Ivf trigger question

So I went in for a monitoring appt today, last night was night 6 of injections (gonal and menopur). I have follicles around 8-12mm in size, so as of right now they do not have me on cetrotide. However, I have a friend who’s on the same dosing as me as well as the exact same day and started her cetrotide 2 days ago. Do these ivf cycles sort of mimic your natural cycle? I naturally ovulate around day 18 of my cycle-is that potentially why my follicles would be smaller right now?