Alpha-fetoprotein (AFP) is used as a tumor marker to help detect and diagnose cancers of the liver, testicles, and ovaries.AFP is normally made by a fetus's liver and yolk sac. It's the main protein during the first three months of development. AFP decreases largely by the time a person reaches age one and is ideally found in adults in very low levels. AFP is found mainly in liver cancer and nonseminomatous germ cell tumors, which are rare. These are found in the pineal gland in the brain.This test may be needed if your doctor suspects you have liver cancer, testicular cancer, or cancers of the brain, mediastinum, or blood.