PCOS cannot be diagnosed by ultrasound because polycystic ovaries are not cysts. They’re follicles or eggs which are normal for the ovary.
It’s normal for all women to sometimes have a higher number of follicles. It’s normal for young women to always have a higher number of follicles because young women have more eggs. That’s why the new international PCOS guidelines state that “ultrasound is now not recommended in diagnosis in those within 8 years of the start of periods.”