The trigger shot, a final injection typically containing Human Chorionic Gonadotropin (hCG) or a GnRH agonist, is a programmed step in fertility treatments such as In Vitro Fertilization (IVF). Its purpose is to synchronize the final stages of egg development and prepare the mature eggs for retrieval. This injection is precisely timed to induce the last phase of maturation in the eggs grown during the preceding stimulation phase. The goal is to prepare the eggs within the ovarian follicles for the scheduled collection procedure, not to release them.
The Mechanism of Final Follicle Maturation
The trigger shot mimics the body’s natural Luteinizing Hormone (LH) surge, which prompts ovulation mid-cycle. Both hCG and GnRH agonists bind to the LH receptor on the ovarian follicle cells, initiating a cascade of biological signals. This induced surge shifts the ovarian environment from promoting follicular growth to focusing on oocyte final maturation.
This hormonal signal causes the egg, or oocyte, to resume meiosis, a specialized type of cell division. The egg has been arrested in the first stage of meiosis since before birth, and the LH-like surge breaks this pause. It causes the oocyte to complete the first meiotic division, expelling half of its chromosomes into the first polar body.
The egg progresses from its immature state to the mature metaphase II stage, which is required for successful fertilization. This final maturation process also involves changes in the surrounding follicular cells. These changes release the egg from the follicle wall, allowing it to float freely in the follicular fluid for successful aspiration during the retrieval procedure.
The trigger shot transitions the egg from a state of growth and arrested development to readiness for fertilization. The structural similarity between hCG and LH allows the administered drug to bind to the same receptors, effectively overriding the body’s natural hormonal cycle to achieve controlled final maturation. This precise biological event takes place over several hours.
Why Not All Follicles Respond to the Trigger Shot
The trigger shot does not cause all follicles to respond. Follicles must reach a specific developmental threshold during the ovarian stimulation phase. Only those that have grown to a certain size and stage possess the necessary internal machinery and receptor density to react to the induced LH surge. Follicles that are too small or insufficiently developed will not complete the final stages of maturation.
For a follicle to contain a mature, retrievable egg, it typically needs to reach a diameter of approximately 15 to 22 millimeters at the time of the trigger shot. Follicles within this size range are considered mature enough to contain an oocyte that is competent to undergo meiotic resumption. Smaller follicles, for example those under 12 millimeters, may contain immature eggs unable to complete the final maturation steps.
The follicle population at the time of the trigger is often heterogeneous, containing follicles of various sizes. Some follicles may be too small, while others may be atretic or dying off. The trigger shot only acts on the cohort of follicles that are developmentally ready. This explains why the number of eggs retrieved often differs from the total number of follicles visible on ultrasound.
The stimulation phase aims to grow a synchronous group of follicles into the mature size range. The subsequent selection of eggs is a biological filtering process, where only the most developed follicles respond successfully to the final hormonal signal. This inherent biological selection explains why the trigger does not cause all follicles to yield a mature egg.
The Critical Window for Egg Retrieval Success
The trigger shot initiates a biological clock, creating a precise and narrow window for the egg retrieval procedure. The timing of the retrieval depends on the duration it takes for the eggs to reach optimal maturity following the injection, while ensuring they are collected before spontaneous ovulation occurs. This window is typically scheduled for 34 to 36 hours after the trigger shot is administered.
If the retrieval is performed too early (under 34 hours), the eggs may be immature, having not completed the necessary meiotic division to be fertilizable. Immature eggs have lower fertilization and developmental potential, negatively impacting the cycle’s success. Conversely, delaying the procedure past the 36-hour mark increases the risk of spontaneous ovulation, where mature eggs are released from the ovary into the abdominal cavity.
The fertility clinic staff must coordinate the patient’s schedule with the laboratory’s resources to adhere to this narrow timeframe. Precise timing is important, as the difference of just a few hours can determine whether the retrieved eggs are optimally mature and available for fertilization. This procedural accuracy maximizes the chances of collecting the highest number of viable eggs.