The alpha-fetoprotein test (AFP test) is where a blood sample is drawn from the mother to check the levels of AFP. AFP is a protein secreted by the fetal liver and excreted in the mother’s blood. It is generally used to provide a screening for neural tube defects like spina bifida and anencephaly. It can also indicate: abdominal wall defects, esophageal and duodenal atresia, some renal and urinary tract anomalies, turner syndrome, some low birth-weight fetuses, placental complications and the presence of Down Syndrome.
The triple screen, also known as a maternal serum screening test or MSS, is a simple and completely safe blood test that measures not only AFP, but hCG and estriol as well. These are all hormones produced by the fetus and passed into the mother’s bloodstream. The test is performed between the 15th and 18th weeks, with the results usually available within one week. Elevated levels of AFP in the mother’s blood can indicate a neural tube defect in the baby such as a deformity of the spinal canal known as spina bifida. Lower levels of AFP suggest a possible child with Down syndrome or other chromosomal defect.
This test is more accurate and screens for additional genetic problems, and is beginning to replace the standard AFP. Generally speaking, any combination of the testing will identify 60% of the babies with Down Syndrome and 80-90% of the babies with neural tube defects. The AFP test is generally most sensitive between the 15th and 17th weeks of pregnancy, while the triple screen can be done a bit earlier.
Some of the controversy surrounding these tests is a result of false positive tests. These are tests that indicate a defect is present in the fetus when in actuality there is not. Only one or two out of 50 women with abnormally high levels will have an affected fetus. Other reasons for abnormal results in the triple screen include carrying twins, that the fetus is either a few weeks older or younger than originally thought, or the tests were performed wrong. Actually, the triple screen cannot diagnose a birth defect; it can only indicate an increased risk. Any abnormal result only indicates that additional testing is required. This leads to other more dangerous tests that may place the mother and child at unnecessary risk. Also, whereas this test is only offered to all pregnant women, many women are made to believe that testing is mandatory. Be sure you educate yourself on the benefits and risks of AFP and triple screen before you make your choice.
Whether or not to have the test is a very hard decision and a very personal one. Some women feel that no matter what is wrong that they would not terminate the pregnancy, and therefore do not want the test. While others are very happy to have the normal results, even knowing that there are no guarantees.
The test results are generally given as percentages, such as: risk of anomaly X in your child is 1 in 500. It is important to remember that being at an increased risk does not mean your baby has the defect. You should also ask that your practitioner explain the results in detail to you. An amniocentesis may be offered to conform the triple screen results.
Revision date: June 22, 2011
Last revised: by Janet A. Staessen, MD, PhD