The trigger shot — what it does, when it's given, and why timing matters so much

If you've reached the trigger shot stage, you know how much significance it carries. The precise timing, the instruction to administer it at a specific hour, the awareness that egg collection is now scheduled and counting down.

Here's what's actually happening — and why the timing is so carefully controlled.

What the trigger shot does

Throughout the stimulation phase, your follicles have been growing but your eggs have not yet completed their maturation process. The trigger shot — usually an injection of hCG or a GnRH agonist — initiates that final stage of maturation, preparing the eggs for collection.

"Collection is timed for approximately 36 hours after the trigger — after maturation is complete but before the body would naturally ovulate and release the eggs."

Why the timing is so precise

If collection happens too early, the eggs won't be fully mature. If it happens too late, ovulation may have already occurred and the eggs may be lost. The 36-hour window is well-established in the research, and your clinic will schedule collection with this in mind — which is why your trigger time is given to the hour.

hCG vs GnRH agonist triggers

For most patients, a standard hCG trigger is used. For patients at higher risk of OHSS — typically those who've produced a large number of follicles — a GnRH agonist trigger may be offered instead. This carries a lower OHSS risk but requires a frozen transfer cycle, as it affects the uterine lining.