fetal pole

https://doi.org/10.1351/goldbook.10715

Thickening on the margin of the yolk sac of a fetus during pregnancy.

Note: Appearance of the fetal pole is used as the earliest sign of a baby in ultrasound examination.
Source:
PAC, 2016, 88, 713. (Glossary of terms used in developmental and reproductive toxicology (IUPAC Recommendations 2016)) on page 755 [Terms] [Paper]