Polycystic ovary syndrome (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 PCOS cannot be diagnosed by ultrasound. At the same time, PCOS cannot be ruled out by ultrasound because it’s possible to have normal-appearing ovaries on ultrasound and still have the hormonal condition PCOS.