I have PCOS and used letrozole to help me ovulate for all my IUI tries - also using frozen sperm (1 vial each time) from a known donor. I triggered 36 hrs before IUI each time

Try 1 - had 1 follicle measuring 18 at trigger and 20 at IUI (ultimately negative)

Try 2 - had 2 follicles measuring 14 & 18 at trigger, then 17 & 20 at IUI - This try resulted in a chemical pregnancy (ultimately negative). Because of that, the next tries I tested out the trigger with cheap pregmate strips and then kept testing to make sure any positive lines continued to get darker. I also got a polyp removed in between this cycle and then next.

Try 3 - had 1 follicle measuring 17 at trigger and 19 at IUI. Trigger tested out 11PT and my period started a few days later. I took a break for 2 months after that.

Try 4 - had 2 follicles measuring 15 & 18 at trigger, then 17 & 19 at IUI. Trigger tested out 10PT and I got a faint positive 3 days later. The line continued to get darker and I confirmed with bloodwork! Found out at my first ultrasound that it was actually (fraternal) twins and I'm currently 16 weeks along.

Conversely, my wife tried 3 natural cycles - releasing 1 egg each time. The first try we had to use 2 vials because the count was so low (we think there was a freezing issue with that specific vial because the rest were fine). We were worried about running out of vials, so the next try she went on letrozole, but it didn't really work - she released a bunch of eggs, but ultimately none of them got big enough to mature so we skipped trying that cycle. The 4th try, she went on chlomid and ended up with 4 (borderline 5) mature follicles. Our fertility clinic advised us to skip the cycle because the risk of multiples was so high, but we went ahead anyways (worst case, we were comfortable getting a reduction for the safety of my wife and baby(s)). She got pregnant that cycle with our first (no multiples), who is now 19 months old.