A trigger shot is an injection that signals your eggs to complete their final stage of maturation so they can be retrieved during an IVF cycle. It mimics the natural hormone surge that normally causes ovulation, but with precise timing controlled by your fertility clinic. The shot is given exactly 35 to 36 hours before your scheduled egg retrieval, making it one of the most time-sensitive steps in the entire IVF process.
How the Trigger Shot Works
In a natural menstrual cycle, a sudden surge of luteinizing hormone (LH) triggers ovulation 36 to 40 hours later. Your eggs undergo their final maturation during that window, completing a critical cell division that makes them capable of being fertilized. The trigger shot replicates this surge artificially, giving your clinic control over exactly when that maturation process begins.
Most trigger shots use human chorionic gonadotropin (hCG), a hormone that activates the same receptors as LH but stays active in your body much longer. LH clears from your bloodstream in under 60 minutes, while hCG has a half-life of more than 24 hours. That prolonged activity helps ensure your eggs have enough hormonal support to fully mature. A successful trigger typically results in more than 75% of retrieved eggs reaching full maturity, meaning they’re ready for fertilization.
Types of Trigger Shots
There are three main approaches, and your clinic will choose based on your response to stimulation medications, your risk of complications, and your overall treatment plan.
hCG Trigger
This is the standard option and has been used since the mid-1970s. The two most common brand names are Ovidrel, which comes as a prefilled syringe injected under the skin, and Pregnyl, which is a powder mixed before injection. hCG triggers are effective and well-studied, but they carry a higher risk of ovarian hyperstimulation syndrome (OHSS) in patients who produce a large number of follicles during stimulation.
GnRH Agonist Trigger (Lupron)
Instead of introducing hCG directly, this approach uses a medication called leuprolide (Lupron) to cause your own pituitary gland to release a natural surge of both LH and FSH. Because the hormones clear your system quickly, the risk of OHSS drops dramatically. In studies of high responders, the rate of clinically significant OHSS was 0.0% with a GnRH agonist trigger alone. The trade-off is that the shorter hormone exposure can sometimes result in fewer mature eggs, and the lining of the uterus may not be as well-supported for a fresh embryo transfer.
Dual Trigger
A dual trigger combines both approaches: a GnRH agonist plus a low dose of hCG given at the same time. The idea is to get the benefits of a more natural hormonal response (including that FSH surge, which hCG alone doesn’t produce) while still providing enough hCG to support egg maturation and the uterine lining. Research has shown improvements in the number of mature eggs retrieved, implantation rates, and clinical pregnancy rates with dual triggers compared to either method alone. This approach is increasingly used for patients with diminished ovarian reserve or those over 35 with low egg counts, as well as a safeguard against the roughly 3% of patients who respond poorly to a GnRH agonist trigger on its own.
Timing Is Critical
The trigger shot is one of the few IVF medications where the exact hour matters. Your clinic will give you a specific time to administer the injection, calculated backward from your scheduled retrieval. If your retrieval is at 10 a.m. on Wednesday, for example, you’ll take the shot at 10 p.m. on Monday night, creating a precise 36-hour window.
This timing exists because eggs need enough hours to complete maturation but must be collected before your body actually ovulates and releases them into the fallopian tubes, where they can’t be retrieved. Being even a few hours off can mean eggs that are either immature or already lost. Most clinics will call you with your trigger time and confirm it, sometimes more than once, because of how consequential this step is.
How the Injection Is Given
Depending on the medication, the trigger shot is injected either subcutaneously (into the fatty tissue just under the skin, usually in the abdomen) or intramuscularly (into the muscle, typically the upper buttock area). Ovidrel comes as a prefilled syringe designed for subcutaneous injection, which most patients can do themselves. Pregnyl may be given intramuscularly, sometimes requiring a partner or someone else to help.
Research comparing the two routes found that subcutaneous injection actually produces higher hormone levels in both the bloodstream and the fluid surrounding the eggs compared to intramuscular injection. Your clinic will specify which route to use based on the medication prescribed.
Side Effects After the Shot
The most common side effects are bloating and pelvic discomfort or pressure. Some tenderness or mild pain at the injection site is also normal. These symptoms overlap with how you may already feel from the stimulation medications, since your ovaries are enlarged and holding multiple mature follicles at this point in the cycle.
For patients at risk of OHSS, the trigger shot can be the tipping point. OHSS causes the ovaries to swell significantly and fluid to leak into the abdomen, leading to pain, nausea, and in severe cases, dangerous complications. This is one of the primary reasons clinics choose a Lupron trigger or a dual trigger for patients who have developed a large number of follicles during stimulation. If your estrogen levels are very high or you have more than the expected number of follicles, your doctor may switch your trigger type at the last minute to reduce this risk.
Storing Your Medication
Trigger shot medications have specific storage requirements that matter for their effectiveness. Ovidrel prefilled syringes need to be kept refrigerated between 36 and 46°F (2 to 8°C) and protected from light in their original packaging. If needed, you can store Ovidrel at room temperature (up to 77°F) for a maximum of 30 days, but it must be used within that window. Pregnyl, which comes as a powder, has its own storage instructions that your pharmacy will provide. Since the trigger shot is typically a single-use medication given on one specific night, most patients simply keep it refrigerated until the moment they need it.
What Happens if the Trigger Doesn’t Work
In rare cases, a trigger shot fails to produce adequate egg maturation. This is more of a concern with GnRH agonist triggers, where a small percentage of patients don’t mount a sufficient hormonal response from their pituitary gland. Clinics monitor for this by checking hormone levels the morning after the trigger. If levels are too low, a rescue dose of hCG can sometimes be given, though the window is tight. The dual trigger protocol was developed partly to address this risk, since the hCG component acts as a backup if the GnRH agonist response is insufficient.
For hCG triggers, failure is less common but can occur if the medication wasn’t stored properly, wasn’t injected correctly, or if there was a timing error. This is why clinics emphasize the importance of following instructions precisely and confirming the injection time.