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The antagonist protocol in IVF
The antagonist protocol is the most common way of running an IVF cycle today. Egg-growth injections start early in the cycle, and a few days later a second injection, a GnRH antagonist such as cetrorelix or ganirelix, is added to stop the eggs from being released too early. It is sometimes called the short protocol.
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In this video our fertility specialist explains the cetrorelix injection and its role in IVF.
Why stopping early ovulation matters
As several follicles grow with the egg-growth injections, the body can release a natural surge of the hormone LH, which would release the eggs before the planned egg collection. The antagonist blocks that surge for as long as it is taken, so the eggs stay in the ovary until the collection day.
How the antagonist protocol runs
- Egg-growth injections (FSH or HMG) start early in the cycle, usually on day 2 or 3.
- The antagonist (cetrorelix injection or similar) is added a few days later, either on a set day or when the scans show the largest follicles growing well, and continued daily.
- The trigger injection is given when the follicles are ready, and the egg collection follows about a day and a half later.
Why it is widely used
- It is shorter than the older long protocol, with fewer days of injections.
- It allows a GnRH agonist trigger instead of hCG, which lowers the risk of ovarian over-stimulation (OHSS) in women who make many follicles, such as many women with PCOS.
- It suits many women, including those with a lower egg reserve.
The choice of protocol is made for each woman after her tests and scans. Other plans, such as the long agonist protocol, still suit some women.
Side effects
The antagonist injection is a small injection under the skin. Redness or itching at the injection site is common and settles. Please do not take these injections on your own: they are timed to your scans.
Fixed-day or flexible start
There are two common ways to decide when the antagonist is added. In the fixed-day approach it starts on a set day of the egg-growth injections, often around the fifth or sixth day. In the flexible approach the start is decided from the scan, once the largest follicle has reached a certain size, or from a blood test for LH and estradiol. Both work well; the choice depends on your response and on how the team prefers to monitor.
A typical antagonist cycle, day by day
- A first scan in the early days of the period, so the team can see the resting ovaries and a thin lining before any injection begins.
- Egg-growth injections every day, usually at the same time each evening.
- Follow-up scans and blood tests every two or three days, to measure the follicles and adjust the plan.
- The antagonist injection added once the follicles are growing, and given daily until the trigger.
- The trigger injection when the largest follicles are mature, at an exact time given by the clinic.
- Egg collection around 36 hours after the trigger, done under short anaesthesia.
From the first injection to egg collection usually takes around ten to fourteen days.
The antagonist protocol and the long protocol
In the long agonist protocol, a different medicine is started in the previous cycle to switch off the body's own hormones for a while before stimulation begins. This takes more weeks and more injections. The antagonist protocol blocks the hormone surge only for the few days it is needed. For many women the chance of pregnancy is similar with either plan, and the shorter plan is easier to follow. Your doctor may still choose the long protocol in some situations, for example with certain cases of endometriosis or when timing needs to be planned well ahead.
Practical tips during the injections
- Keep the injections as instructed on the pack, and check whether they need the fridge.
- Write down the time of each injection, and set a phone alarm.
- Bring your injection chart to every scan.
- Mild bloating and tiredness are common as the follicles grow. A rapidly swelling or tight belly, severe pain, vomiting, breathlessness or passing very little urine, during the injections or in the days after egg collection, need an urgent call to the clinic.
See also IVF injections and the trigger injection.
Questions about the antagonist protocol
What does the cetrorelix injection do in IVF?
It blocks the natural LH surge, so the eggs are not released before the planned egg collection.
When is the antagonist injection started?
A few days after the egg-growth injections begin, either on a set day or when the scans show the largest follicles growing well. Your doctor tells you the day.
Is the antagonist protocol the same as the short protocol?
Often, yes. The antagonist protocol is widely known as the short protocol, because it takes fewer days than the long protocol.
Why is the antagonist protocol used in PCOS?
It allows an agonist trigger instead of hCG, which lowers the risk of OHSS in women who grow many follicles.
What if I miss an antagonist injection?
Call your clinic at once. A missed dose can let ovulation happen early, so the plan may need to change.
How many days does the antagonist protocol take?
From the first injection to egg collection usually takes around ten to fourteen days, depending on how the follicles grow.
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What you read here explains fertility care in general terms and cannot replace a consultation. Decisions about your own care should be made with a doctor who knows your history and has seen you. We do not carry out or offer sex determination of any kind, which is prohibited by law in India under the PCPNDT Act.