Antral follicles are small follicles (2 to 8 mm in size) that are visible on the ovaries via ultrasound. They are also known as resting follicles. They appear in the beginning of the menstrual cycle, and their number can indicate the amount of microscopic primordial follicles contained within the ovary. An ultrasound exam to count the number of antral follicles may be performed as a part of fertility testing.
A low number of antral follicles may indicate poor ovarian reserves. In other words, the total number of eggs left in the ovaries may be low. Low antral follicle counts may also predict a poorer response to fertility drugs meant to stimulate ovulation.
An abnormally high number of antral follicles may indicate PCOS.