In vitro fertilization (IVF) involves stimulating the ovaries to produce multiple eggs. The successful outcome of an IVF cycle is closely tied to the number of eggs retrieved. During ovarian stimulation, the medical team aims to maximize the number of mature eggs, increasing the chances of creating viable embryos for transfer. Monitoring the growth and quantity of fluid-filled sacs called follicles is the central method doctors use to track progress and determine the best timing for egg retrieval.
Follicles Defined: The Role in IVF Monitoring
An ovarian follicle is a small, fluid-filled sac within the ovaries that holds the potential to contain an immature egg (oocyte). While only one follicle typically matures during a natural cycle, IVF uses hormonal medications to encourage a cohort of follicles to develop simultaneously. Follicles are monitored in two phases: before and during the stimulation process.
Before treatment, a transvaginal ultrasound counts the antral follicles, which are small, resting follicles measuring two to nine millimeters in diameter. The Antral Follicle Count (AFC) estimates the patient’s ovarian reserve and helps predict how the ovaries will respond to stimulation medication. Once stimulation starts, developing follicles are measured and counted every few days via ultrasound to track their growth. This ensures the follicles are progressing toward the size required to contain a mature egg.
Identifying the Optimal Follicle Count
The optimal follicle count balances maximizing egg retrieval with patient safety. Most fertility specialists target between six and 15 mature follicles at the time of the trigger shot. This range is ideal because it provides sufficient eggs for fertilization and embryo selection without the risks associated with overstimulation.
Cycles resulting in 10 to 15 eggs are statistically linked to the best cumulative live birth rates. Conversely, a low count of five or fewer follicles significantly reduces the probability of a live birth. For patients aged 35–40, the live birth rate was less than 20% with one to three follicles but increased to 25–40% with four or five follicles. The follicle count directly correlates with the number of eggs retrieved, which determines the pool of embryos available for transfer.
Beyond the Count: Follicle Size and Maturity
While the number of follicles is important, IVF success hinges on the maturity of the eggs they contain. Follicles must reach a specific size range to yield a mature, retrievable egg. The ideal size for a mature follicle is between 16 and 22 millimeters in diameter on the day of the trigger shot.
Follicles under 10 millimeters are unlikely to contain a mature egg, and those between 12 and 15 millimeters may yield immature oocytes. Follicles exceeding 22 millimeters may contain post-mature eggs or be cystic, compromising egg quality. The timing of the trigger shot—a final hormone injection to induce maturation—is precisely calculated. This requires at least two or three of the largest follicles to reach the threshold size of 17 to 18 millimeters.
Factors Influencing Ovarian Response
The number of follicles produced in response to stimulation is determined by pre-existing and adjustable factors. Maternal age is the most important predictor of egg number and quality, as ovarian reserve naturally declines over time. Older patients generally have fewer antral follicles, require higher medication doses, and result in fewer eggs retrieved.
Ovarian reserve is estimated using two markers: the Anti-Müllerian Hormone (AMH) level and the Antral Follicle Count (AFC). AMH is secreted by small follicles, and its blood level correlates with the remaining egg supply. A low AMH or AFC count suggests a diminished ovarian reserve and predicts a low response to stimulation. The stimulation protocol is then tailored to the patient’s profile, adjusting the type and dosage of hormonal medication (e.g., GnRH antagonist or agonist) to optimize the ovarian response.
Low and High Responses: Risks and Management
When ovarian response falls outside the desired range, specific risks and management strategies are necessary. A low response (retrieving three or fewer eggs) indicates diminished ovarian reserve and risks cycle cancellation before retrieval. Physicians may modify future protocols by increasing stimulation medication or employing double ovarian stimulation (two stimulations in one cycle). Accumulating eggs from multiple low-response cycles into one large batch of embryos is also used to improve success rates.
Conversely, a high response (more than 20 mature follicles or 15 eggs retrieved) increases the risk of Ovarian Hyperstimulation Syndrome (OHSS). OHSS is a serious complication where the ovaries swell and fluid leaks into the abdomen and chest, causing severe bloating and dehydration. To mitigate this risk, physicians often use a GnRH agonist instead of a traditional hCG trigger shot. They may also recommend a “freeze-all” cycle, freezing all embryos for later transfer, since pregnancy hormones can worsen OHSS.