There are several different definitions of polycystic ovarian syndrome (PCOS). The most commonly used definition is the Rotterdam criteria, which states that the diagnosis of polycystic ovarian syndrome requires at least two of the following three criteria to be present: (1) oligo- or anovulation, (2) clinical and/or biochemical signs of hyperandrogenism, and (3) polycystic ovaries. The definition requires that all other possible causes of the aforementioned features must be excluded prior to the diagnosis of PCOS being made. Only one of the features required for the diagnosis of PCOS can be diagnosed on ultrasound, and other aetiologies for the required features cannot be excluded by ultrasound. Therefore it is not possible to diagnose a woman as having PCOS by ultrasound alone.