A trigger shot is the final injection of a stimulated cycle — given once your follicles have grown to the right size on IUI or IVF monitoring scans. It delivers the hormonal signal (hCG, or sometimes a GnRH-agonist) that tells the eggs inside those follicles to complete maturation, the same signal a natural LH surge would send. Ovulation, or egg retrieval in an IVF cycle, is then scheduled 36 to 40 hours later — which is why the dose and exact time are fixed by the clinic and never adjusted at home. What is a trigger shot, and why does its timing have to be so exact? covers the full explanation, including the warning signs to watch for afterward.