Second trimester prenatal screening may include several blood tests. These are 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. This is done between the 15th and 20th weeks of pregnancy. (Between the 16th to 18th is ideal.) The multiple markers include:

• Alpha-fetoprotein screening (AFP). This blood test measures the level of alpha-fetoprotein in the mother's blood during pregnancy. AFP is a protein normally made by the fetal liver. It is present in the fluid surrounding the fetus (amniotic fluid). It 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, such as spina bifida
  • Down syndrome
  • 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
  • Increased risk of preeclampsia or fetal growth lag in the third trimester
• 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. It also looks 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. It 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. These indicate a problem when the fetus is actually healthy. Or there can be false-negative results. These indicate 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 find an abnormality is greater than using just one screening independently. Most cases of Down syndrome can be found when both first and second trimester screening are used.

A cell-free DNA screening test, which is usually done in the first trimester, may also be used. This test can find some genetic abnormalities not found by the multiple marker test.