What Does Slice 10 Anechoic Cystic Formation Of 3 To 7mm Mean?

hello doctors my sister have presence of slice 10 anechoic cystic formation from 3 to 7 mm in the right side ovary and her period didnot come since 3 months we consolted a doctor about it but didnot understand so much about it can you tell me more about it she is 20 years old thank you