Alpha fetoprotein (AFP) is a protein found in human blood. Its level is elevated in a pregnant woman's blood when there are problems with her unborn child such as fetal spinal cord defects (anencephaly, spina bifida, myelomeningocele), a high risk of miscarriage, and abruptio placentae. It's also elevated when a woman is carrying more than one fetus. Pregnant women are given a blood screening test for AFP at around 16-18 weeks of pregnancy. The AFP level is also measured in the amniotic fluid obtained in amniocentesis to help identify abnormal fetal development.
AFP is also a useful marker for the diagnosis and assessment of certain tumors (teratomas ) of the testis. AFP is also elevated in patients with primary liver cancer (hepatocellular carcinoma) and in women with some types of germ cell ovarian tumors.
The normal range in adults is less than 40 micrograms of AFP per liter of blood.
The information in this writeup was taken from the science dictionary at http://biotech.icmb.utexas.edu/; I oversaw the development of the dictionary (the website was mothballed in 1998) and I believe I wrote the entry this is based on.