Alpha-Fetoprotein (AFP) Test

The alpha-fetoprotein (AFP) test is available to women between their 15th and 20th week of pregnancy to screen for fetal abnormalities. This simple blood test measures the level of a protein called alpha-fetoprotein which is secreted by the liver of the fetus and enters the mother's blood stream. Elevated levels of AFP may indicate an increased risk of neural tube defects such as spina bifida or anencephaly, while low levels suggest Down Syndrome or other chromosomal defects.

The AFP test does not determine the existence of these genetic disorders; it only predicts their likelihood. And while the AFP test is most often used to screen for these defects, it can also be used to identify abdominal wall defects, some renal and urinary tract abnormalities, Turner syndrome, low birth weight, and placental complications. An incorrectly-calculated gestational age and multiple fetuses can also cause abnormal AFP levels.

Results are usually available about a week after testing and are given in the form of either a positive/negative or a risk ranking, such as 1:270 chance of having a baby with Down Syndrome. If your test results indicate an elevated risk, your doctor will recommend you see a genetic counselor and have additional testing such as a second AFP test, triple or quad marker testing, amniocentesis or CVS (Chorionic Villi Sampling).

The AFP test is often part of other screening tests, including the triple screen test and quad marker test. The triple screen test measures AFP, Human Chorionic Gonadotropin (hCG) and unconjugated estriol (an estrogen produced by the fetus and placenta). The triple screen is replacing the standard AFP test in many areas because it is more accurate (69 percent accuracy rate) and screens for additional genetic problems. The quad marker test measures AFP, hCG, unconjugated estriol, and inhibin-A (a hormone produced by the placenta). The quad marker test can predict approximately 75 to 80 percent of neural tube defects and Down Syndrome cases.

One of the drawbacks of the AFP test is the high rate of false-positive results, or incorrect results that indicate a potential genetic problem in a healthy fetus. These results often prompt the woman to submit to additional tests, such as amniocentesis, which carry a risk of miscarriage or harm to the fetus. It is estimated that in more than 80 percent of abnormal results the baby turns out to be perfectly healthy.

The decision to have an AFP test performed is a very personal and difficult one to make. Knowing that a defect exists may affect prenatal care, delivery methods, and postnatal treatment. In addition, some women want to prepare themselves for what may lie ahead, or terminate the pregnancy early if a problem is found. Others know they would never terminate the pregnancy and for them, the testing is unnecessary.