The antral follicle count is a method for ovarian reserve testing using transvaginal ultrasound.

Ovarian follicles are the name given to the small ovarian cysts that contain eggs. At the beginning of a menstrual cycle, using ultrasound you can visualize and even measure these tiny follicles in the ovary. A normal follicle will be less than 9 or 10 mm in diameter.

Studies have demonstrated that as a woman ages, the number of these follicles that are visible at the beginning of the menstrual cycle decreases significantly. It likely represents a decrease in the number of viable eggs that remain in the ovary.

In addition, younger women with a decreased ovarian reserve may also have a smaller number of these follicles. This is demonstrated by the fact that women with a diminished antral follicle count respond poorly to fertility medications, have a greater likelihood of having IVF cycles canceled for poor response, and when they do complete IVF cycles, their pregnancy rates are lower

Studies differ on the exact number of follicles that would be considered decreased but most would be in agreement that a total number of less than four is very low and therefore predictive of decreased ovarian reserve.