In vitro fertilization (IVF) involves combining egg and sperm outside the body to create an embryo. The success of this procedure relies on manipulating the body’s natural reproductive hormone cycle to achieve a precise sequence of events. Injections are the primary delivery method for the high doses of hormones necessary to override and control the ovaries’ monthly activity. This hormonal control ensures the exact timing of egg maturation and produces a greater number of eggs than a natural cycle. The goal of using these injected hormones is to maximize the yield of healthy, mature eggs for retrieval.
Controlling the Natural Cycle
Before stimulating the ovaries to produce multiple eggs, the natural reproductive cycle must be temporarily silenced, a step often termed suppression or down-regulation. This is accomplished using injections of GnRH (Gonadotropin-releasing hormone) analogs. These medications prevent the pituitary gland from sending its own signals to the ovaries.
The prevention of a spontaneous surge of Luteinizing Hormone (LH) is the primary objective of this stage. A natural LH surge causes premature ovulation, which would release the developing eggs before they can be collected during the retrieval procedure. GnRH agonists, such as leuprolide, initially cause a temporary release of FSH and LH, known as a “flare,” before completely suppressing the pituitary gland’s function over several days.
GnRH antagonists, like ganirelix, offer an alternative approach by immediately blocking the GnRH receptors. This instant blockade prevents the release of FSH and LH, providing quicker suppression without the initial “flare” effect seen with agonists. The choice between an agonist (long protocol) or an antagonist (short protocol) determines the duration and timing of the suppression phase, but both stop premature ovulation.
Maximizing Egg Production
Once the natural cycle is suppressed, the focus shifts to controlled ovarian stimulation, maturing multiple follicles simultaneously. This is achieved by injecting high doses of synthetic hormones called gonadotropins, which include Follicle-Stimulating Hormone (FSH) analogs and sometimes Luteinizing Hormone (LH) analogs. IVF requires the growth of many follicles, unlike a natural cycle which recruits only one.
FSH is the primary hormone responsible for promoting the growth and development of the ovarian follicles, the fluid-filled sacs containing the eggs. Recombinant FSH, a synthetic version manufactured in a lab, is frequently used because it is highly purified and allows for precise dosing. The injections encourage several small follicles to continue growing past the point where the body would normally select only one dominant follicle.
LH works in concert with FSH by stimulating theca cells to produce androgens, which are then converted into estrogen. While some protocols use pure FSH, others incorporate LH activity, often through recombinant LH or Human Menopausal Gonadotropin (hMG), to ensure optimal follicular development and egg quality. These daily injections typically last 8 to 14 days, with progress monitored closely through blood tests and ultrasound.
Timing the Final Maturation
The final, time-sensitive injection is the “trigger shot,” administered once the follicles have reached the appropriate size, usually around 17 to 20 millimeters in diameter. This injection is given approximately 34 to 36 hours before the planned egg retrieval procedure. Precise timing ensures the eggs complete their final maturation but are retrieved just before they would naturally ovulate.
The trigger shot typically involves a large dose of Human Chorionic Gonadotropin (hCG), a hormone that closely mimics the action of the natural LH surge. The sudden spike in this LH-like activity causes the eggs inside the follicles to undergo meiosis, completing the final stages of maturation and loosening their attachment to the follicle wall. This prepares them for easy aspiration during the retrieval.
Alternatively, a GnRH agonist, like leuprolide, can be used as the trigger, especially in patients at high risk for Ovarian Hyperstimulation Syndrome (OHSS). The agonist trigger causes a massive, short-lived surge of both FSH and LH from the pituitary gland. This surge accomplishes the final maturation but, due to its rapid clearance from the body, significantly reduces the risk of severe OHSS.
Supporting Early Pregnancy
Following the egg retrieval, an injection regimen is used to support the uterine lining in preparation for embryo implantation and early pregnancy. The intense hormonal stimulation and the physical act of aspirating the follicular fluid often impair the natural function of the corpus luteum, the structure that produces progesterone. Because the corpus luteum is compromised, the body cannot produce sufficient levels of progesterone on its own.
Progesterone is necessary for maintaining the uterine lining, or endometrium, making it receptive to the implanting embryo. Without external supplementation, the uterine environment would be unsuitable for sustaining a pregnancy. This luteal phase support is most commonly administered via oil-based injections, usually intramuscularly, or through vaginal gels and suppositories.
Intramuscular injections of progesterone in oil provide high, stable levels of the hormone. This supplementation continues into the first trimester, typically until the placenta is mature enough to produce sufficient progesterone, usually around 8 to 10 weeks of gestation. Consistent administration of progesterone ensures the endometrium remains stable and nourished, maximizing the chance of successful implantation and ongoing pregnancy.