The fetal pole is a thickening on the margin of the yolk sac of a fetus during pregnancy. It is usually identified at 6.5 weeks with abdominal ultrasound imaging, and 6 weeks with vaginal ultrasound imaging. The fetal pole may be seen at 2-4mm crown-rump length (CRL), and heart motion is often detected when it is seen. In the embryo, the heartbeat is seen as a regular flutter, which should be first evident at 5mm CRL. If the embryo is less than 5mm CRL, it is possible for it to be healthy without showing a heartbeat, though a followup study in 5-7 days will almost always demonstrate the heartbeat.