Second Trimester Prenatal Screening Tests
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
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 mother’s blood during pregnancy. AFP is a
protein normally produced by the fetal liver and 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
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
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
of the pregnancy and to look at the fetal spine and other body parts for defects.
amniocentesis may be needed for accurate diagnosis.
Multiple marker screening is not
diagnostic. This means it is not 100% accurate, and it is only a screening test to
determine who in the population should be offered more testing for their pregnancy.
can be false-positive results, indicating a problem when the fetus is actually healthy,
false negative results, indicating a normal result when the fetus actually does have
When a woman has both first and second
trimester screening tests done, the ability of the tests to detect an abnormality
greater than using just one screening independently. Most cases of Down syndrome can
detected 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 detect some
abnormalities not detected by the multiple marker test.