By the stage of a cancer we try to express how far the disease has spread. It is crucial, as treatment is mostly decided depending on the stage of a cancer. For ovarian cancer, doctors use a simple I-IV staging system called the FIGO (International Federation of Gynecology and Obstetrics) system.

Stage I means the cancer is confined to the ovaries. In stage IA, the cancer is confined to one ovary, while in IB the cancer is present in both ovaries. In stage IC, in addition to the cancer being present in either one or both of the ovaries, cancer cells may be present on the outer surfaces of one or both ovaries, or in fluid taken from inside the abdomen; or, the outer wall of a cystic ovarian tumor may have burst.

By stage II we mean the cancer has grown outside the ovary or ovaries, but it is inside the pelvis. In stage IIA, the cancer has reached the fallopian tubes or the womb, while IIB means the cancer has grown into other tissues in the pelvis, such as the bladder or rectum. Stage IIC indicates that in addition to stages IIA and IIB, either some cancer is present on the surface of at least one ovary or cancer cells are found in fluid taken from inside the abdomen during surgery, or the ovary ruptures before or during surgery.

Stage III means the cancer has spread outside the pelvis into the abdominal cavity. It is also stage III if cancer is found in the lymph nodes in the upper abdomen, groin or behind the womb. In stage IIIA, cancer can be seen under the microscope in tissue taken from the lining of the abdomen, while in IIIB, small tumor growths are found on the lining of the abdomen. In IIIC, tumor growths larger than 2cm are found on the lining of the abdomen; the lymph nodes in the upper abdomen, groin or behind the womb contain cancer.

Stage IV, the most advanced of all, means the cancer has spread into other body organs such as the liver or lungs.