Pregnyl is an injectable form of human chorionic gonadotropin (hCG) used primarily as the “trigger shot” in IVF. Its main job is to force your eggs to complete their final stage of maturation so they can be retrieved at exactly the right moment. It’s typically given as a single injection of 5,000 to 10,000 units, and the timing of that shot determines when your egg retrieval happens.
How Pregnyl Triggers Egg Maturation
In a natural menstrual cycle, a surge of luteinizing hormone (LH) from the pituitary gland tells the ovaries it’s time to release a mature egg. Pregnyl mimics that LH surge. The hCG molecule shares the same structural backbone as LH and about 85% of the same amino acid sequence in its key region, which means it locks onto the same receptors on your ovarian cells and sends the same signal.
That signal does three things simultaneously. First, it restarts the process of meiosis in your eggs, the final cell division that brings each egg from an immature state to a fully mature one ready for fertilization. Second, it triggers the physical changes that would normally lead to ovulation, where the follicle ruptures and releases the egg. Third, it transforms the emptied follicle into a structure called the corpus luteum, which begins producing progesterone, the hormone needed to prepare the uterine lining for an embryo.
In IVF, the goal is to capture those mature eggs before they’re actually released from the ovary. That’s why the retrieval is scheduled with such precision after the Pregnyl injection.
The 36-Hour Window
Egg retrieval is typically performed 35 to 36 hours after the Pregnyl injection, roughly 1 to 2 hours before ovulation would naturally occur. This timing is critical. Too early, and the eggs haven’t finished maturing. Too late, and the eggs have already been released into the pelvic cavity where they can’t be collected.
Your clinic will give you a very specific time to administer the injection, often late at night, so the retrieval falls during regular morning or early afternoon hours. Because the window is tight, even a delay of a few hours can affect results. This is why clinics emphasize that the trigger shot is the one injection in the entire IVF cycle where exact timing matters most.
When Your Follicles Are Ready
Your fertility team won’t administer Pregnyl until your follicles have reached the right size on ultrasound monitoring. Research on optimal follicle size shows that a lead follicle of 21 to 22 millimeters is associated with the best outcomes. In one analysis of over 1,600 treatment cycles, the odds of clinical pregnancy were about 2.3 times higher when the lead follicle measured 21 to 22 mm compared to 19 to 20 mm. Triggering too early, when follicles are still small, reduces the chances that the eggs inside have matured enough for successful fertilization.
Pregnyl’s Role After Retrieval
The trigger shot’s effects don’t end at retrieval. Because hCG has a long half-life in the body, the single dose continues to stimulate the corpus luteum for several days, keeping progesterone production going. This matters because progesterone is what makes the uterine lining receptive to an embryo.
However, there’s a gap in progesterone support that can develop during the second half of the luteal phase. The stimulatory effect of the Pregnyl injection eventually wears off, but it’s too early for an implanting embryo to produce enough of its own hCG to take over. Fertility specialists call this the “luteal gap.” It’s one of the main reasons most IVF patients are prescribed supplemental progesterone after retrieval, to bridge that gap until a pregnancy can sustain itself hormonally.
How Pregnyl Compares to Ovidrel
Pregnyl is derived from the urine of pregnant women (called urinary hCG), while Ovidrel is a lab-manufactured recombinant version. Both serve the same function, but they aren’t identical in outcomes. A matched comparison of 112 patients per group found that urinary hCG like Pregnyl produced significantly more eggs per follicle, a higher ratio of mature eggs, and more embryos overall. The ongoing pregnancy and delivery rate was 49.5% with urinary hCG compared to 34.0% with Ovidrel.
Ovidrel does have practical advantages: it comes pre-mixed in a prefilled syringe and is given as a smaller subcutaneous injection. Pregnyl, by contrast, comes as a powder that must be mixed with a sterile liquid before injection and is typically given as an intramuscular shot. Some clinics still prefer Pregnyl for its clinical track record, while others use Ovidrel for convenience. Your clinic’s choice often depends on your specific protocol and risk factors.
Mixing and Storing Pregnyl
Pregnyl arrives as a freeze-dried powder with a separate vial of sterile liquid. You’ll draw the liquid into a syringe, inject it into the powder vial, and gently swirl until dissolved. The unmixed powder can be stored at room temperature (between 59°F and 86°F). Once reconstituted, the solution stays stable for 60 days in the refrigerator, though in IVF you’ll almost always use it immediately.
Side Effects and OHSS Risk
The most significant risk associated with Pregnyl in IVF is ovarian hyperstimulation syndrome (OHSS). The hCG in Pregnyl can cause ovarian blood vessels to become abnormally leaky, leading to fluid shifting out of the bloodstream and into the abdomen. OHSS typically develops within a week of the injection.
Mild to moderate symptoms include abdominal bloating, nausea, vomiting, diarrhea, and tenderness around the ovaries. These are relatively common and usually resolve on their own. Severe OHSS is less frequent but more dangerous, with symptoms like rapid weight gain (more than 2.2 pounds in 24 hours), severe abdominal pain, shortness of breath, decreased urination, and in rare cases, blood clots.
Certain factors increase OHSS risk: polycystic ovary syndrome, being under 35, having a low body weight, a large number of follicles developing during stimulation, high estrogen levels before the trigger, and any previous episodes of OHSS. For patients at high risk, some clinics will use a GnRH agonist trigger instead of Pregnyl to reduce the chance of hyperstimulation.
False Positive Pregnancy Tests
Because Pregnyl is literally the same hormone that pregnancy tests detect, it will cause a positive result on any test taken too soon after the injection. The hCG from the shot typically takes about 10 days to clear from your system, though this varies with your metabolism and the dose used. Testing before that window can show a false positive that reflects the medication rather than a pregnancy. Most clinics schedule a blood test around 10 to 14 days after embryo transfer to get an accurate reading.