Prenatal screening tests are a set of procedures that are performed during pregnancy on expectant mothers to determine whether a baby is likely to have specific birth defects

The following are some of the more common tests performed during pregnancy:

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 or thickening.

• Two maternal serum (blood) tests. The blood tests measure two substances found in the blood of all pregnant women:

o 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.

o 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 21) and trisomy 18.

If the results of these first trimester screening tests are abnormal, genetic counseling is recommended. Additional testing such as chorionic villus sampling, amniocentesis, cell-free fetal DNA, or other ultrasounds 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). 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 the following:

    o Open neural tube defects (ONTD), such as spina bifida o Down syndrome o Other chromosomal abnormalities

    o Defects in the abdominal wall of the fetus

    o Twins--more than one fetus is making the protein

    o A miscalculated due date, as the levels vary throughout pregnancy

• hCG. This is human chorionic gonadotropin hormone (a hormone produced by the placenta)

• Estriol. This is a hormone produced by the placenta

• Inhibin. This is 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 needed for accurate diagnosis. Multiple marker screening is not diagnostic. 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

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. Nearly all cases of Down Syndrome can be detected when both first and second trimester screening are used

 

What is an amniocentesis?

An amniocentesis is a procedure used to obtain a small sample of the amniotic fluid that surrounds the fetus to diagnose chromosomal disorders and open neural tube defects (ONTDs), such as spina bifida. Testing is available for other genetic defects and disorders depending on the family history and availability of laboratory testing at the time of the procedure.

An amniocentesis is generally offered to women between the 14th and 16th weeks of pregnancy who are at increased risk for chromosome abnormalities, such as women who are over age 35 years of age at delivery, or those who have had an abnormal maternal serum screening test, indicating an increased risk for a chromosomal abnormality or neural tube defect.

What is a chorionic villus sampling (CVS)? Chorionic villus sampling (CVS) is a test that can find certain problems with your fetus. These include many diseases that run in families (genetic disorders) and chromosome defects. It is done during early pregnancy, most often between the 10th and 12th weeks.

CVS can find many genetic disorders, such as Tay-Sachs disease and hemophilia. It can also find chromosome defects, such as Down syndrome. CVS can't find neural tube defects. CVS is done earlier in pregnancy than amniocentesis (usually done at 14-16 weeks). And the results of CVS are available sooner. This gives you more time to make decisions about your pregnancy or plan for the future.

Why It Is Done?

CVS may be done during your first trimester of pregnancy to find certain birth defects. You may choose to have this test because:

• Of your age. As you get older, you have a greater chance of having a baby with a birth defect.

• You or the baby's father carries an abnormal gene that can cause an inherited disease, such as Tay-Sachs disease, sickle cell anemia, or cystic fibrosis.

• You or the baby's father has a family history of a genetic disorder or birth defect.

• You have already had a child who has Down syndrome or another chromosome defect.

• Other tests suggest that your chance of having a baby with a genetic disorder or birth defect is higher than average.