The Antral Follicle Count (AFC) is an initial measure used during fertility monitoring to estimate a woman’s potential response to ovarian stimulation. The AFC, observed via ultrasound, serves as a predictive tool for ovarian reserve—the remaining supply of eggs available. This count provides a baseline understanding of how many egg-containing sacs might be recruited and grown during an in vitro fertilization (IVF) cycle. Specialists use this initial count to tailor medication protocols, though the final number of eggs retrieved is influenced by complex biological processes.
Understanding Ovarian Follicles
Ovarian follicles are small, fluid-filled sacs within the ovaries, each containing an immature egg (oocyte). These structures support the egg through its development until it is ready for release. At birth, ovaries contain millions of primordial follicles, which are the earliest stage of development.
The Antral Follicle Count (AFC) specifically measures follicles that have developed a small, fluid-filled cavity called the antrum. These antral follicles are typically 2 to 10 millimeters in diameter and are visible via transvaginal ultrasound early in the menstrual cycle.
The AFC indicates the pool of eggs available for recruitment during IVF stimulation. While a natural cycle selects only one dominant follicle, fertility medication stimulates multiple antral follicles to grow simultaneously, aiming to bring several eggs to maturity for retrieval.
The Relationship Between Follicles and Egg Retrieval
Determining the number of eggs retrieved from a specific follicle count depends on size and selection. When eight follicles are observed at the start of a stimulation cycle, this represents the potential pool of structures that may grow. During the stimulation phase, these follicles are monitored to ensure they respond to medication and increase in size.
For a follicle to yield an egg ready for retrieval, it must reach a mature diameter, typically 16 to 22 millimeters. Since not all follicles grow at the same rate, the initial count rarely equals the final number of mature follicles. Even if all eight follicles reach the optimal size, the number of eggs retrieved is usually lower than the total follicle count.
Clinical data shows that retrieval yields an egg from 60 to 80 percent of the mature follicles aspirated. Therefore, if eight follicles are successfully stimulated, a woman could expect to retrieve approximately five to seven eggs. This discrepancy happens because some follicles may not contain an egg, or the egg may remain adhered to the follicular wall during aspiration.
Factors Influencing Egg Yield and Maturity
The final egg yield depends heavily on dynamic factors toward the end of the stimulation phase, especially the timing of the final hormone injection. The most important step before retrieval is the administration of the “trigger shot,” given approximately 34 to 36 hours prior to the procedure.
This injection, often containing human chorionic gonadotropin (hCG), mimics the natural luteinizing hormone surge. The trigger shot induces the final stage of egg maturation (meiosis) and causes the egg to detach from the follicle lining. If the injection is mistimed, retrieved eggs may be too immature to fertilize or may have already begun to degrade.
Follicle size uniformity also affects the outcome; if the eight follicles vary widely in size, the trigger shot timing will only be optimal for those within the 16 to 22 millimeter range. Technical factors during the retrieval procedure also influence the final count. Even if a follicle appears the correct size, the aspiration needle may not successfully draw out the egg due to adhesion issues or difficulty accessing the ovary.
Beyond the Count: Egg Quality and Fertilization
While the number of eggs retrieved is important, the ultimate measure of success shifts to the intrinsic quality and viability of those eggs. Not every egg collected will be suitable for the next stages of the process, as they must first be assessed for maturity by the embryologist.
Even if five to seven eggs are retrieved from the eight initial follicles, typically only about 80 percent of those will be mature enough to proceed with fertilization. Once maturity is confirmed, the eggs are combined with sperm, and the fertilization process begins. On average, only about 80 percent of the mature eggs successfully fertilize to form a zygote. This means that there is a natural attrition rate that narrows the pool of viable embryos.
The process of development continues for several days in the laboratory, during which the fertilized eggs must progress through various cell divisions. Not all embryos continue to grow, and only about 30 to 50 percent of fertilized eggs typically reach the blastocyst stage, which is the preferred stage for transfer or cryopreservation. This natural selection highlights why quality frequently outweighs quantity in fertility treatment, as the goal is to produce the highest quality embryo possible.