Updated by the Progyny Clinical Team — August 2025.
You’ve finished your consultation and all the testing. Now you’re ready to begin treatment.
Based on your results, you’ve made a plan with your reproductive endocrinologist (REI) to start in vitro fertilization (IVF) using an antagonist protocol.
What is the antagonist protocol?
There are several IVF protocols that can be used. Each one is designed to help your ovaries grow the optimal number of mature eggs that can be fertilized. Hopefully, this leads to a healthy pregnancy.
The antagonist protocol is the most common protocol to start with. Your REI will personalize your medication doses, visits, and schedule. It’s called the “antagonist protocol” because of the medication that is used to prevent ovulation while the eggs are growing. These medications — Cetrotide or Ganirelix — block signals from your brain that would normally tell your body to ovulate (release the eggs).
How does it work?
As with all IVF cycles, you’ll take injectable hormone medications called gonadotropins. These help your ovaries grow follicles — small fluid-filled sacs that each hold an egg. As the follicles grow, your body produces estrogen.
Once your estrogen levels rise and/or your follicles are a certain size, your brain may try to trigger ovulation. This is where the antagonist medication comes in. Your REI will decide the right time to start this based on your progress during the treatment cycle. It’s important to prevent ovulation so that the follicles can keep growing until they reach the optimal size and maturity for retrieval.
What’s the general timeline?
- Your clinic will schedule a baseline blood test and ultrasound. This is usually on day 2 or 3 of your period, or after taking birth control pills for a short time.
- If everything looks good, you’ll start your gonadotropin shots that evening to start growing the follicles.
- After 2 to 4 days of the injections, you’ll return to the clinic for more blood work and an ultrasound to check your progress. Your REI may adjust your dose based on how things are going.
- Most people start the antagonist medication on days 4 to 7 of gonadotropin injections. You’ll continue taking gonadotropins and the antagonist medications until it’s time for your trigger shot — the final shot that gets your eggs ready for retrieval.
- A typical cycle involves 9 to 12 days total of injections with ultrasound and bloodwork visits every one to two days.
What are the benefits of the antagonist protocol?
- Antagonist medications work quickly, so you can begin them exactly when your body is ready.
- Since you don’t start them right away, your body is able to use some of its natural gonadotropins early in the cycle.
- It lowers the risk of ovarian hyperstimulation syndrome (OHSS), a possible side effect of IVF. Your REI can choose the safest type of trigger shot for you, including Lupron instead of human chorionic gonadotropin, to help lower the chance of OHSS.
What else do you need to know?
- Cetrotide and Ganirelix can sometimes cause a reaction at the injection site, like a small, itchy bump or hives. This is very common and usually goes away in a few hours. You can take allergy medicine if needed but always check with your fertility care team if you’re concerned.
- It’s very important not to miss a dose of the antagonist medication. Missing a dose could lead to early ovulation, which means you might lose the eggs you’ve worked so hard to grow.
- Once you begin antagonist injections, you’ll need to take one every day. Because the timing of your trigger shot can be unpredictable, stay in touch with your fertility team and pharmacy — to make sure you have enough medication on hand.
- Your REI may increase your gonadotropin dose after you start the antagonist. This helps the follicles keep growing. Keep track of your supply for this too, in case you need a refill.
If you have questions or concerns, Progyny is here for you. Please contact your Progyny Care Advocate for support.
Disclaimer: The information provided by Progyny is for educational purposes only and is not medical advice. Always consult a qualified healthcare provider for medical guidance.