The HCG Trigger Shot: What It Does and Why You Take It

Your reproductive endocrinologist has asked you to take a trigger shot, and you are now holding either an Ovidrel pen or a small vial of Novarel, trying to understand why a single injection is needed when your body is supposed to do this on its own. This post walks through what the hCG trigger shot actually does, why it has become a fixture of medicated cycles and IUI, and what you can reasonably expect in the 36 to 40 hours that follow.

The short version is this. The trigger shot is a synthetic version of the luteinising hormone (LH) surge that normally tells a mature follicle to release its egg. Instead of waiting for your own pituitary to deliver that signal, your clinic delivers it on a known clock, so the rest of the cycle, whether that means timed intercourse, an IUI, or simply knowing when to expect ovulation, can be planned with precision. And yes, Ovidrel is a trigger shot, the most commonly prescribed one in fertility clinics today.

Is Ovidrel a trigger shot, and what does hCG do?

A trigger shot is a dose of human chorionic gonadotropin (hCG), or in some cases a recombinant equivalent.¹ The drug is given as a single subcutaneous injection, usually into the fat layer of the lower abdomen or the front of the thigh. The brand names you are most likely to see are Ovidrel (recombinant choriogonadotropin alfa, supplied as a prefilled 250 microgram pen), Novarel, and Pregnyl (urinary-derived hCG, supplied as a powder you reconstitute with a diluent).⁶

hCG is structurally very similar to LH. The two hormones share a common alpha subunit and bind the same receptor on the ovary, which is the LH/hCG receptor sitting on the mature follicle. From the follicle's point of view, an hCG injection looks essentially identical to a natural LH surge, only larger and longer-lasting.¹ That single biological fact, that hCG and LH share a receptor, is the reason the trigger shot works at all.

When I explain this in clinic, I usually describe it as a synthetic LH surge in a syringe. Your body normally produces a brief burst of LH from the pituitary gland once a follicle is mature enough to ovulate. The burst lasts roughly 24 to 36 hours, the follicle responds by finalising the egg's last cell division and then rupturing to release it. The trigger shot does the same job, with one important difference: we control the timing. The injection goes in at a specific hour your clinic has chosen, and ovulation follows on a predictable timeline.

Why is a trigger shot used at all?

The first question many patients ask, fairly, is why we need to add a drug to a cycle when ovulation could happen on its own. There are several real reasons.

The first is timing precision. If you are doing intrauterine insemination (IUI), the sperm sample has to be in the uterus in the narrow window when the egg is released. Without a trigger, the clinic has to chase your own LH surge using ovulation predictor kits (OPKs), and the IUI gets scheduled reactively, often at inconvenient hours. With a trigger, the IUI slot is booked in advance and the timing margin is tight but reliable.³

The second is surge reliability. Many people with polycystic ovary syndrome (PCOS) do not get a clean, detectable LH surge on OPKs. The LH baseline in PCOS is often elevated, which can cause OPK strips to read positive when no surge has actually happened, or to never read clearly positive even when ovulation is approaching. In these cycles, waiting for an OPK is unreliable; the trigger removes the ambiguity.

The third is mature follicle, no rupture. Sometimes a follicle reaches mature size, around 18 to 22mm, but does not rupture on its own. This shows up on a follow-up scan as a persistent dominant follicle that has stopped growing but is still intact. The trigger shot rescues those cycles by forcing rupture so the egg is released.

The fourth, and the one I find clinicians sometimes underplay, is the corpus luteum effect. After ovulation, the empty follicle reorganises into the corpus luteum, which produces progesterone to support the luteal phase. Because hCG continues binding the LH/hCG receptor for several days, it gives the corpus luteum extra support during early luteal phase, which can be useful in some patients.

For a person on letrozole or clomid with timed intercourse, whether to add a trigger is a clinic-by-clinic decision. Some reproductive endocrinologists use a trigger in every medicated cycle. Others reserve it for cycles where the OPK has been unhelpful or the follicle is slow to release. I add a trigger in most PCOS letrozole cycles because the OPK noise is so high; for ovulatory patients on letrozole, the case is weaker. For IUI cycles, trigger is essentially universal.³

What does hCG do inside the ovary?

Once the trigger shot is injected, hCG enters the bloodstream and binds the LH/hCG receptor on the dominant follicle. Three biological events follow.

The first is resumption of meiosis. The egg has been arrested in the middle of cell division since before you were born. The LH surge, or in this case the hCG signal, releases that arrest. Meiosis I completes, and the egg becomes a mature secondary oocyte ready for fertilisation.¹

The second is follicular rupture. Over the next 36 to 40 hours, the follicle wall thins, prostaglandins and proteolytic enzymes weaken the connective tissue at the apex, and the follicle releases the egg into the fallopian tube. The exact timing varies slightly between people, but the window is reproducible enough that clinics use it to schedule procedures.⁴

The third is corpus luteum formation. The empty follicle collapses and reorganises into a yellow-pigmented endocrine gland called the corpus luteum. This structure produces progesterone (and some oestrogen) for the rest of the luteal phase, which is what prepares the uterine lining for implantation. The hCG you injected continues to bind LH receptors on the corpus luteum for several days after the shot, contributing to luteal-phase progesterone.

This last point matters for two reasons. It is part of why some clinics do not routinely give additional luteal support after a trigger, and it is also why the trigger shot itself shows up on a home pregnancy test for the next 7 to 14 days, a topic I cover in a dedicated post.

Who needs a trigger shot and who doesn't?

There is no single rule, but the rough pattern in practice looks like this.

IUI cycles: almost always triggered. The IUI slot is scheduled around the trigger, not the other way around.

Letrozole or clomid with timed intercourse: variable. Many clinics use a trigger when the patient has PCOS or when previous cycles had unreliable OPKs. Other clinics let ovulation happen naturally and confirm it with a day-21 progesterone draw.

Stim cycles for IVF: always triggered, but usually with higher hCG doses, sometimes a Lupron-only trigger, and sometimes a dual trigger combining both.

Unmedicated natural cycles: rarely triggered, since the whole point of a natural cycle is to let the body select and release a follicle on its own clock.

If you are reading this for a first letrozole cycle, the most likely reason a trigger has been added is that your RE wants either tight scheduling for IUI or removal of OPK ambiguity for timed intercourse. Both are reasonable. The shot does not replace your follicle, it just sets the alarm clock for it.

What trigger shot doses might I be prescribed?

There are two main categories of hCG used for triggering, and the dose terminology can be confusing because they are measured in different units.

Recombinant hCG (Ovidrel) comes as a prefilled 250 microgram pen. Recombinant simply means the drug is manufactured by genetically engineered cell cultures rather than purified from the urine of pregnant women, which is how the older formulations were made.⁴ The 250 microgram dose is biologically roughly equivalent to 5,000 to 10,000 international units (IU) of urinary hCG, and it is the standard dose for almost all ovulation induction and IUI cycles.

Urinary hCG (Novarel, Pregnyl) is dosed in IU. The standard ovulation induction dose is 5,000 to 10,000 IU, given as a single intramuscular or subcutaneous injection. Higher doses, 10,000 IU and above, are more typical of IVF cycles where the larger follicle cohort needs a stronger signal.

For comparative efficacy, large studies have found no meaningful difference between recombinant and urinary hCG in pregnancy outcomes.⁴ Ovidrel is more expensive and easier to use (prefilled, no mixing). Urinary hCG is cheaper but requires reconstitution. In the UK and most of Europe, urinary hCG is still common. In US clinics, recombinant is more typical because of convenience.

A note on storage. Ovidrel pens are refrigerated until use, but they tolerate brief time at room temperature, which matters if you are bringing the pen home from the pharmacy on a hot day. Always check the specific instructions on your prescription label or with your pharmacist. If a pen has been left at room temperature for longer than the manufacturer allows, call the clinic before injecting.

What does the trigger shot injection feel like?

Most patients I see report that the injection itself is unremarkable. The needle is short, the volume of fluid is small, and it goes into the subcutaneous fat layer of the lower abdomen, not into muscle. Pinch the skin a couple of inches off the navel, insert the needle at 90 degrees, push the plunger slowly, and discard the pen immediately into a sharps container or a sturdy capped plastic bottle.

You may feel a brief sting at the injection site, similar to many other subcutaneous injections. The site is rarely sore afterwards. Mild redness in a small patch around the injection point is normal and usually fades within a few hours.

What people more commonly notice is what happens 12 to 48 hours later. Some patients feel mild abdominal bloating as the follicle does its final rupture work. A few report a low pelvic ache or a one-sided pulling sensation around the time ovulation actually occurs, which is the same sensation some people experience naturally as mittelschmerz. Breast tenderness in the days after the shot is usually from rising progesterone in the early luteal phase, not from the trigger itself.

These symptoms are mild for the vast majority. Severe pelvic pain, rapid abdominal distension, calf swelling, shortness of breath, or significantly reduced urine output are not normal post-trigger sensations and warrant a call to your clinic, because they raise the concern of ovarian hyperstimulation syndrome (OHSS) or, much more rarely, a clot. OHSS is uncommon in oral medication cycles, where typically only one or two follicles are recruited. It is more of a concern in IVF stim cycles with many follicles, which is why your RE counts mature follicles on the pre-trigger scan and cancels the trigger if too many are recruited.

When does ovulation happen after the trigger shot?

This is the question every patient asks, and the answer is reassuringly precise.

Across most studies, ovulation occurs approximately 36 to 40 hours after the trigger injection.⁶ Some patients ovulate a little earlier, some a little later, but the window is tight enough that clinics use it to schedule IUI for 24 to 36 hours post-trigger and to advise intercourse on the night of trigger plus the following two nights.

Why the window is wider than a single hour comes down to follicle biology. Once hCG binds the receptor, the cascade of meiosis resumption, prostaglandin release, follicular wall thinning, and rupture all take time. Individual follicles vary slightly in how quickly each step happens, especially with respect to size at the moment of trigger. A 22mm follicle may rupture closer to 36 hours; a 17mm follicle may take longer.

For practical timing, the rule I give patients is simple. Day 0 is trigger night. Day 1, the next morning to evening, the egg is finishing maturation. Sometime between the night of Day 1 and the early hours of Day 2, the egg releases. The egg is viable for fertilisation for roughly 12 to 24 hours after release. Sperm survives in fertile cervical mucus for 3 to 5 days. The fertile window after a trigger is therefore the night of the trigger plus the following 48 hours, with the strongest probability concentrated in the first 36 hours after the shot.²

I have a dedicated post on optimal timing for sex or IUI after a trigger, including what to do if your IUI is the morning after trigger versus 36 hours later. Most patients want both clarity and a margin for error, and the truth is the timing has both built in.

How long does the trigger shot stay in your body?

This is also the question that produces the most anxiety, because it is tied to the urge to test early for pregnancy.

hCG has a half-life of roughly 24 to 36 hours in circulation.² After a 250 microgram Ovidrel dose (equivalent to about 6,500 IU), the drug clears from the bloodstream and the urine over approximately 7 to 14 days. Higher doses, such as the 10,000 IU urinary hCG used in some protocols, can take a few days longer.²

For a home pregnancy test (HPT), what this means is that any positive result in the first week after a trigger is most likely the drug itself, not a pregnancy. Lines fade over days 7 to 10, and the trigger is usually fully out by day 10 to 14. This is exactly why your RE typically schedules the beta-hCG blood test at 14 days post-trigger. By that point, the trigger has cleared and any hCG remaining in your blood is your own.

Some patients "test out the trigger" by taking a home test every morning starting the day after the shot, watching the line fade, and then waiting for it to redarken if a real pregnancy is implanting. This is one approach, and for some people the data reduces anxiety. For others, watching the line fade only to wait for it to come back is a recipe for two weeks of preoccupation. There is no medical reason to test before 14 days, and I usually advise patients to wait for the lab beta. I have a separate post on this if you want the longer version.

What does a typical trigger shot cycle look like?

If your clinic uses a trigger, the rhythm of the cycle looks something like this. Day 1 is the first day of a real period. From day 3 you take the oral medication, letrozole or clomid, for five days. Around days 11 to 13 you go in for a transvaginal ultrasound to check follicle size and uterine lining. If the follicle is mature, your clinic will give you the trigger shot or send you home with an Ovidrel pen and an exact time to take it that evening. From the moment of the trigger, the clock starts. Twenty-four to thirty-six hours later is the IUI or the start of the intercourse window. Two weeks after that, the beta-hCG blood test tells you whether the cycle worked.

That sounds tidy when written down. In practice it can feel like a series of scans, drives back and forth to the clinic, alarms set for awkward injection times, and a great deal of waiting. The hCG trigger shot is one of the more concrete moments in the cycle, partly because you actually do something with your hands, and partly because the timeline that follows is the most predictable forty hours of the whole month. So yes, Ovidrel is a trigger shot, and once you understand what it does, the rest of the cycle gets a lot easier to read.

What's next

• If your trigger is Ovidrel and you want side-effect and injection detail: Ovidrel: Side Effects, Timing, and What to Expect
• If you took the shot tonight and need to plan the next 72 hours: When to Time Sex or IUI After Trigger Shot
• If you are tempted to test early and want to understand the false-positive risk: Trigger Shot False Positives on Pregnancy Tests
• If you have heard of a Lupron trigger and want to know whether it applies to you: Lupron Trigger vs HCG Trigger
• If this cycle moves you toward IUI: IUI Explained

Sources

1. Practice Committee of the American Society for Reproductive Medicine. Evidence-based treatments for couples with unexplained infertility: a guideline. Fertility and Sterility 2020;113(2):305-322. https://doi.org/10.1016/j.fertnstert.2019.10.014
2. Damewood MD, Shen W, Zacur HA, Schlaff WD, Rock JA, Wallach EE. Disappearance of exogenously administered human chorionic gonadotropin. Fertility and Sterility 1989;52(3):398-400. https://pubmed.ncbi.nlm.nih.gov/2670598/
3. Cantineau AEP, Cohlen BJ, Heineman MJ. Ovarian stimulation protocols (anti-oestrogens, gonadotrophins with and without GnRH agonists/antagonists) for intrauterine insemination (IUI) in women with subfertility. Cochrane Database of Systematic Reviews 2007;(2):CD005356. https://doi.org/10.1002/14651858.CD005356.pub2
4. Driscoll GL, Tyler JPP, Hangan JT, Fisher PR, Birdsall MA, Knight DC. A prospective, randomized, controlled, double-blind, double-dummy comparison of recombinant and urinary HCG for inducing oocyte maturation and follicular luteinization in ovarian stimulation. Human Reproduction 2000;15(6):1305-1310. https://doi.org/10.1093/humrep/15.6.1305
5. Andersen AN, Devroey P, Arce JC. Clinical outcome following stimulation with highly purified hMG or recombinant FSH in patients undergoing IVF: a randomized assessor-blind controlled trial. Human Reproduction 2006;21(12):3217-3227. https://doi.org/10.1093/humrep/del284
6. U.S. Food and Drug Administration. Ovidrel (choriogonadotropin alfa) Prescribing Information. https://www.accessdata.fda.gov/scripts/cder/daf/index.cfm?event=overview.process&ApplNo=020726