Unmedicated iui with trigger shot

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I am doing first round <a href="https://glowing.com/glow-fertility-program">iui</a> unmedicated but my doctor has mentioned that when they check the ultrasound in 3 days time if I haven’t gotten my surge they would recommend a trigger shot to force ovulation 2 days after. He says he doesn’t want the follicle to get too big. Questions- any side effects to a trigger shot? Why would you be worried about follicle size?