In vitro fertilization (IVF) is a highly regulated medical process that manages the reproductive cycle. It involves several distinct stages, from controlled ovarian stimulation to embryo transfer. The initial phase encourages the ovaries to develop multiple mature eggs through careful hormonal management. This process relies on one precisely timed intervention known as the “trigger shot,” which signals the conclusion of the stimulation phase. The trigger is a time-sensitive step, initiating an irreversible biological countdown to egg collection.
Defining the Trigger Shot
The trigger shot is a single, carefully timed injection administered subcutaneously or intramuscularly at the end of ovarian stimulation. Its purpose is to induce the final phase of egg development before collection by the fertility specialist. This injection acts as the precise command for final maturation.
The medication used is typically one of two main hormonal compounds. The most common substance is human chorionic gonadotropin (hCG), which is chemically similar to a naturally occurring hormone. Alternatively, a Gonadotropin-Releasing Hormone (GnRH) agonist, such as leuprolide acetate, may be used, often for patients at risk of ovarian hyperstimulation syndrome (OHSS).
The injection is named the “trigger” because it sets off a rapid cascade of events within the ovarian follicles. This action is an all-or-nothing event; once administered, the final maturation process begins and cannot be reversed. The timing is critical because the eggs must be retrieved before the body naturally releases them through ovulation.
Biological Mechanism and Critical Timing
The fundamental action of the trigger shot is to mimic the body’s natural pre-ovulatory Luteinizing Hormone (LH) surge. In a natural cycle, this surge signals the dominant follicle to release its egg. In IVF, the medication ensures multiple stimulated eggs complete their development.
Mechanism of hCG
Human chorionic gonadotropin (hCG) works by directly binding to the same ovarian receptors normally activated by LH. This binding initiates the resumption of meiosis, a cell division process where the egg sheds half of its chromosomes. This final genetic maturation is necessary for the egg to be viable for fertilization.
Mechanism of GnRH Agonists
When a GnRH agonist is used, the mechanism differs slightly. It causes the pituitary gland to release a substantial, short-lived surge of the body’s own LH and Follicle-Stimulating Hormone (FSH). This endogenous surge acts on the ovarian follicles to achieve the same final maturation.
Critical Timing
The precise timing of the injection is paramount to the cycle’s success and is generally set at 34 to 36 hours before the scheduled egg retrieval. If administered too early, the eggs may not complete maturation, leading to the collection of immature eggs. If administered too late, the body may spontaneously ovulate, releasing the mature eggs where they cannot be retrieved.
The Necessary Follow-Up Procedure
The biological changes initiated by the trigger shot directly lead to the scheduling of the egg retrieval. This procedure is a minor surgical operation performed under light sedation or monitored anesthesia. It is designed to collect the now-matured eggs before they are released from the ovaries.
During the retrieval, the fertility specialist uses a transvaginal ultrasound probe to visualize the ovaries and the fluid-filled follicles. A thin needle is guided through the vaginal wall into each follicle to gently aspirate the follicular fluid containing the eggs. The entire collection process is brief, typically lasting only 15 to 30 minutes.
The patient receives specific instructions leading up to the procedure, including a mandatory fasting period to ensure safety under sedation. Following the retrieval, the collected eggs are immediately transferred to the embryology laboratory for assessment and fertilization. Without the precise action of the trigger shot and the timely follow-up retrieval, the entire stimulation phase of the IVF cycle would be ineffective.