AFP test during pregnancy, also known as alpha-fetoprotein test, is frequently done to detect various genetic disorders such as Down syndrome, trisomy 18, trisomy 21 and neural tube defects. Down syndrome and trisomy 18 are genetic abnormality caused by DNA altered structure, and AFP pregnancy test detects on average about 70 percent of confirmed cases. Neural tube defects is a malformation of
. fetuses' brain structures, which results in high alpha-fetoprotein levels because nervous system tissues contain high amounts of this molecule. The test is done by drawing blood from a pregnant woman at 15 to 20 weeks of her pregnancy and measuring of AFP levels. The results are then compared with normal results adjusted for the pregnancy or gestational age and mother's weight. This is a very important step, because if the gestational age is evaluated incorrectly, false positive or false negative results may occur. For example, if the actual gestational age of the baby is actually older that the doctor's estimation, then a positive result is actually a false positive, and is probably not an indicator of a genetic abnormality. Adjustment for race and mother's age are also very important to avoid false positive results. Even when all the corrections and adjustments are done properly, and the test results are accurate, a positive result does not always indicate a genetic abnormality. It rather shows that there is a possibility of a problematic pregnancy, including maternal elevated blood pressure, retarded child growth or pre-term birth. An elevated AFP results should be always correlated with other tests for an accurate prediction, because when used alone it cannot be employed in establishment of a certain diagnosis. Smoking and mother's liver disease are also frequently responsible for false positive AFP results. Smoking increases AFP levels by about 10 percent. Because the liver produces variable amounts of alpha-fetoprotein, a disease such a hepatitis or cirrhosis may cause false positive AFP pregnancy test results. http://www.ncbi.nlm.nih.gov/pubmed/11920904 or http://www.ncbi.nlm.nih.gov/pubmed/21952001
