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Polycystic ovary. Coloured 3-D ultrasound scan of a polycystic ovary (centre). Polycystic ovarian disease (or Stein-Leventhal syndrome) causes multiple cysts (black) to form in the ovaries. A cyst is an abnormal fluid-filled cavity. Ovarian cysts can form when follicles fail to ovulate. Symptoms of polycystic ovarian disease are irregular or absent menstruation, obesity and hirsuitism (excessive hairiness). The condition is caused by hormone imbalances. Ultrasound is a diagnostic technique that sends high frequency sound waves into the body via a transducer. The returning echoes are recorded and used to build an image of an internal structure. (KEYSTONE/SCIENCE PHOTO LIBRARY/DR NAJEEB LAYYOUS)