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If you’re new to fertility treatments, you’re probably curious about how many and what kinds of needles you may have to take. A common injection is what’s known as the “trigger shot,” which doctors can prescribe to bring on the final stages of ovulation.
What is the trigger shot?
The trigger shot is an injection of human choriogonadotropin (hCG), often under the name Ovidrel or Pregnyl. hCG is the same as the human pregnancy hormone, but it looks just like luteinizing hormone (LH), and they both act on the same LH receptor which is responsible for releasing an egg during ovulation.
LH typically surges mid-cycle, and has two important jobs: to mature the egg within the dominant follicle (the sac that holds the egg) and to release the egg from the follicle wall so that it’s now ready to burst from the ovary on its way to the fallopian tube.
In fertility treatment, doctors can use a trigger shot with hCG to play the role that LH usually would in an unmedicated cycle and kickstart ovulation. (There are also non-hCG medications that can act as a trigger shot. These are called GnRH agonist medications (brand names include Suprefact and Decapeptyl) and are sometimes used in IVF cycles as a strategy to decrease the risk of developing ovarian hyperstimulation syndrome.)
What does a trigger shot do?
Trigger shots aren’t always used in fertility treatment—it will depend on your reproductive health and your doctor’s approach. It can spur the final stage of ovulation once a dominant follicle has grown if you don’t have your own LH surge, or it’s commonly used to help your doctor line up timing for sex, at-home insemination, an IUI, or an egg retrieval (for IVF or egg freezing).
Why use a trigger shot?
- If you’re not able to have an LH surge on your own (for example, some patients with PCOS, or those who are hypogonadotropic hypogonadism, which can be caused by low body mass, excessive exercise, a genetic condition, etc.)
- To control ovulation for timed intercourse or IUI
- To time the release of eggs for an egg retrieval (for IVF or egg freezing)
When to take the trigger shot?
In timed intercourse and IUI cycles, your doctor will monitor the growth of your ovarian follicles. Once the biggest one (called the dominant follicle) is above 16 to 18 millimeters (this may vary clinic to clinic), your doctor will instruct you when to take the trigger shot and when to have sex/at-home inseminate or when your IUI(s) will be scheduled. This process makes it easier to time treatment, so that sperm can be waiting in the fallopian tube for the egg’s arrival 24 to 36 hours after the trigger shot.
In IVF cycles, the trigger shot can also be used to time an egg retrieval. This is to help your doctor get the highest number of mature eggs in one go. Your doctor will typically tell you to take your trigger shot about 34 to 36 hours before your scheduled retrieval (again, this is clinic-dependent).
Getting that timing just right is critical. Triggering too early can mean that many follicles don’t have eggs released (they’re still stuck to the wall) or are immature (have not completed the maturation process spurred by the trigger). If it’s too late, the follicles will have already ovulated leaving no eggs to be retrieved. So it’s important to follow your doctor’s precise guidance on when to inject. It can be helpful to set an alarm and to make sure you’ve read through the instructions in advance. And watch out for daylight saving time—a notorious culprit for causing timing mishaps for fertility patients.
How long after trigger shot do you ovulate?
You won’t ovulate right away after taking the trigger shot. It typically takes 24 to 36 hours for the eggs to be released after you inject.
How long does the trigger shot stay in system?
The half-life of an hCG injection is about 29 to 48 hours, which means that by this time, the concentration remaining in the body is down to only half. That said, hCG can still be detectable for up to 10 days post-trigger, and even longer for some (translation: the lingering hormones from the injection can cause a pregnancy test to give a false positive—more on that below). How fast hCG gets cleared from the body comes down to your metabolism, body weight, the dose injected and injection type (subcutaneous versus intramuscular).
How long after trigger shot can you test for pregnancy?
Here’s where things can get tricky. During pregnancy, hCG is secreted by the implanted embryo to help maintain levels of estrogen and progesterone produced by the corpus luteum (the leftover follicle shell that’s now releasing important hormones). It’s hCG that an at-home pregnancy test looks for in urine. But because a trigger shot contains hCG, testing too soon can lead to a false positive.
- Ovulation happens roughly 36 hours after the trigger shot.
- If an egg is fertilized, it takes about six days for the developing embryo to reach the uterus and start to implant.
- Around this time, the pregnancy hormone hCG starts to be produced—but it takes a few more days after implantation for hCG levels to be high enough to get picked up on an at-home pregnancy test. (Though an early-results test like ours can spot pregnancy five days before your expected period, if you’re taking a trigger shot, its hCG could still be picked up at that point so it’s best to wait the full 14 days post-injection.)
(We’ve got a full guide on when and how to take a pregnancy test if you’re looking for extra info.)
Sometimes you’ll see people in the TTC (trying to conceive) community “testing out” the trigger shot, where they take daily pregnancy tests to watch the hCG from the trigger shot fade away and then look for the line to get dark again to indicate HCG from a pregnancy. Meanwhile, others prefer to wait until they’re in the clear for an accurate result, be it with an at-home urine test or a blood test at their fertility clinic.
What are the trigger shot side effects?
The most common side effects from the trigger shot are lower abdominal tenderness or pain due to the ovaries being slightly enlarged (this is likely from the ovulation process itself). You might also notice tenderness and/or bruising around the needle spot.
Rarer side effects (reported by less than two percent of patients) include breast pain, flatulence, abdominal bloating, nausea, sore throat, upper respiratory tract infection and itching.
Can a trigger shot increase pregnancy success rate?
Fertility treatment doesn’t always require using a trigger shot. Your doctor will review your individual case to determine your treatment plan.
When it comes to boosting your odds of success, there seems to be no significant difference in clinical pregnancy rates when you ovulate on your own versus using an hCG trigger shot.
Rather, the difference between a cycle with and without a trigger is that “natural” ovulation (sans trigger) may require more frequent clinic visits so that your doctor can keep an eye on when ovulation is coming. Or, your doctor may ask you to use at-home ovulation tests to monitor on your own. (Learn all about ovulation signs and symptoms to help you know what to expect.)
Whether it comes down to a trigger shot or careful monitoring, the goal is to sync up when the egg and sperm meet, or to time the release of your eggs if they’re being collected for freezing or IVF. To get it just right, it’s key to follow your doctor’s instructions as closely as you can.
