The fetal pole 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).
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Fetal pole Wikipedia(Text) CC BY-SA