AFP
The alpha fetoprotein (MSAFP) is not a conclusive diagnostic test, but a screening test only. This test, conducted by drawing blood, measures the amount of protein in the amniotic fluid coming from the fetal urine. An elevated level may indicate conditions like neural tube defects, such as spina bifida - an opening in the spine, and sometimes also including water-on-the-brain (hydrocephaly) or a lack of brain development (anencephaly). A low level may be associated with Down Syndrome. (AFP testing of the amniotic fluid is also possible.)The best time to test is between 15 and 18 weeks gestation. Results are back in 1-2 weeks, and if your test comes back positive (low or high), this does not necessarily signify a problem, but simply means that further testing should be done. Common reasons for false positives include a discrepancy between the assumed fetal age and the actual age (i.e. thinking you're 16 weeks pregnant when you're actually only 14.5 weeks along) or if you are carrying more than one baby. In twin or higher multiple pregnancies, AFP results are not reliable because there is no way to determine the alpha fetoprotein levels from each separate baby. Further testing is usually recommended.
Elevated levels of AFP are observed in 80-90% of affected pregnancies. On the other hand, according to R. Harold Holbrook Jr. M.D., Associate Professor, Division of Maternal-Fetal Medicine at Department of Gynecology and Obstetrics Stanford University School of Medicine, "About 90% or greater of 'screen positives' turn out to be false positives because of the way the test is set up - it is only a screening test."