- First Trimester Prenatal Screening Tests
First trimester screening is a combination of fetal ultrasound and maternal blood testing performed during the first trimester of pregnancy. This screening process can help to determine the risk of the fetus having certain birth defects. Screening tests may be used alone or in combination with other tests.
There are three parts of first trimester screening:
ultrasound test for fetal nuchal translucency (NT)
Nuchal translucency screening uses an ultrasound test to examine the area at the back of the fetal neck for increased fluid.
two maternal serum (blood) tests
The blood tests measure two substances found in the blood of all pregnant women:
pregnancy-associated plasma protein screening (PAPP-A) - a protein produced by the placenta in early pregnancy. Abnormal levels are associated with an increased risk for chromosome abnormality.
human chorionic gonadotropin (hCG) - a hormone produced by the placenta in early pregnancy. Abnormal levels are associated with an increased risk for chromosome abnormality.
When used together as first trimester screening tests, nuchal translucency screening and maternal blood tests have a greater ability to determine if the fetus might have a birth defect, such as Down syndrome, trisomy 18, or trisomy 13.
If the results of these first trimester screening tests are abnormal, genetic counseling is recommended. Additional testing such as chorionic villus sampling or amniocentesis may be needed for accurate diagnosis.
- 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 performed 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) - a blood test that 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 the following:
open neural tube defects (ONTD) 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
hCG - human chorionic gonadotropin hormone (a hormone produced by the placenta)
estriol - a hormone produced by the placenta
inhibin - a hormone produced by the placenta
Abnormal test results of AFP and other markers may indicate the need for additional testing. Usually an ultrasound is performed to confirm the dates of the pregnancy and to look at the fetal spine and other body parts for defects. An amniocentesis may be performed to assess for elevated amniotic fluid levels of AFP and other chemicals which may indicate the presence of spina bifida.
Multiple marker screening is not diagnostic. It 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. There can also be “false negative” results, indicating a no abnormality when the fetus actually does have a health problem.
When a woman has both first and second trimester screening tests performed, the ability of the tests to detect an abnormality is greater than using just one screening independently. Over 80 percent of fetuses affected with Down Syndrome can be detected when both first and second trimester screening are used.
What is a chorionic villus sampling (CVS)
Read more:
What is an amniocentesis
What is a chorionic villus sampling (CVS)
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