An HCG trigger shot is an injection of human chorionic gonadotropin, a hormone that signals your ovaries to release a mature egg. It’s one of the most common steps in fertility treatments like IUI (intrauterine insemination) and IVF (in vitro fertilization), used to control the exact timing of ovulation so that procedures or intercourse can be scheduled during your most fertile window. After the injection, ovulation typically occurs 36 to 48 hours later.
How the HCG Trigger Shot Works
In a natural menstrual cycle, your brain releases a surge of luteinizing hormone (LH) that tells a mature follicle in your ovary to rupture and release its egg. HCG is structurally almost identical to LH and binds to the same receptor, so injecting it creates an artificial version of that surge. Your ovary can’t tell the difference between the two hormones.
This is useful because natural LH surges are unpredictable. They can happen in the middle of the night or vary by a day or two from cycle to cycle. The trigger shot replaces that uncertainty with a precise countdown: follicular rupture and ovulation usually happen 36 to 38 hours after injection, giving your fertility team a reliable window to schedule egg retrieval or insemination.
Beyond triggering ovulation, HCG also supports the corpus luteum, the structure left behind after the follicle releases its egg. The corpus luteum produces progesterone, which thickens the uterine lining and helps sustain an early pregnancy. HCG’s longer-lasting activity compared to natural LH means it continues supporting that process for days after injection.
When and Why It’s Used
The trigger shot comes at the end of a stimulation phase. For most fertility protocols, you’ll spend several days taking medications that encourage your ovaries to develop one or more mature follicles. Your doctor monitors follicle growth through ultrasounds and blood work. Once the lead follicle (or follicles) reaches the right size, typically around 18 to 20 millimeters, you’re told to administer the trigger shot at a very specific time.
That timing matters. If you’re having an IUI, the insemination is scheduled to coincide with ovulation, usually 36 to 42 hours after the shot. For IVF, egg retrieval is performed just before the eggs would naturally release, typically at the 36-hour mark. Even in simpler treatment cycles where you’re timing intercourse at home, the shot gives you a clear ovulation window to work with.
Dosage and How to Inject It
The standard dose for triggering ovulation is 5,000 to 10,000 international units (IU), with 10,000 IU being the most commonly prescribed. The medication comes as a powder that you mix with sterile water before injecting. Your clinic will give you exact instructions on your dose and timing.
Most patients inject the shot subcutaneously, meaning just under the skin of the abdomen. Despite some packaging that says “intramuscular only,” many clinics direct patients to use the subcutaneous route with a smaller needle. The preparation process involves drawing up 1 mL of bacteriostatic water with a larger mixing needle, injecting it into the vial of HCG powder, swirling gently until dissolved, then drawing the solution back into the syringe. You then switch to a thinner, shorter needle (typically 27 gauge) for the actual injection.
To inject, clean a spot on your abdomen with an alcohol swab, let it dry, pinch up the skin, and insert the needle straight in (not at an angle). Push the plunger all the way down, wait about five seconds, then withdraw. The whole process takes a few minutes once you’re comfortable with it. Many clinics offer instructional videos or have nurses walk you through the first injection.
Side Effects and Risks
The most common side effects are mild: bloating, tenderness near the ovaries, and minor discomfort at the injection site. These resolve on their own within a few days for most people.
The more serious risk is ovarian hyperstimulation syndrome (OHSS). HCG plays a direct role in triggering OHSS because its long-lasting activity can overstimulate the ovaries, especially in women who developed many follicles during the stimulation phase. Mild OHSS causes abdominal bloating, nausea, and pelvic discomfort, and it typically resolves within about a week. If pregnancy occurs, symptoms can linger longer because the body starts producing its own HCG.
Severe OHSS is uncommon but can be dangerous. Warning signs include rapid weight gain (more than about two pounds in 24 hours), severe nausea and vomiting, shortness of breath, decreased urination, and a visibly swollen abdomen. Women with polycystic ovary syndrome (PCOS) and those who produce a large number of follicles during stimulation are at higher risk.
HCG vs. GnRH Agonist Triggers
For patients at high risk of OHSS, particularly those with PCOS, some fertility protocols use a different type of trigger called a GnRH agonist instead of HCG. Rather than directly mimicking LH, a GnRH agonist causes your brain to release its own natural LH surge. Because that natural surge is shorter-lived than injected HCG, it nearly eliminates the risk of hyperstimulation.
The trade-off is that the shorter hormonal support can make the uterine lining less receptive to implantation in the same cycle. To work around this, clinics using a GnRH agonist trigger often freeze all embryos and transfer them in a later cycle. This freeze-all approach has become increasingly popular for high-risk patients, offering comparable pregnancy rates with significantly better safety profiles.
What to Know About Pregnancy Tests After the Shot
Because HCG is the same hormone that pregnancy tests detect, the trigger shot can cause a false positive for up to 10 to 14 days after injection. The hormone needs time to clear your system completely. If you test too early, you may see a positive result that reflects the medication rather than a pregnancy. Your clinic will typically schedule a blood test at a specific point in your cycle to get an accurate reading, since blood tests can measure exact HCG levels and distinguish between residual medication and a rising level that indicates implantation.