Pregnant women need to undergo screening tests in the second semester of pregnancy. Mostly, these include several blood tests, called multiple markers. These blood tests provide information about a woman’s risk of having a baby with genetic disorders or birth defects. Such screening is usually performed by taking a sample of the mother’s blood between the 15th and 20th weeks of pregnancy. The multiple markers (blood tests) include:
- Alpha-fetoprotein screening (AFP): This blood test measures the level of alpha-fetoprotein in the mothers’ blood during pregnancy. AFP is a protein normally produced by the fetal liver and is present in the fluid surrounding the fetus (amniotic fluid). The AFP blood test is also called MSAFP (maternal serum AFP). Abnormal levels of AFP may indicate the following:
- Open neural tube defects (ONTD), such as spina bifida
- Defects in the abdominal wall of the fetus
- Twins–more than one fetus is making the protein
- Down syndrome
- Other chromosomal abnormalities
- A miscalculated due date, as the levels vary throughout pregnancy
Abnormal test results of AFP and other markers may indicate the need for additional testing.
- Usually an ultrasound is performed to confirm the pregnancy dates and to look at the fetal spine and other body parts for defects.
One needs to understand that multiple marker screening tests is not diagnostic in nature. This means it is not 100 percent accurate, and is only a screening test to determine who in the population should be offered additional testing for their pregnancy. There can be false-positive results–indicating a problem when the fetus is actually healthy or false negative results–indicating a normal result when the fetus actually does have a health problem.