The process of in vitro fertilization (IVF) involves a precisely orchestrated series of steps, culminating in the induction of egg maturation. Ovarian stimulation medication encourages multiple fluid-filled sacs, called follicles, to grow within the ovaries, each potentially containing an egg. When the follicles reach a suitable size, a single, carefully timed injection is administered to trigger the final maturation process. This trigger shot is the definitive step that prepares the eggs for collection and determines the success of the retrieval procedure.
Understanding the Trigger Shot Mechanism
The trigger shot is a hormone injection designed to mimic the body’s natural Luteinizing Hormone (LH) surge, which signals the final stage of egg development. The most common medication used is human chorionic gonadotropin (hCG), which is structurally similar to LH and binds to the same receptors. A gonadotropin-releasing hormone agonist (GnRH agonist), such as leuprolide, is also used as an alternative trigger, working differently by prompting the patient’s own pituitary gland to release a burst of LH and Follicle-Stimulating Hormone (FSH).
Regardless of the type used, the injection initiates the final maturation division of the egg, known as Meiosis I, allowing it to become a mature metaphase II oocyte ready for fertilization. This hormonal surge sets a fixed biological clock, making the timing of the egg retrieval procedure crucial. Retrieval is scheduled approximately 34 to 36 hours after the trigger shot to collect the eggs before they naturally ovulate into the abdominal cavity, ensuring the highest number of mature eggs are available.
The Post-Trigger Follicle Growth Rate
Follicles continue to enlarge significantly during the 34 to 36-hour window between the trigger shot and egg retrieval. This final growth is primarily due to the rapid accumulation of follicular fluid rather than a continued proliferation of cells. Research indicates that follicles typically expand by an additional 2 to 4 millimeters (mm) in total diameter during this short period. This means a follicle measuring 16 mm at the time of the trigger shot could be expected to measure 18 to 20 mm at the time of retrieval.
Calculating this over the 36-hour interval, the post-trigger growth rate is approximately 0.05 to 0.11 mm per hour, or about 1 to 2 mm per day, continuing the pace observed during the stimulation phase. This final increase in size is an important metric, as the fluid volume increases significantly, making the follicle easier to visualize and access during the retrieval procedure. The trigger shot provides this final boost in size while simultaneously completing the maturation of the egg within the sac.
Optimal Follicle Size for Egg Retrieval
Follicles that are between 16 mm and 22 mm in diameter on the day of egg retrieval are considered the optimal size for yielding a mature oocyte. This range is the clinical target, which is why the trigger shot is typically administered when the leading follicles measure between 17 mm and 18 mm.
Follicles smaller than 14 mm at the time of retrieval are unlikely to contain a mature egg, as they have not completed the necessary growth and maturation steps. Conversely, follicles exceeding 24 mm may contain post-mature or aged eggs, or may be cystic and not contain an egg. Therefore, a successful cycle depends on achieving a uniform cohort of follicles within this narrow size window, as follicle size indicates the maturity of the egg inside.
Variables Influencing Final Maturation
While the trigger shot provides a standardized hormonal surge, the final maturation and growth of follicles vary based on several patient-specific factors. The initial size of the follicle at the time of the trigger injection is a significant predictor of success. Follicles already in the 12 mm to 19 mm range on trigger day have the highest likelihood of achieving final maturity and yielding a quality egg.
Patient age and ovarian reserve, often assessed by Anti-Müllerian Hormone (AMH) levels, also play a role in the responsiveness of the follicles. The specific ovarian stimulation protocol used influences the homogeneity of the follicle cohort. Patients with an asynchronous group of follicles (a wider range of sizes) may have fewer follicles reach the optimal retrieval size. Fertility specialists closely monitor hormone levels and follicle dimensions to optimize the timing of this crucial final injection.