The trigger shot is a final, precisely timed injection that completes the ovarian stimulation phase of In Vitro Fertilization (IVF). This medication is a defining moment in an IVF cycle, as its correct timing is paramount for the success of the subsequent egg retrieval. Administering this shot at the exact minute specified by the clinic dictates the entire timeline of the procedure. Without this hormonal signal, the eggs would not reach the necessary level of maturity for collection and fertilization.
The Role of the Trigger Shot in IVF
The trigger shot is an artificial mechanism used to mimic the body’s natural Luteinizing Hormone (LH) surge. This surge naturally prompts the final maturation and release of an egg from its follicle. In IVF, the trigger shot ensures this process is controlled and synchronized across all developing follicles.
The injection typically contains Human Chorionic Gonadotropin (hCG), which is structurally similar to LH. This allows it to bind to ovarian cell receptors, forcing the eggs to undergo the final stages of maturation, a process called meiosis. The goal is to collect the eggs in this mature state just before they are spontaneously released, or ovulated.
The hCG also initiates luteinization, which prepares the follicular lining for later hormone production. Other options, such as a GnRH agonist like leuprolide, may be used depending on the patient’s risk profile, particularly the risk of Ovarian Hyperstimulation Syndrome (OHSS).
Determining the Optimal Timing
Determining the exact time for the trigger injection relies on a two-part assessment conducted by the clinical team. The first part is the readiness assessment, involving frequent monitoring of hormone levels and follicular development through blood tests and transvaginal ultrasounds. Follicles are fluid-filled sacs in the ovaries that house the developing eggs.
The specialist looks for the largest follicles to reach an ideal target diameter, typically between 16 and 22 millimeters. Blood tests simultaneously measure the Estradiol (E2) hormone level, which is produced by the developing follicles. A rising E2 level indicates that the follicles are healthy and producing mature eggs.
Once the ovaries meet these maturity criteria, the clinic schedules the injection time. The trigger shot must be administered precisely 34 to 36 hours before the scheduled egg retrieval procedure. For example, a retrieval scheduled for 10:00 AM on Wednesday requires the trigger to be taken at 10:00 PM on Monday.
This narrow window is critical because mistiming carries significant risks. If the shot is taken too early, the eggs may not have completed final maturation, resulting in the retrieval of immature eggs. If the injection is delayed, the body may spontaneously ovulate before the retrieval, releasing the eggs where they cannot be collected.
Immediate Steps Following Injection
Once the trigger shot is administered, a 34-to-36-hour waiting period begins. This window allows the medication to fully act on the eggs, ensuring they reach peak maturity just before retrieval. Patients should not take any further injectable medications during this time.
Practical instructions during this period often include a confirmation call to the clinic to verify the injection time. Patients must also begin fasting several hours before the procedure, typically instructed not to eat or drink anything for about seven hours, as the retrieval is performed under sedation. Additionally, certain over-the-counter pain medications, like ibuprofen, are often stopped, with acetaminophen being the preferred alternative.
The final step is the egg retrieval, a minor, outpatient surgical procedure performed under light sedation. A thin needle is guided by ultrasound through the vaginal wall to gently aspirate the fluid containing the mature eggs from each follicle. The precision of the trigger shot timing directly determines the success of this collection, maximizing the number of viable eggs available for fertilization in the laboratory.