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You can see the article here: Wikipedia:Fetal pole
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 transabdominal ultrasound imaging, and 6 weeks with transvaginal ultrasound imaging. It may be seen at 2-4mm Crown-Rump Length (CRL - Longest Axis), 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.