A CA 125 test measures the amount of the protein CA 125 (cancer antigen 125) in your blood.A CA 125 test may be used to monitor certain cancers during and after treatment. In some cases, a CA 125 test may be used to look for early signs of ovarian cancer in women with a very high risk of the disease.A CA 125 test isn't accurate enough to use for ovarian cancer screening in all women because many noncancerous conditions can increase the CA 125 level.Many different conditions can cause an increase in CA 125, including normal conditions, such as menstruation, and noncancerous conditions, such as uterine fibroids. Certain cancers may also cause an increased level of CA 125, including ovarian, endometrial, peritoneal and fallopian tube cancers.