As the pregnancy advances, the embryo is easier to see, and accurate measurements can be obtained with transabdominal ultrasound. A crown-rump length (CRL) measurement. This ultrasound scan shows the CRL, which is the average of discrete fetal measurements from the tip of the head end to the tip of the rump end in the midsagittal plane of the embryo. This is highly accurate for pregnancy dating. Images courtesy of RSNA