I had 1 at 14mm on day 6, it was a very pesky leading follicle! They let it go all the way to 26 mm (day 11) to wait for the next cohort to come up. It was pretty painful in the end, felt like ohss, and was glad I did a freeze all! But the good news is we got 5 pgd normals out of that cycle, and am currently 7 weeks preg with first fet, so it can work out. I did do bcp to start, btw, but in my case leading follicle happened anyway.