The trigger shot is an injectable hormone given during fertility treatment to cause your eggs to finish maturing and release from the ovaries on a predictable schedule. It mimics the natural surge of luteinizing hormone (LH) that your body produces mid-cycle to trigger ovulation. By controlling exactly when ovulation happens, your fertility team can time an egg retrieval or intrauterine insemination (IUI) with precision, typically scheduling the procedure 34 to 40 hours after the injection.
How the Trigger Shot Works
In a natural menstrual cycle, rising estrogen levels signal your pituitary gland to release a sudden surge of LH. That surge tells the largest follicle in your ovary to complete its final stage of maturation and release the egg about 36 to 40 hours later. The trigger shot replicates this process with an external hormone so your medical team knows the exact hour ovulation will occur.
The most common form of trigger shot uses human chorionic gonadotropin (hCG), a hormone that behaves almost identically to LH but stays active in your body much longer. The other option is a GnRH agonist, which works differently: instead of acting directly on the ovary, it tells your own pituitary gland to release a burst of both LH and FSH. That burst is shorter-lived, which becomes important when managing side effects.
Types of Trigger Shots
There are three main approaches, and which one you receive depends on your response to stimulation medications and your risk profile.
- hCG trigger: The most widely used option since the 1970s. It comes as either a recombinant form (brand name Ovitrelle) or a urinary-derived form (Pregnyl). Because hCG has a long half-life, it provides strong, sustained support for egg maturation and early embryo implantation. The tradeoff is a higher risk of ovarian hyperstimulation syndrome (OHSS).
- GnRH agonist trigger: Medications like leuprolide (Lupron), triptorelin, or buserelin cause your pituitary to release its own natural LH and FSH surge. Because that surge is shorter in duration, the risk of OHSS drops significantly. This option is only available if you’ve been on a GnRH antagonist protocol during stimulation.
- Dual trigger: A combination of both, using a lower dose of hCG alongside a GnRH agonist. This strategy aims to reduce OHSS risk while still providing enough hormonal support for a fresh embryo transfer. Studies have found that dual trigger improves clinical pregnancy rates compared to using a GnRH agonist alone, and it can also outperform hCG-only triggers in normal responders.
When the Shot Is Given
Your fertility clinic monitors your follicles with ultrasound and blood work throughout your stimulation cycle. The trigger shot is administered once at least one follicle reaches approximately 17 mm in diameter. On that same day, your clinic typically checks estradiol, LH, and progesterone levels to confirm your body is ready.
Progesterone levels on trigger day matter more than you might expect. If progesterone rises too high (above about 1.0 to 2.0 ng/ml), it can signal that the uterine lining is advancing too quickly for an embryo to implant successfully. In those cases, your team may recommend freezing all embryos rather than doing a fresh transfer.
You’ll receive very specific timing instructions, often down to the exact hour. Egg retrieval is scheduled 34 to 40 hours after the injection. For IUI cycles, insemination is usually timed within a similar window. Even being off by a few hours can affect results, so clinics treat this timing seriously.
How to Take the Injection
Trigger shots are given either subcutaneously (into the fatty tissue of the abdomen) or intramuscularly (into the upper outer area of the buttock). The route depends on the specific medication. Recombinant hCG like Ovitrelle is typically subcutaneous, while urinary-derived hCG at higher doses has traditionally been given intramuscularly. Research comparing the two routes found that subcutaneous injection actually produces higher hCG levels in both the blood and the fluid surrounding the eggs at the time of retrieval, so both methods are effective.
GnRH agonist triggers like leuprolide are given subcutaneously. If you’re doing a dual trigger, you’ll give yourself two injections at the same time, both under the skin of the abdomen.
Side Effects and OHSS Risk
Mild side effects after the trigger shot are common and can include bloating, abdominal tenderness near the ovaries, nausea, and general discomfort. These symptoms overlap with the effects of the stimulation medications you’ve already been taking, so they may simply feel like a continuation of what you’ve experienced during your cycle.
The more serious concern is ovarian hyperstimulation syndrome. OHSS typically develops within a week after an hCG injection and occurs because hCG’s long-lasting activity continues to stimulate already-enlarged ovaries. Mild to moderate OHSS causes abdominal bloating, pain, nausea, vomiting, and diarrhea. Severe OHSS is less common but more dangerous, potentially causing rapid weight gain (more than 2.2 pounds in 24 hours), severe abdominal pain, shortness of breath, decreased urination, and blood clots.
This is the primary reason GnRH agonist triggers were developed. Because the natural LH surge they produce is shorter-lived than injected hCG, ovarian stimulation subsides more quickly. If your clinic sees that you’re a high responder with many large follicles, they’ll often switch to a GnRH agonist or dual trigger to lower your OHSS risk.
Pregnancy Tests After the Trigger
Because the trigger shot itself contains or stimulates hCG, the same hormone that pregnancy tests detect, testing too early will give you a false positive. The injected hCG needs time to clear your system before a pregnancy test can give a reliable result. This clearance period is generally around 10 days but varies depending on the dose you received and your individual metabolism. Your clinic will tell you exactly when to test or will schedule a blood draw to confirm pregnancy at the right time.
If you received a GnRH agonist trigger without any hCG, this isn’t a concern, since the medication doesn’t contain hCG directly. However, most protocols that include a fresh transfer use at least some hCG, so the waiting period applies to the majority of patients.