Second trimester prenatal screening may include several blood tests, called multiple markers. These markers provide information about a woman's risk of having a baby with certain genetic conditions or birth defects. Screening is usually done by taking a sample of the mother's blood between the 15th and 20th weeks of pregnancy (16th to 18th is ideal). The multiple markers 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), and crosses the placenta into the mother's blood. The AFP blood test is also called MSAFP (maternal serum AFP).Abnormal levels of AFP may signal:
Open neural tube defects (ONTD), such as spina bifida
Other chromosomal abnormalities
Defects in the abdominal wall of the fetus
Twins. More than one fetus is making the protein
A miscalculated due date, as the levels vary throughout pregnancy
hCG. Human chorionic gonadotropin hormone (a hormone made by the placenta).
Estriol. A hormone made by the placenta.
Inhibin. A hormone made by the placenta.
Abnormal test results of AFP and other markers may mean more testing is needed. Usually an ultrasound is done to confirm the dates of the pregnancy and to look at the fetal spine and other body parts for defects. An amniocentesis may be needed for accurate diagnosis.
Multiple marker screening is not diagnostic. This means it is not 100% accurate, and is only a screening test to determine who in the population should be offered more 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.
When a woman has both first and second trimester screening tests done, the ability of the tests to detect an abnormality is greater than using just one screening independently. Most cases of Down syndrome can be detected when both first and second trimester screening are used.
A test that is usually done in the first trimester may also be used called cell-free DNA. This test can detect some genetic abnormalities not detected by the multiple marker test.