A trigger shot is a precisely timed injection given near the end of an IVF stimulation cycle to push your eggs through their final stage of maturation. Without it, the eggs inside your follicles aren’t ready to be fertilized. The shot mimics the natural hormone surge that normally causes ovulation, giving your fertility team a narrow, predictable window to retrieve mature eggs before your body releases them on its own.
Why the Trigger Shot Is Necessary
During a natural menstrual cycle, a surge of luteinizing hormone (LH) signals the eggs to complete their final phase of development. Eggs are paused at an immature stage and need this hormonal push to become mature enough for fertilization. In IVF, fertility medications stimulate multiple follicles to grow at once, but those eggs remain stuck in that immature holding pattern until something tells them to finish maturing.
The trigger shot provides that signal. It either introduces a hormone that acts like LH or causes your body to release its own LH surge. Once the shot is given, the eggs resume their development and reach maturity within roughly 36 hours, at which point the retrieval is performed.
Types of Trigger Shots
There are three main approaches, and your clinic will choose based on your response to stimulation medications and your risk profile.
hCG trigger: This is the traditional option. Human chorionic gonadotropin (hCG) has a molecular structure similar to LH and actually binds to the same receptors more strongly, making it roughly five times more potent at activating the cellular machinery that pushes eggs to maturity. It doesn’t raise your LH levels directly. Instead, it does the job LH would do, just more powerfully. Doses typically range from 5,000 to 10,000 IU, though doses as low as 2,500 IU and as high as 15,000 IU have been used depending on follicle count, estrogen levels, and the perceived risk of overstimulation. Patients with fewer follicles sometimes receive higher doses, while those with a strong response may get lower ones.
GnRH agonist trigger (Lupron trigger): Instead of introducing an LH-like hormone from outside, this approach causes your pituitary gland to release its own natural surge of both LH and FSH. This produces a more physiological response, closer to what happens in a natural cycle. It’s often preferred for patients at higher risk of ovarian hyperstimulation syndrome (OHSS), because the natural surge is shorter-lived and less intense than the prolonged stimulation from hCG. This option only works in cycles using a specific type of stimulation protocol (antagonist cycles).
Dual trigger: This combines both approaches, pairing a GnRH agonist with a dose of hCG. The idea is to get the benefits of both: the natural LH and FSH surge from the agonist plus the strong receptor activation from hCG. No standardized dose exists for the dual trigger, and clinics use varying combinations. Some use a full dose of hCG alongside the agonist, while others use a reduced hCG dose.
Timing Between Trigger and Egg Retrieval
This is one of the most time-sensitive steps in the entire IVF process. Most women would naturally ovulate and release eggs from their follicles 37 to 38 hours after the trigger shot. Your fertility team schedules the retrieval one to two hours before that would happen, typically 35 to 36 hours after the injection. This window is tight because the eggs need to be collected while they’re mature but still inside the follicles.
As a practical example: if you give yourself the trigger shot at 10 p.m. on a Monday, your egg retrieval would be scheduled for around 10 a.m. on Wednesday morning. Your clinic will give you a specific time to administer the injection, often down to the minute, and it’s important to follow that precisely.
There’s a subtle difference in how quickly each trigger type works at the cellular level. Blood levels of LH peak about 4 hours after a GnRH agonist trigger, while hCG levels peak about 15 hours after an hCG trigger. Despite the slower rise, hCG’s stronger receptor activation may actually start the maturation process earlier. Your clinic accounts for these differences when scheduling your retrieval.
How the Injection Is Given
Trigger shots are given either subcutaneously (into the fatty tissue just under the skin, usually in the abdomen) or intramuscularly (into the muscle, typically the upper outer area of the buttock). Subcutaneous injections use a smaller needle and are easier to self-administer. Research comparing the two routes found that subcutaneous injection actually resulted in higher hCG levels in both the blood and the fluid surrounding the eggs at the time of retrieval. Many clinics now default to the subcutaneous route for recombinant hCG formulations.
Confirming the Shot Worked
Some clinics will have you come in for a blood draw about 12 hours after the trigger injection to confirm that enough hCG was absorbed. This matters because if levels are too low, the eggs may not mature properly. In one study, a blood hCG level above roughly 90 mIU/mL at the 12-hour mark predicted better egg maturation rates. The average level at 12 hours was about 102 mIU/mL. Higher body weight can affect absorption and result in lower levels, which is one reason clinics sometimes adjust the dose.
Side Effects After the Trigger
By the time you reach the trigger shot, you’ve already been on stimulation medications for about 8 to 12 days, so your ovaries are enlarged and producing high levels of estrogen. The trigger shot can intensify symptoms you’re already experiencing. The most commonly reported side effects include bloating, breast tenderness, headaches, nausea, hot flashes, and bruising at the injection site. Bloating in particular tends to get noticeably worse between the trigger shot and the retrieval, because the follicles are at their largest.
The more serious concern is OHSS, a condition where the ovaries overreact and fluid shifts out of the bloodstream, causing abdominal swelling, pain, and in severe cases, dangerous complications. hCG is the primary driver of OHSS because it provides prolonged, intense stimulation to the ovaries. This is exactly why the GnRH agonist trigger was developed as an alternative for high-risk patients: it produces a shorter, more controlled hormonal surge that is far less likely to set off this chain reaction.
Why Your Clinic Chooses One Type Over Another
The decision comes down to balancing egg maturation with safety. An hCG trigger is powerful and reliable, producing consistently high maturation rates, which is why it remains the default for most patients. But if your estrogen levels are very high or you’ve developed a large number of follicles, the risk of OHSS goes up significantly with hCG, and your clinic may switch to a GnRH agonist trigger or a dual trigger instead.
The dual trigger has gained popularity as a middle ground. By including a lower dose of hCG alongside the agonist, clinics aim to maintain strong egg maturation rates while still reducing OHSS risk compared to a full hCG dose alone. It’s also sometimes used for patients who have had poor maturation rates with a single trigger type in previous cycles. The tradeoff is that dosing isn’t standardized, so protocols vary significantly from clinic to clinic.