The trigger shot is the final injection in an IVF stimulation cycle, given to mature your eggs and prepare them for retrieval. After days of stimulating your ovaries to grow multiple follicles, this single shot signals the eggs to complete their final stage of development so they can be collected about 35 to 36 hours later. Without it, the eggs inside your follicles wouldn’t be mature enough to fertilize.
How the Trigger Shot Works
During a natural menstrual cycle, your brain releases a surge of luteinizing hormone (LH) that tells a mature egg to detach from the follicle wall and ovulate. The trigger shot replicates that surge artificially, but in a controlled, precisely timed way so your doctor can retrieve the eggs before they release on their own.
When the trigger medication reaches the ovaries, it activates receptors on the cells surrounding each egg. This kicks off a chain of events: the egg resumes the final stage of cell division it had been paused in, the supportive tissue around it expands, and the egg loosens from the follicle wall. These steps are essential. An egg that hasn’t completed this maturation process can’t be fertilized successfully, and the quality of this signaling is directly correlated with embryo quality down the line.
Types of Trigger Medications
There are three main approaches, and the one your clinic recommends depends on your protocol and your risk of complications.
hCG trigger: The most traditional option. Human chorionic gonadotropin (hCG) is structurally similar to LH, so it binds to the same receptors and sends the same maturation signal. The key difference from natural LH is that hCG has a much longer half-life. Its biological effect can last for several days, which provides strong, sustained support for egg maturation and the early luteal phase (the stretch after retrieval when your body would normally prepare for pregnancy). The downside of that long activity is a higher risk of ovarian hyperstimulation syndrome (OHSS), especially in women who produced a large number of follicles.
GnRH agonist trigger: Instead of directly mimicking LH, this medication (commonly leuprolide, sometimes called by brand names like Lupron) works on the brain’s pituitary gland, causing it to release a burst of both LH and FSH. This more closely mimics the natural midcycle surge. The LH spike it produces is shorter, lasting roughly 24 to 36 hours compared to the multi-day effect of hCG. That shorter duration dramatically reduces the risk of OHSS but can leave the luteal phase undersupported, which is associated with lower pregnancy rates and higher miscarriage rates if extra hormonal support isn’t added.
Dual trigger: A combination of both, using a GnRH agonist alongside a lower dose of hCG. The idea is to get the benefits of the natural-like FSH and LH surge from the agonist while the smaller hCG dose provides enough luteal support to maintain pregnancy rates. Studies show the dual trigger produces a significantly higher LH peak than hCG alone, plus a distinct FSH surge that hCG on its own doesn’t generate. This approach is increasingly common, particularly in freeze-all cycles.
When Your Doctor Decides to Trigger
Throughout your stimulation cycle, your clinic monitors follicle growth with ultrasounds and blood work. The trigger is typically administered once two to three follicles reach at least 17 to 18 mm in diameter. Research shows that follicles between 12 and 19 mm on trigger day contribute the most to the number of eggs actually retrieved, so your doctor is balancing the need for the lead follicles to be large enough while giving smaller follicles time to catch up.
Blood tests on trigger day usually measure estrogen, progesterone, and LH levels. Estrogen reflects how many follicles are actively growing (higher levels generally mean more follicles), while progesterone and LH help confirm the timing is right and that you haven’t started to ovulate on your own.
Timing Is Critical
Most women would ovulate naturally 37 to 38 hours after their LH surge. Your retrieval is scheduled 35 to 36 hours after the trigger injection, giving your team a one-to-two-hour window to collect the eggs before they release. This is why clinics are so specific about the exact time you take the shot, often down to the minute. Injecting too early or too late can mean eggs that aren’t fully mature or, worse, eggs that have already ovulated and can’t be retrieved.
You’ll typically receive a very precise time from your nurse, often late in the evening, so that retrieval falls during normal operating hours the following morning (roughly a day and a half later).
How the Injection Works
The trigger shot is a subcutaneous injection, meaning it goes into the fatty tissue just under the skin, usually in the abdomen. One of the most commonly used forms, a prefilled syringe of recombinant hCG, should be stored in the refrigerator but can be left at room temperature for up to 30 days. Take it out 10 to 15 minutes before injecting so it warms slightly, which can make the injection more comfortable.
Some formulations require mixing a powder with a liquid before injecting. Your clinic will walk you through the preparation. The injection itself takes seconds and uses a small needle similar to the ones you’ve been using throughout stimulation.
OHSS Risk and How the Trigger Affects It
Ovarian hyperstimulation syndrome is the most significant complication tied to the trigger shot, and hCG plays a central role in causing it. The long-lasting effect of hCG promotes the release of vascular growth factors that increase blood vessel permeability, allowing fluid to leak from the bloodstream into the abdomen and, in severe cases, the chest. This leads to bloating, pain, nausea, and in rare serious cases, blood clots or kidney problems.
For women at high risk, typically those with a very high follicle count or polycystic ovary syndrome, several strategies can reduce the danger. Switching to a GnRH agonist trigger significantly lowers or can even eliminate OHSS in most cases. Reducing the hCG dose (using around 2,000 to 3,250 IU instead of the standard 5,000 to 10,000 IU) has also been shown to reduce severe OHSS while still maturing enough eggs. Combining a GnRH agonist trigger with a freeze-all approach, where all embryos are frozen and transferred in a later cycle, is one of the most effective prevention strategies available.
It’s worth noting that rare cases of severe OHSS have still occurred even with a GnRH agonist trigger and freeze-all protocol, so no strategy eliminates risk entirely.
What to Expect After the Trigger
After the injection, you may feel some bloating, mild cramping, or breast tenderness. These are normal responses to the hormonal surge. Because hCG is the same hormone detected by pregnancy tests, a trigger shot will cause a false positive on a home pregnancy test for up to about two weeks afterward. Your clinic will schedule a blood-based pregnancy test at the appropriate time to avoid confusion.
Some clinics draw blood the morning after the trigger to confirm it worked, checking that hormone levels have shifted appropriately. If the trigger fails, which is uncommon, eggs may not mature properly and retrieval results can be poor. This is more of a concern with GnRH agonist triggers, where the pituitary surge can occasionally be insufficient.
Between the trigger and retrieval, your clinic will ask you to avoid strenuous activity. Your ovaries are enlarged from stimulation at this point, and vigorous movement increases the risk of ovarian torsion, where the ovary twists on itself. Staying hydrated and resting is the standard guidance for this short waiting period.