Ovarian follicles are small, fluid-filled sacs within the ovaries, each containing an immature egg, or oocyte. While the end result of a menstrual cycle is typically the release of a single mature egg, the biological process leading up to this event involves the development of many follicles. This cyclical event, known as folliculogenesis, begins long before the egg is released. Hormonal signals determine which follicles will survive and which will not. The number of follicles involved provides insight into a person’s reproductive health.
The Follicular Starting Pool (Recruitment)
The initiation of follicular growth each month is known as recruitment, where a group of resting follicles begins the final stages of development. This process is triggered by a subtle rise in Follicle-Stimulating Hormone (FSH) released by the pituitary gland at the start of the follicular phase of the cycle. FSH acts as the primary signal, causing a cohort of small, early-stage follicles to start growing simultaneously.
The size of this initial cohort typically involves 15 to 20 or more follicles. These are recruited from the ovarian reserve, which is the total pool of remaining non-growing follicles. Recruitment is a continuous process that occurs regardless of whether ovulation was successful in the previous cycle.
Follicles that enter this recruitment stage are often in the early antral stage. These growing sacs become increasingly dependent on FSH for their continued development and survival. The existence of this large starting pool explains why the body prepares multiple follicles each month, even though the goal is usually a single ovulation.
All recruited follicles start to produce hormones like estrogen and inhibin. This simultaneous growth creates a competitive environment where each follicle vies for the hormonal support necessary to continue maturing. The number of follicles in this starting pool reflects the overall size of the remaining ovarian reserve.
Selection of the Dominant Follicle
The process of selection narrows the field down to one. As the cohort of growing follicles secretes increasing amounts of estrogen and inhibin, these hormones feed back to the pituitary gland. This feedback mechanism causes a progressive decline in the circulating levels of FSH in the bloodstream.
This drop in FSH creates a critical threshold that only the most developed and robust follicle can survive. The follicle that possesses the highest number of FSH receptors is able to continue growing despite the diminishing hormonal signal. This follicle is designated the dominant follicle.
The non-dominant follicles cannot survive the reduced FSH environment. These follicles undergo a process called atresia, which is a form of programmed cellular death. They break down and are reabsorbed by the body within the same cycle.
The dominant follicle continues to grow, maturing to a size of about 18 to 25 millimeters in diameter. It simultaneously transitions its dependence from FSH to Luteinizing Hormone (LH). The mature follicle produces a final surge of estrogen, which triggers a massive release of LH from the pituitary gland. This LH surge causes the dominant follicle to rupture, releasing the mature egg into the fallopian tube in the event known as ovulation.
Clinical Assessment: Antral Follicle Count
The Antral Follicle Count (AFC) is a clinical measurement used to assess the number of small, developing follicles present in the ovaries at the beginning of a cycle. This count is performed using a transvaginal ultrasound, typically between day two and day five of the menstrual cycle. The AFC counts visible follicles, generally those measuring between two and ten millimeters in diameter.
The number of follicles counted during an AFC provides an estimate of a person’s ovarian reserve, or the remaining supply of eggs. Since these small antral follicles represent the cohort that has been recruited for the current cycle, their number reflects the overall size of the dormant primordial follicle pool. A higher AFC suggests a larger ovarian reserve, while a lower count indicates a diminished reserve.
An AFC totaling 10 to 20 follicles across both ovaries is often considered a normal range, particularly for younger individuals. Counts below 4 to 7 are associated with a reduced ovarian reserve, while counts exceeding 25 may suggest conditions like Polycystic Ovary Syndrome (PCOS). These numbers are used to predict how a person might respond to ovarian stimulation medications used in fertility treatments.
The AFC only measures the follicles currently recruited and visible on the ultrasound, not the total number of eggs remaining in the ovaries. The overall ovarian reserve includes millions of microscopic, non-growing follicles, which cannot be directly counted. Therefore, the AFC serves as a valuable snapshot of the available pool of eggs that can be activated for growth.