Does letrozole lengthen cycles?

This is our first IUI cycle - I usually ovulate around day 12-13 on my own. This month I took letrozole days 3-7. Went in for an ultrasound cycle day 9 and had 2 15mm follicles and 1 13mm. They told me to trigger day 13 and come in day 15 for IUI based off of the follicle sizes - however I’m nervous I’m going to ovulate on my own before the IUI since I always do before cycle day 15! Just looking for some hope here I guess that we’ll still be alright, I don’t want to waste the trigger shot if we aren’t even going to end up doing the IUI.