The fetal pole (sometimes referred to as a tom) is a thickening on the margin of the yolk sac of a fetus during pregnancy. It is usually identified at 8 weeks with abdominal ultrasound imaging, and 6 weeks with vaginal ultrasound imaging. However it is quite normal for the fetal pole to not be visible until about 9 weeks. 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.