The trigger shot is a shot of HCG which will make you ovulate within 24-36 hours (we did the trigger shot 24 hours before IUI the first cycle we tried it, and 36 hours before IUI the second cycle we tried it). It is used in otherwise-ovulating people in order to time the IUI precisely. If a trigger shot is used, there's really no reason to do more than one IUI in a cycle (we did anyway one time because the trigger shot was a last-minute decision and we already had two vials of sperm on hand). Generally, trigger shots are used in conjunction with ultrasound monitoring (to make sure the follicle is big enough). We were able to have the ultrasounds and the trigger shots covered by insurance (even though the IUIs and sperm were not covered). It might be a sort of extreme measure to take during a first attempt; we waited until the third cycle of TTC #3 to try it. We did use it during the first attempt at TTC #4, but that's just because we only had one vial of sperm from our older boys' donor left, and wanted to increase our chances of pregnancy. And it worked!

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One nice thing about the trigger shot is that it DOES NOT increase your chances of multiples (twins, triplets, etc.) when used in the absence of other fertility drugs. One bummer thing about it is that it makes you feel pregnant (since HCG is the pregnancy hormone) even if you're not (although both times we used it, we did get pregnant, so I'm not sure what symptoms were from the shot and what were from the new pregnancy hormones).
HTH!
Lex