Merck Manual

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Sonogram of a dominant follicle, mare
dominant-follicle-sonogram-horse-sertich-sized
Sonogram of a dominant follicle, mare

    Usually one follicle becomes dominant during estrus. Ultrasonography of the ovaries can allow accurate measurement of the follicle. The anechoic (black) follicle is 37.2 mm in diameter.

Courtesy of Dr. Patricia Sertich.