I don't have PCOS but did the whole ultrasound & tracking ovulation whilst on clomid, later FSH and later again IVF. Speaking from a non-PCOS perspective there is usually a predictable growth pattern. I remember my FS knowing when to get me back in for another ultrasound and being able to tell when the follicle was ready for O - indicating some predictablity. If I was close-ish and she wasn't sure she sent me for a BT and they could tell more from that.

When I was on FSH for the purpose of IVF I did have some surprise follicles which the FS didn't predict were going to be there. I remember having about 11 eggs ready and then when I came back a few days later I had 15, so perhaps there can be some surprises but usually it is predictable. Interestingly enough I only had 11 eggs fertilise so I will never know if the late arrivals were able to be fertilised or not.

Wishing you the best of luck. I know it's not easy.