Understanding Prenatal Testing for Some Birth Defects

In 2007, the American College of Obstetrics and Gynecology (ACOG) recommended that all pregnant women, regardless of age, be offered screening for Down syndrome as well as the option of diagnostic testing. Screening and diagnostic testing options should be discussed with a doctor and/or genetic counselor.

What is a Maternal Blood Screening Test?

Maternal Blood Screening tests are performed during a pregnancy at specific gestational ages. Depending on the test used, it tells the likelihood that a pregnancy may be affected with a disorder, such as Down syndrome, Trisomy 18 or an open neural tube defect.

  • It does not determine if the pregnancy is actually affected with these disorders.
  • It sometimes predicts a “high risk” for a pregnancy to be affected with one of these conditions, even when the pregnancy is not affected.
  • It leads to the detection of about 80% to 90% of pregnancies with these three conditions, depending on the particular screening test used and the gestational age of the pregnancy. Therefore, approximately 10% to 20% of pregnancies with these conditions are missed by screening.
  • It does not provide any information about other genetic conditions.
  • It is not associated with a risk of miscarriage.

Common examples of screening options include:

1. First trimester screening:

This test screens for pregnancies at increased risk to be affected with Down syndrome and Trisomy 18 (some labs also provide a screening risk for Trisomy 13). This test is performed in the first trimester between approximately 10 weeks and 13 weeks gestation. A small amount of a pregnant woman’s blood is drawn, and the levels of a protein (PAPP-A) and a hormone (hCG) are measured in the sample. The final screening result is calculated by combining the results of this maternal blood test with the results of a special fetal ultrasound measurement called nuchal translucency (NT). As this test does not screen for open neural tube defects, a separate maternal blood test called MSAFP is available between approximately 15 weeks and 21 weeks gestation to screen for open neural tube defects.

2. Quadruple marker screening (also known as the “quad screen”):

This test screens for an increased risk for Down syndrome, Trisomy 18 and open neural tube defects. This test is performed in the second trimester between approximately 15 weeks and 21 weeks gestation. A small amount of a pregnant woman’s blood is drawn, and four proteins/hormones (AFP, hCG, unconjugated estriol and inhibin A) are measured in the sample.

3. Sequential screening:

This is a two-part test that screens for an increased risk for Down syndrome, Trisomy 18 and open neural tube defects. This test gives both a preliminary result in the first trimester (for Down syndrome and Trisomy 18) and a final result in the second trimester (for Down syndrome, Trisomy 18 and open neural tube defects). A small amount of a pregnant woman’s blood is drawn in the first trimester. The first trimester preliminary result is calculated by combining the results of this first trimester blood test with the fetal nuchal translucency (NT) measurement. If the results in the first trimester indicate an increased risk for Down syndrome and/or Trisomy 18, diagnostic testing options are available. However, if the results in the first trimester are within a normal range, a small portion of maternal blood is drawn in the second trimester. The results of this second blood test are combined with the first trimester preliminary result to yield a final result.

4. Integrated screening:

This test is a two-part test that screens for an increased risk for Down syndrome, Trisomy 18 and open neural tube defects. A small amount of a pregnant woman’s blood is drawn in both the first trimester and second trimester. The screening result is only available in the second trimester and is calculated by combining the second trimester blood test with both the first trimester blood test and the first trimester nuchal translucency (NT) measurement. Of note, unlike sequential screening, only one result is received from this test.

The American College of Obstetricians and Gynecologists recommends that women with a “high risk” screening test result be offered diagnostic testing.

What is a Diagnostic Test?

It tells (with greater than 99% accuracy) whether a pregnancy is affected or is not affected with Down syndrome, Trisomy 18 and many other chromosome abnormalities.

It can sometimes test for other genetic diseases (if indicated). For example, if a couple has additional risks for a specific genetic disease due to a known family history of an inherited condition; if there are ultrasound abnormalities or findings/markers suggestive of a particular genetic condition; if one or both parents have a genetic disease; or if one or both parents are identified as carriers of a particular genetic disorder. The detection rate for other genetic diseases varies by condition.

It is typically associated with a risk of miscarriage. Examples of common diagnostic testing procedures include:

1. Chorionic villus sampling (CVS):

A diagnostic testing procedure typically performed between 11 weeks and 13 weeks 6 days gestation for the diagnosis of fetal chromosome abnormalities and/or other genetic disorders (if indicated). This procedure involves the removal of small pieces of chorionic villi from the placenta either through a needle inserted into the lower abdomen (transabdominal) or through a very thin tube inserted into the cervix (transcervical). The average risk for miscarriage is 1 in 100. If performed before 9 weeks gestation, there is an additional risk for fetal limb defects. As CVS does not test for open neural tube defects, a separate maternal blood test called MSAFP is available between approximately 15 weeks and 21 weeks gestation to screen for open neural tube defects.

2. Amniocentesis:

A diagnostic testing procedure typically performed between 15 weeks and 23 weeks gestation for the diagnosis of fetal chromosome abnormalities, open neural tube defects and/or other genetic diseases (if indicated). It tells (with greater than 90% accuracy) whether a pregnancy is affected with an open neural tube defect. This procedure involves the removal of a small amount of amniotic fluid from the uterus through a needle inserted into the lower abdomen. The average risk for miscarriage is less than or equal to 0.5%. If performed before 15 weeks gestation, the risk of miscarriage may be increased.

No diagnostic test can detect all birth defects and genetic diseases. Even when all of the results of a diagnostic test are normal, every pregnancy still has a risk for problems that cannot be detected before birth.

Ultrasound Exams

There are different types of ultrasound used throughout a pregnancy, which vary in their ability to detect structural abnormalities in the fetus depending on the type of equipment and gestational age of the pregnancy.

Examples of ultrasound types used to detect birth defects and/or markers of genetic disease include:

1. Comprehensive Ultrasound (also known as “level II ultrasound”):

An ultrasound examination performed after 18 weeks gestation to evaluate a fetus for major structural malformations/birth defects.

2. Fetal Echocardiogram:

A specialized ultrasound examination performed after 18 weeks gestation to evaluate a fetus for a congenital heart defect.

Ultrasound is not diagnostic of genetic disease. In addition, ultrasound cannot detect all birth defects. It also cannot detect mental disabilities, such as autism and mental retardation.

Parental Testing Options

Genetic testing options are typically based on a couple’s ethnic ancestry, personal medical history as well as family history. Certain parental blood tests called “carrier tests” can determine if an individual is a carrier of a genetic condition with autosomal recessive inheritance or X-linked recessive inheritance. In autosomal recessive conditions, both parents need to be carriers of the same condition to have a chance to have an affected child. In X-linked recessive conditions, females are carriers, while males are affected with condition. Carriers typically do not have any symptoms of the disease.

Examples of common carrier tests include (but are not limited to):

  • Cystic fibrosis
  • Spinal muscular atrophy
  • Sickle cell disease (and other hemoglobinopathies such as beta-thalassemia and alpha-thalassemia)
  • Fragile X syndrome
  • Ashkenazi Jewish diseases (including Tay-Sachs disease and several other diseases)

Parental genetic testing options should be discussed with a doctor and/or genetic counselor.

Source: AccessDNA.com

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