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The trigger injection: timing ovulation for IUI and IVF
A trigger injection is usually a single injection that completes the ripening of the egg and sets the time of ovulation. It is given when the follicle is the right size on the scan. After an hCG trigger, ovulation usually happens about a day and a half later, so IUI or the IVF egg collection is timed from the moment the injection is given.
Watch the videos
In these videos our fertility specialist explains which injections are used to trigger ovulation; the second video is in Hindi.
Types of trigger injection
- hCG injection for ovulation: the most common trigger in IUI and many IVF cycles. It acts like the body's own surge of LH.
- GnRH agonist trigger: used in some IVF cycles that use antagonist injections, especially when many follicles have grown. It lowers the risk of ovarian over-stimulation (OHSS).
- Dual trigger: hCG and an agonist together, used in some IVF cycles to help egg maturity.
When is IUI done after the trigger?
IUI is usually planned for about one to one and a half days after an hCG trigger, close to the time of ovulation. With timed intercourse, couples are usually advised to have intercourse on the day after the trigger and sometimes the day after that. Your clinic gives you the exact plan.
Trigger timing in IVF
In IVF the egg collection is booked about a day and a half after the trigger, before the eggs are released. That is why the injection must be taken at the exact time given, often late at night.
Things to know
- An hCG trigger can make a home pregnancy test look positive for several days afterwards, so the pregnancy test is done on the day your clinic advises.
- Mild bloating or tenderness can follow. Seek help at once for a swollen, tight belly, breathlessness or passing very little urine.
- Do not take a trigger injection on your own: it is timed to a scan.
How the trigger works
In a natural cycle, a surge of the hormone LH tells the egg to finish maturing and the follicle to release it. The trigger injection copies this surge at a time chosen from the scan. In IVF, the eggs are collected around 36 hours after the trigger, just before they would be released. In IUI or timed intercourse, ovulation usually follows about a day and a half later, sometimes up to two days.
The dual trigger
In some IVF cycles, two triggers are given together: hCG and a GnRH agonist. Your doctor may suggest it in particular cases, for instance after a cycle in which the eggs collected were not fully mature. Your doctor will explain if it is part of your plan.
Taking the trigger at the right time
- Write down the exact time you are given, and set two alarms.
- Check that you have the right medicine and know how to prepare it, well before the time.
- If you are late or unsure, call the clinic at once rather than guessing.
- Tell the clinic the exact time you took it, especially before egg collection.
The trigger and the pregnancy test
An hCG trigger can stay in the blood for several days and give a false positive on a home test. This is why the pregnancy blood test is done on the date your clinic gives you, and it is wiser not to rely on early home tests. An agonist trigger does not cause this.
The trigger in a frozen embryo transfer
In a natural-cycle frozen transfer, the scan follows a follicle in your own cycle, and an hCG trigger may be given to time ovulation. The embryo transfer is then planned a set number of days later, matched to the age of the embryo.
The trigger and over-stimulation
In a cycle using antagonist injections, if the scans show a very large number of follicles, an agonist trigger can be used in place of hCG; its effect is shorter, so the ovaries are less likely to over-react. Often the embryos are then frozen, with the transfer planned for a later month once the ovaries have returned to normal size. Freezing is a routine, well-established part of IVF.
Read more about ovulation induction and IUI treatment.
Questions about the trigger injection
How long after the hCG trigger does ovulation happen?
Usually about a day and a half later, which is why IUI or the egg collection is timed from the moment the injection is given.
When should IUI be done after the trigger injection?
Usually about one to one and a half days after an hCG trigger. Your clinic gives you the exact time.
What is an agonist trigger?
A different trigger used in some IVF cycles with antagonist injections, especially when many follicles have grown. It lowers the risk of OHSS.
Can the trigger injection give a false positive pregnancy test?
Yes. hCG from the trigger can stay in the body for several days, so test only on the day your clinic advises.
What if I miss the trigger time?
Call your clinic straight away. The timing matters, and the plan may need to change.
Is an early home pregnancy test reliable after the trigger?
Usually not. The hCG in the trigger is the same hormone a home test picks up, and it can linger for several days, so an early test may look positive when there is no pregnancy. The blood test on your planned date gives the real answer.
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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.