Merck Manual

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Sonogram of 13-day conceptus, mare
13-day-conceptus-sonogram-mare-sertich-sized
Sonogram of 13-day conceptus, mare

    The anechoic (black) yolk sac moves throughout the uterine horns and body from day 6–16 of gestation. A specular reflector (white dash) is imaged at the dorsal and ventral aspect of the yolk sac.

Courtesy of Dr. Patricia Sertich.