A trigger shot is an injectable fertility medication that forces your ovaries to release mature eggs at a precise, predictable time. It’s used during fertility treatments like IVF, IUI, and timed intercourse cycles to give doctors control over exactly when ovulation happens, typically 36 to 40 hours after the injection. This timing precision is what makes procedures like egg retrieval and insemination possible.
How a Trigger Shot Works
In a natural menstrual cycle, your brain releases a surge of luteinizing hormone (LH) that signals your ovaries to release a mature egg. Ovulation follows roughly 24 to 56 hours after that surge begins, with an average around 32 hours. The problem for fertility treatments is that this natural surge is unpredictable, making it nearly impossible to schedule a procedure at the right moment.
A trigger shot solves this by replacing your natural LH surge with an injection of human chorionic gonadotropin (hCG), a hormone that binds to the same receptor as LH but is about five times more potent. This stronger signal ensures the eggs inside your follicles complete their final stage of maturation and prepare for release. Common brand names include Ovidrel, Pregnyl, and Novarel. Some clinics use a different type of trigger, a GnRH agonist (sometimes called a Lupron trigger), which works by prompting your own pituitary gland to release a burst of LH rather than providing an external hormone directly.
When Trigger Shots Are Used
Trigger shots come into play across several fertility scenarios, each with slightly different goals:
- IVF (in vitro fertilization): The trigger shot matures your eggs so they can be surgically retrieved before ovulation occurs. Timing is critical here, since doctors need to collect the eggs while they’re still inside the follicles.
- IUI (intrauterine insemination): The shot ensures ovulation happens within a known window so sperm can be placed in the uterus at the optimal time. Most clinics perform insemination approximately 24 and 36 hours after the injection.
- Timed intercourse: For couples using ovulation-stimulating medications without a procedure, the trigger shot pinpoints the fertile window so intercourse can be timed effectively.
In IUI cycles specifically, hCG-triggered ovulation has been linked to improved pregnancy rates. One study found that pregnancy rates nearly doubled, from 10.9% to 19.6%, when the timing of the hCG injection was optimized relative to the insemination procedure.
Follicle Size and Timing Criteria
You won’t receive a trigger shot on a set calendar day. Instead, your doctor monitors your ovarian follicles with ultrasound, watching them grow over several days. Most IVF centers administer the trigger once two to three follicles reach at least 17 to 18 millimeters in diameter. Follicles between 12 and 19 mm on the morning of the trigger are the most likely to yield a mature egg.
Once the shot is given, the clock starts. In IVF, egg retrieval is typically scheduled 36 hours later, though research shows oocyte maturity is reliably achieved by that point and no spontaneous ovulation occurs before about 39 hours. This gives clinics a workable window. For IUI and timed intercourse, the expectation is that ovulation will occur somewhere between 36 and 40 hours after injection in stimulated cycles, so procedures and timing are planned accordingly.
How the Injection Is Given
Trigger shots are given either subcutaneously (into the fat layer just under the skin, usually in the abdomen) or intramuscularly (into a larger muscle, typically the upper buttock or thigh). Many patients self-administer the subcutaneous version at home. Research comparing the two routes found that subcutaneous injection actually produces higher hormone levels in both the bloodstream and the follicular fluid surrounding the eggs, so both methods are effective.
Your clinic will specify the exact time to inject, often late in the evening, so that the retrieval or insemination falls during normal clinic hours the following day or two days later. Precision matters here. Being even an hour or two off can affect the outcome, so most clinics emphasize injecting at the exact minute prescribed.
hCG Trigger vs. Lupron Trigger
The standard hCG trigger shot is the most common choice, but it carries a higher risk of ovarian hyperstimulation syndrome (OHSS), particularly in patients who have responded strongly to fertility medications and developed many follicles. For these high-risk patients, clinics often use a GnRH agonist trigger instead. Because the GnRH agonist prompts a shorter, more natural LH surge with a shorter half-life, the ovaries are less likely to become dangerously overstimulated.
The tradeoff is that GnRH agonist triggers have been associated with lower pregnancy rates in fresh embryo transfer cycles, likely because the shorter hormone surge doesn’t support the uterine lining as well in the days following ovulation. To get around this, many clinics now use a “dual trigger,” combining a small dose of hCG with the GnRH agonist, or they plan to freeze all embryos and transfer them in a later cycle when the uterine lining can be properly prepared.
Side Effects and OHSS Risk
Mild side effects from the trigger shot itself are common and usually short-lived: bloating, breast tenderness, mild pelvic discomfort, and soreness at the injection site. These are expected responses to the hormonal shift.
The more serious concern is OHSS, which causes the ovaries to swell significantly. Symptoms typically begin within a week of the injection. Mild to moderate OHSS involves abdominal pain, bloating, nausea, vomiting, and diarrhea. A mild form affects a meaningful number of women using injectable fertility drugs and usually resolves within about a week, though symptoms can worsen and linger if pregnancy occurs.
Severe OHSS is uncommon but potentially life-threatening. Warning signs include rapid weight gain of more than 2.2 pounds in 24 hours, severe and persistent nausea and vomiting, decreased urination, shortness of breath, and a noticeably tight or enlarged abdomen. Complications can include fluid buildup in the abdomen and chest, blood clots, kidney failure, and ovarian torsion (twisting). This is one of the main reasons doctors carefully monitor follicle count and estrogen levels before deciding to proceed with the trigger.
False Positive Pregnancy Tests
Because the trigger shot is made of hCG, the exact same hormone that pregnancy tests detect, it will cause a positive test result for days after injection even if you’re not pregnant. The medication takes 10 to 14 days to fully clear your system. Testing before that window closes will give you an unreliable result. Most clinics recommend waiting at least two weeks after the trigger shot before taking a home pregnancy test, or they’ll schedule a blood test at the appropriate time to get a definitive answer.
Some patients “test out” their trigger by taking cheap pregnancy tests daily and watching the line fade as the hCG leaves their body. Once the test turns completely negative, any new positive result is more likely to reflect an actual pregnancy. This approach isn’t officially recommended by most clinics, but it’s a widely used strategy among fertility patients who want earlier clarity.