As with all other cancers, staging in ovarian cancer is a way of describing the size of the tumour and how far it has spread at the time of diagnosis. Ovarian cancer is largely staged by what the surgeon finds at the initial operation. The findings of the pathologist who examines the tumour tissue and other samples (biopsies) under the microscope are also important in deciding the stage of each tumour. Staging is then used by your specialists to help make the right decisions regarding your treatment, and it also gives a rough guide to the outlook.

For ovarian cancer, stage one (Stage I - Roman numerals are conventionally used) means the tumour is confined to the ovary. Stage II means the tumour has extended onto surrounding tissues and structures in the pelvis such as the uterus. Stage III means that the tumour has spread outside the pelvis (the lower part of the abdominal cavity) and into the main part of the abdomen. What the surgeon usually finds are several nodules of tumour attached to the surface of the bowel and other organs and their anchoring membranes, and in the omentum. The omentum is a large fatty membrane draped over the bowel just inside the abdomen. If possible, this will be removed at surgery, as it often harbours the bulk of the cancer cells from an ovarian cancer. Stage III is the stage that most women have at diagnosis.

In stage IV ovarian cancer there is evidence of cancer in the liver or outside the abdomen, usually in the outer lining of the lung. For each stage there are substages, which are given letters (e.g. Stage Ia, Ib, Ic, IIa etc) depending on a variety of other things about the spread of the cancer, such as the size of any deposits, and whether any cancer cells were seen in the abdominal fluid.

It is also important to remember that the staging system is only applied to cancers when they are first diagnosed, it is not used if they recur.