Antral follicle count (AFC) is an ovarian reserve test performed by transvaginal ultrasound. It is generally only performed by fertility specialists. The test requires 2D-transvaginal ultrasound of ≥7MHz frequency to visualize the ovaries. The small follicles in the ovary (2-9mm) are counted. The result can be imprecise in the presence of ovarian pathology such as endometriomas or other ovarian cysts. It is subject to significant physiologic cycle-to-cycle variation as well as inter-observer variability.

AFC is a ‘functional’ ovarian reserve test intended to estimate the ‘true’ ovarian reserve: the number of primordial follicles remaining. The number of follicles available for gonadotropin stimulation each month (antral follicles) is proportionate to the number of primordial follicles (oogonia) remaining. Pathology studies confirm that AFC provides a reasonable estimate of ovarian reserve. In the setting of in vitro fertilization, AFC can be helpful to estimate the number of oocytes expected during ovarian stimulation. The normal range for AFC is highly dependent on age. A total AFC of 5-7 is considered low. A finding of more than 12 follicles on each ovary is termed ‘polycystic’ although that is not sufficient to meet the diagnostic criteria for PCOS.