Trigger Shot: Definition, How it Works, Side Effects, & More

By Dr. Kenosha Gleaton, OBGYN

There are a lot of acronyms and terms involved in the fertility journey, especially for those pursuing assisted reproductive technology (ART). For those undergoing in vitro fertilization (IVF), intrauterine insemination (IUI), or timed intercourse, “trigger shot” is likely a familiar term. Let’s talk more about what a trigger shot is, how it works, and what to expect. 

What Is a Trigger Shot?

A trigger shot is an injection of human chorionic gonadotropin (hCG), sometimes referred to as the pregnancy hormone. [1] A trigger shot is used to help achieve pregnancy by encouraging the final growth and release of mature eggs from the ovaries (aka ovulation). Trigger shots are often used alongside other gonadotropins, like FSH or LH. [1-2]

Some examples of standard trigger shots include Ovidrel, Novarel, and Pregnyl. [3] There is also an agonist trigger shot that is called Lupron. Lupron doesn’t use hCG, which may be better suited for those who are at a higher risk of developing ovarian hyperstimulation syndrome (OHSS) or have other concerns with using hCG. [4] Lupron can also have the opposite effect of a trigger shot, depending on how it’s used. Need more information? Read our beginner’s guide to fertility drugs → 

The Science Behind Trigger Shots

Trigger shots are the result of years of research surrounding conception, fertility, and hormones. To give you a brief overview, pregnancy requires a mature egg, sperm, and a path for the two to meet. For a mature egg and sperm to meet, those trying to conceive (TTC) are told to time sex around the fertile window, which is a few-day period surrounding ovulation. Unfortunately, TTC is not always this simple. Many factors can influence someone’s ability to get pregnant, including sperm health, egg health, and ovulatory dysfunction (interesting fact- the most common overall cause of female fertility is the failure to ovulate). [5] This is where fertility treatments and hormonal therapies, like hCG injections, can help. Find out if you're ovulating with the ovulation test kit. 

HCG, the hormone found in most standard trigger shots, has many roles in the body. In people assigned male at birth (AMAB), hCG aids in the production of testosterone and sperm. [6] In people assigned female at birth (AFAB), hCG is crucial for sustaining a healthy pregnancy by thickening the uterine lining and preventing the body from menstruating. [7] 

What Does the Trigger Shot Do?

Depending on how a trigger shot is used, it can help facilitate egg maturation, ovulation, and reproductive division in eggs, a process known as meiosis. 

Trigger Shots in Intrauterine Insemination (IUI)

IUI is a procedure that involves a provider delivering concentrated sperm into the uterus through a catheter. When undergoing IUI, a healthcare provider will monitor the ovarian follicles via ultrasound to ensure the follicles are growing as expected. The provider will instruct the patient on when exactly to administer the shot at home, often 24-36 hours before the insemination procedure will take place. [1-2] The shot will encourage ovulation, improving the likelihood of sperm fertilizing an egg. 

Read our breakdown of IUI vs IVF 

Trigger Shots in In Vitro Fertilization (IVF)

When used with IVF, the trigger shot is given before eggs are retrieved from the uterus. The hCG injection is used to encourage eggs to divide and mature. This process, known as meiosis, results in a decrease in the number of chromosomes found in the eggs. [1] Similarly to IUI, the healthcare provider will instruct the patient when to administer the shot. Afterward, the healthcare provider can confirm the eggs are mature through microscopic visualization before retrieving the eggs. [1-2] Once retrieved and fertilized, the provider will then transfer the embryos back into the uterus for implantation. 

How Long After the Trigger Shot Will I Ovulate?

Healthcare providers and researchers state that ovulation will occur within about 36-42 hours after an hCG shot is given. [1,8] Your healthcare provider can administer blood tests to measure your hCG levels following injections, and they can talk with you about when to use early pregnancy test strips.

Side Effects and Considerations

Some potential side effects may arise, including [2-3]:

• Allergic reactions: skin rash, itching, hives, swelling
• Abdominal discomfort
• Nausea, vomiting
• Pelvic pain or bloating
• OHSS

OHSS can be life-threatening, so it’s important to let your healthcare provider know if you are experiencing any symptoms. [4] Symptoms can appear up to two weeks after a trigger shot is given. Symptoms may include abdominal pain, nausea, vomiting, diarrhea, bloating, blood clots, decreased urination, weight gain, and shortness of breath. [4] 

Gonadotropin therapy can also increase the risk of having a multiple pregnancy, especially when used for IUI or timed intercourse. [2] You should speak with your provider about any other risks or considerations to keep in mind regarding this treatment.  

Success Rates and Expectations

The trigger shot is often just one part of a larger fertility treatment plan, so it can be difficult to accurately assess success rates. Individual factors can also greatly influence fertility treatments. We do know that the trigger shot is a necessary part of fertility treatments like IVF, as it encourages the division and maturation of eggs prior to retrieval. [1] One study conducted on IUI cycles with and without the use of a trigger shot found that the injection greatly increased the pregnancy rate, from 5.8% to 18.2%. [9] When the trigger shot was used in combination with a natural LH surge, the pregnancy rate was over 30%. [9] 

Other research confirms the benefits of the trigger shot, with implantation and pregnancy rates greatly improving with the use of an hCG injection. [10] 

This being said, with all medical procedures and treatments, it’s important to speak directly with your healthcare provider to better understand your chances of success. Other factors- like your age and underlying conditions- can play a role in your success rates. 

Natalist's Role in Your Fertility Journey

Everyone’s fertility journey is different. Whether you’re new to the TTC community or you’re well-versed in all things fertility and pregnancy, we at Natalist are here to help. Get products designed to help you on your path to parenthood, from women's prenatal vitamin to self-care items and books. 

References:

1. Trigger Shot for IVF and Its Importance in Egg Quality. Coastal Fertility. January 2021. https://coastalfertility.com/ivf-trigger-shot/
2. Fauser, Bart. Barbieri, Robert. Martin, Kathryn. Patient education: Infertility treatment with gonadotropins (Beyond the Basics). Up to Date. April 2023. https://www.uptodate.com/contents/infertility-treatment-with-gonadotropins-beyond-the-basics#H2995902611
3. Human Chorionic Gonadotropin, HCG injection. Cleveland Clinic. Accessed December 2023. https://my.clevelandclinic.org/health/drugs/18559-human-chorionic-gonadotropin-hcg-injection
4. Ovarian hyperstimulation syndrome. Mayo Clinic. November 2021. https://www.mayoclinic.org/diseases-conditions/ovarian-hyperstimulation-syndrome-ohss/symptoms-causes/syc-20354697
5. What are some possible causes of female infertility? NIH Office of Communications. January 2017. https://www.nichd.nih.gov/health/topics/infertility/conditioninfo/causes/causes-female
6. Cole LA. Biological functions of hCG and hCG-related molecules. Reprod Biol Endocrinol. 2010;8:102. Published 2010 Aug 24. doi:10.1186/1477-7827-8-102
7. Human Chorionic Gonadotropin. Cleveland Clinic. March 2022. https://my.clevelandclinic.org/health/articles/22489-human-chorionic-gonadotropin
8. Firouz M, Noori N, Ghasemi M, Dashipour A, Keikha N. Comparing the Effectiveness of Doing Intra-uterine Insemination 36 and 42 Hours After Human Chorionic Gonadotropin (HCG) Injection on Pregnancy Rate: A Randomized Clinical Trial. J Family Reprod Health. 2020;14(3):173-179. doi:10.18502/jfrh.v14i3.4670
9. Taerk E, Hughes E, Greenberg C, et al. Controlled Ovarian Hyperstimulation with Intrauterine Insemination Is More Successful After r-hCG Administration Than Spontaneous LH Surge. J Reprod Infertil. 2017;18(3):316-322.
10. Jahanshahi M, Aleyasin A, Aghahosseini M, Najafian A, Shabani Nashtaei M, Hosseinimousa S. The effect of intrauterine hCG injection before embryo transfer on pregnancy rate in frozen embryo transfer cycles. Ann Med Surg (Lond). 2022;79:104091. Published 2022 Jun 25. doi:10.1016/j.amsu.2022.104091

Dr. Kenosha Gleaton is board-certified in gynecology and obstetrics and is the Medical Advisor of Natalist. She received her MD from MUSC and completed her residency at Carolinas Medical Center in Charlotte, NC.

Dr. Gleaton is passionate about women, health equity, and mentoring. She is the CEO of The EpiCentre, an OBGYN spa-like practice, and is a Clinical faculty member of Charleston Southern University. She is also a member of the American College of Obstetrics & Gynecology, the American Association of Gynecologic Laparoscopists, and the American Association of Professional Women