Alpha-fetoprotein test. A measurement of the level of alpha-fetoprotein (AFP) in a pregnant woman’s blood; it reflects the amount of AFP produced by a developing fetus. The test is used to detect birth defects. A high level of AFP in a woman’s blood sometimes indicates that the fetus has a neural tube defect, a birth defect in which the skull, brain, or spinal cord does not develop properly. AFP testing combined with other biochemical and hormonal measurements can also screen for other fetal abnormalities, including Down syndrome.