is the single most accurate measure of gestational age. Charts have been developed for this purpose, but some simple rules of thumb can also be effectively used. From 6 to 11 weeks gestational age, the fetal CRL grows at a rate of about 1 mm per day. Crown Rump Length: Gestational age = 6 weeks plus (CRL x days). For example, a CRL of 23 mm would correspond to a gestational age of 9 weeks and two days (6 weeks plus 23 days = 9 weeks and 2 days). Accuracy of CRL after 12 weeks in predicting gestational age diminishes and is replaced by measurement of the width of the fetal head (biparietal diameter or BPD).
