Pregnyl works by mimicking luteinizing hormone (LH) in your body — it binds to the same receptors and triggers the same biological responses, whether that's releasing an egg from the ovary, producing testosterone, or helping testes descend in young boys.

What Pregnyl Does in Your Body

Pregnyl contains human chorionic gonadotropin (hCG), a hormone that's naturally produced during pregnancy. What makes hCG useful as a medication is that it looks almost identical to luteinizing hormone (LH) at the molecular level. Your body's LH receptors can't tell the difference.

Think of it like a master key. LH is the key your body normally uses to unlock certain hormonal processes. hCG is a copy of that key — it fits the same locks and opens the same doors.

Here's what happens depending on who's taking it:

In Women: Triggering Ovulation

During a normal menstrual cycle, your pituitary gland releases a surge of LH that tells the ovary to release a mature egg. This is ovulation. In fertility treatment, doctors use medications like Gonal-F or Menopur to grow multiple follicles (the fluid-filled sacs that contain eggs), and then use Pregnyl as the "trigger shot" to cause those follicles to release their eggs.

The process works like this:

1. Ovarian stimulation medications grow follicles over 8-14 days
2. Your doctor monitors follicle development with ultrasounds and blood tests
3. When follicles are mature (typically 18-22mm), you receive a Pregnyl injection
4. Pregnyl mimics the LH surge, triggering ovulation approximately 36-40 hours later
5. Egg retrieval (IVF) or timed intercourse/insemination (IUI) is scheduled accordingly

Pregnyl also supports the corpus luteum — the structure left behind after the egg is released — which produces progesterone to help maintain an early pregnancy.

In Men: Stimulating Testosterone Production

In men with hypogonadotropic hypogonadism (where the pituitary gland doesn't send enough LH signal), Pregnyl steps in as a substitute. It binds to LH receptors on the Leydig cells in the testes, telling them to produce testosterone.

It's like having a backup alarm system. If the brain's normal signal (LH) isn't reaching the testes, Pregnyl delivers the same message directly through injection.

This is important for two reasons:

• Testosterone production: Men need testosterone for energy, muscle mass, bone density, mood, and sexual function
• Fertility preservation: Unlike external testosterone (which can actually shut down sperm production), hCG stimulates the testes to make testosterone internally, which keeps sperm production running

In Boys: Promoting Testicular Descent

In prepubertal boys with cryptorchidism (undescended testes), Pregnyl stimulates testosterone production in the testes, which can promote descent into the scrotum. The testosterone essentially helps complete a developmental process that didn't happen on its own.

How Long Does It Take to Work?

The timeline depends on what Pregnyl is being used for:

• Ovulation trigger: Ovulation occurs approximately 36-40 hours after injection. This is one of the most precisely timed events in fertility treatment.
• Testosterone stimulation in men: Testosterone levels begin to rise within days, but it may take several weeks of regular injections (2-3 times per week) to reach therapeutic levels.
• Cryptorchidism: Treatment protocols typically run 3-6 weeks. Descent, if it's going to happen, usually occurs during this treatment window.

How Long Does Pregnyl Last in Your System?

hCG has a half-life of approximately 24-36 hours, meaning it takes about 1-1.5 days for half the dose to be cleared from your body. It's generally detectable in blood for about 10-14 days after injection.

This is important to know because:

• Pregnancy testing: If you take a pregnancy test too soon after a Pregnyl injection, you may get a false positive. Most doctors recommend waiting at least 10-14 days after the trigger shot before testing.
• Multiple injections: For men on ongoing hCG therapy, regular dosing (2-3 times per week) maintains steady levels.

What Makes Pregnyl Different from Similar Medications?

There are several hCG products on the market. Here's how Pregnyl compares:

Pregnyl vs. Ovidrel

• Pregnyl is urinary-derived hCG (extracted from the urine of pregnant women) and given as an intramuscular injection
• Ovidrel (choriogonadotropin alfa) is recombinant hCG (made in a lab using DNA technology) and given as a subcutaneous injection with a pre-filled syringe
• Ovidrel is more convenient (no mixing, easier injection), but Pregnyl allows more flexible dosing since it comes in a multi-dose vial

Pregnyl vs. Novarel

• Both are urinary-derived hCG products given by IM injection
• They're essentially interchangeable — same active ingredient, same route, same dosing
• Availability and price may differ, making one a practical substitute for the other during shortages

Pregnyl vs. Lupron Trigger

• Some IVF protocols use Lupron (leuprolide), a GnRH agonist, instead of hCG as the ovulation trigger
• Lupron triggers cause a natural LH/FSH surge from the pituitary gland rather than providing exogenous hCG
• Lupron triggers carry a lower risk of OHSS, making them preferred for patients at high risk for hyperstimulation
• However, Lupron triggers may result in a weaker luteal phase, so they're not suitable for all protocols

For more details on alternatives, see our full guide on alternatives to Pregnyl.

Final Thoughts

Pregnyl works because hCG is nature's LH lookalike. It unlocks the same biological processes — ovulation, testosterone production, testicular descent — by fitting into the same receptors. It's a straightforward mechanism, and understanding it helps you make sense of your treatment plan.

If you're currently taking Pregnyl or about to start, you may also want to read about side effects to watch for, drug interactions, and how to save money on your prescription.