Cervical cancer staging refers to the extent of the disease. Doctors stage the disease based on the size of the tumor and whether it has spread to nearby tissues or other organs in the body. Stage 0 is not considered invasive because the cancer cells are found only in the surface layer of cells of the cervix. Stage 1a is considered invasive cancer because the microscopic tumor grows through the surface layer of cells into the underlying cervical tissue and is 3 to 5 mm deep by 7 mm wide. Stage 1b cervical cancer has two stages: one where the tumor can still be seen only with a microscope but is more than 5 mm deep and 7 mm wide in the second stage. For stage 1b, the cancer can be seen without a microscope and is more than 4 cm in diameter in stage 2a. The tumor extends beyond the cervix to the upper two-thirds of the vagina but does not reach the tissues surrounding the uterus in stage 2b. The cancer has spread to the tissues surrounding the uterus in stage 3a. The tumor extends to the lower third of the vagina in stage 3b. Cancer spreads to the pelvic wall (the lining of the abdominal wall cavity between the hips) or it may spread to the ureters (tubes that carry urine from the kidneys to the bladder). If the flow of urine is blocked, the kidneys may become enlarged or stop working. In stage 4a, the tumor invades nearby pelvic organs such as the bladder or rectum and may spread to lymph nodes in the pelvis. In stage 4b, cancer spreads beyond the lymph nodes in the pelvis to other places in the body such as the liver, intestines, or lungs.