Pregnyl is a brand-name injectable form of human chorionic gonadotropin (hCG), a hormone that mimics the action of luteinizing hormone (LH) in the body. It has three FDA-approved uses: triggering ovulation in women undergoing fertility treatment, treating low testosterone caused by a pituitary problem in men, and encouraging testicular descent in boys with undescended testes. Most people encounter Pregnyl in the context of fertility care, where it plays a central role in timed ovulation.
How Pregnyl Works in the Body
Your pituitary gland naturally produces luteinizing hormone, which tells your reproductive organs to make sex hormones and, in women, triggers the release of a mature egg each month. Pregnyl’s active ingredient, hCG, is nearly identical to LH in how it behaves at the cellular level. When injected, it latches onto the same receptors and sends the same signals.
In women, this means hCG can stand in for the natural mid-cycle LH surge that causes ovulation. In men, it stimulates the cells in the testes responsible for producing testosterone. It also carries a small amount of activity similar to follicle-stimulating hormone (FSH), which supports sperm production and egg development. This dual action is what makes Pregnyl useful across several different conditions.
Triggering Ovulation in Fertility Treatment
The most common reason women receive Pregnyl is as a “trigger shot” during fertility treatment. After follicle-stimulating medications have helped one or more eggs mature inside the ovaries, a single injection of Pregnyl mimics the LH surge that would normally cause ovulation. This gives fertility specialists precise control over when the egg is released, which is critical for timing intrauterine insemination (IUI) or egg retrieval during IVF.
Ovulation typically occurs about 36 hours after the injection, so procedures are scheduled around that window. Pregnyl is approved specifically for women whose inability to ovulate stems from a hormonal issue rather than from the ovaries themselves failing. If the ovaries have stopped responding entirely (primary ovarian failure), hCG will not help because there are no mature follicles to release.
After ovulation, Pregnyl also supports the early stages of pregnancy by stimulating the tissue left behind after the egg is released (the corpus luteum) to produce progesterone. Progesterone thickens the uterine lining and helps a fertilized egg implant, so this secondary effect is an important part of why Pregnyl is used in fertility cycles.
Treating Low Testosterone in Men
Pregnyl is approved for a condition called hypogonadotropic hypogonadism, which is a way of saying the testes aren’t producing enough testosterone because the brain isn’t sending the right signals. The problem originates in the pituitary gland or hypothalamus, not the testes themselves. Because hCG acts like LH, injecting it bypasses the faulty signal and directly stimulates the testes to ramp up testosterone production.
This approach has a practical advantage over straight testosterone replacement. Testosterone injections, gels, or patches raise hormone levels but also suppress the brain’s signals to the testes, which can shrink them and reduce sperm production. Pregnyl preserves testicular function because it works through the same pathway the body uses naturally. For men who want to maintain fertility while treating low testosterone, this distinction matters a great deal. Treatment typically involves injections given multiple times per week over a period of months, with testosterone and sperm levels monitored along the way.
Undescended Testes in Boys
In some boys, one or both testes fail to move down into the scrotum before birth. When this happens and no physical blockage is causing it, Pregnyl can sometimes encourage descent. The hormone stimulates the testes to produce androgens, which play a role in the natural descent process that would otherwise happen during puberty.
Treatment is typically started between the ages of 4 and 9. The response is not always permanent. In some cases the testes descend and stay in place, but more often the effect is temporary. One practical benefit of trying Pregnyl first is that it helps predict whether the child will eventually need surgery (orchiopexy) to move the testes into the correct position. If the testes respond to hCG and descend even temporarily, that’s useful diagnostic information.
What About hCG for Weight Loss?
You may have heard of hCG being marketed as a weight-loss aid, often paired with extremely low-calorie diets of 500 to 800 calories per day. This is not an approved use. The FDA has specifically warned against hCG weight-loss products, and Pregnyl’s own prescribing label states it is not intended for this purpose.
There is no evidence that hCG itself causes weight loss. Any pounds lost on these programs come from severe calorie restriction, which carries its own risks: gallstones, irregular heartbeat, vitamin and mineral deficiencies, and dangerous shifts in electrolyte levels. The Mayo Clinic notes that common side effects of hCG diets include depression, extreme fatigue, headaches, and fluid retention. In rarer cases, blood clots and ovarian cysts have been reported. hCG weight-loss products have also been linked to worsening certain cancers, including prostate cancer, because the hormone can stimulate androgen production that fuels tumor growth.
How Pregnyl Is Given
Pregnyl comes as a powder that is mixed with a sterile liquid before injection. It is given as an intramuscular shot, typically in the upper outer area of the buttock or thigh. In fertility settings, many patients learn to administer the injection at home after being shown the technique by a nurse. The timing of the injection is often very specific, particularly for ovulation triggering, so clinics will give you an exact time to inject.
Once mixed, the solution needs to be refrigerated and used within a set timeframe, as the hormone breaks down relatively quickly in liquid form. Unused powder vials are stored at room temperature. Your pharmacy or clinic will provide specific storage instructions with your prescription.
Side Effects and Risks
The most significant risk for women using Pregnyl during fertility treatment is ovarian hyperstimulation syndrome (OHSS). This happens when the ovaries overreact to hormonal stimulation and swell, sometimes releasing fluid into the abdomen. Mild OHSS causes bloating, nausea, and pelvic discomfort. Severe cases, though uncommon, can lead to rapid weight gain, difficulty breathing, and dangerous fluid shifts that require medical attention. Your fertility team monitors follicle development with ultrasound before giving the trigger shot specifically to minimize this risk.
Other side effects tend to be mild: injection site soreness, headaches, irritability, and fatigue. In men, fluid retention and breast tenderness can occur. In boys being treated for undescended testes, early signs of puberty (such as increased body hair or genital growth) sometimes appear during treatment but typically resolve once the injections stop.
Pregnyl is not appropriate for everyone. People with hormone-sensitive cancers (such as prostate, breast, or ovarian cancer), unexplained uterine bleeding, or an allergy to hCG products should not use it. Women with ovarian cysts unrelated to polycystic ovary syndrome are also advised against its use, as the hormone could enlarge them further.