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 2-4 mm 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 5 mm CRL. If the embryo is less than 5 mm CRL, it is possible for it to be healthy without showing a heartbeat, though a follow up study in 5-7 days will almost always demonstrate the heartbeat.

Embryology