Chicago Center for Jewish Genetics Disorders - About Jewish Genetic Disorders

Prenatal Testing

What is it: Prenatal testing is performed during pregnancy to assess the health status of the fetus. Testing can be of two types: prenatal screening or prenatal diagnostic testing:

Prenatal screening leads to a new estimation of the risk to the fetus for certain disorders but it is NOT diagnostic. Types of prenatal screening are:
- Triple marker screening: This test is performed at 16-18 weeks gestation and involves taking a blood sample from the pregnant mother. The blood is analyzed to measure the levels of three chemicals associated with pregnancy: alpha fetoprotein, uE3 (uncongujated estriol), and hCG (human chorionic gonadotrophin). It then uses these three levels, the gestational age of the fetus, and maternal factors such as age, weight, and being affected with diabetes to calculate the risk for the fetus to have certain genetic disorders. These disorders are Down syndrome (Trisomy 21), Trisomy 18, and spina bifida (an opening anywhere along the spine of the fetus). If the test is abnormal, this does NOT mean the fetus is affected, but rather that the fetus is at an increased risk for the indicated disorder. Several factors can cause inaccurate results, such as the fetus being younger or older than estimated or the presence of twins or triplets. With an abnormal triple marker result, a couple might choose to have ultrasound or diagnostic prenatal testing done (see the following sections) for accurate dating of the pregnancy, to look for specific fetal characteristics, or to test for specific genetic disorders. It has been estimated that 10% of all women who have triple marker screening have a positive result for the above disorders and of these, only 2-3% will have a fetus with a birth defect. This means that overall it is estimated that 0.2-0.3% of all pregnancies in which triple screen is performed detect a fetus with a birth defect. Triple screening leads to the prenatal diagnosis of 95 percent of cases of anencephaly (no brain development), 80 percent of cases of serious spina bifida, as well as 60 percent of cases of Down syndrome. The American College of Obstetricians and Gynecologists have stated that they believe this test should be offered to all pregnant women, regardless of maternal age. It is not required that a pregnant woman have this test done.
- Ultrasound: An ultrasound examination uses high frequency sound waves to create images on a video monitor. This non-invasive technique is widely used to examine developing fetuses for structural defects and other problems. It is regarded as a highly effective, safe, and useful screening procedure and is often done after an abnormal triple screen result. Ultrasound can also be used to determine the gestational age of the fetus, and to show if a woman is carrying twins. If either of these factors accounts for the abnormal triple screen test result, no further testing is needed. Ultrasound can also detect some serious birth defects. If the ultrasound does not provide an explanation for an abnormal triple screen test result, additional testing is recommended.
  For example, if the triple screen shows that a woman is at increased risk of having a baby with a neural tube defect (NTD) such as spina bifida or anencephaly, her provider may recommend a high-resolution (level 2) ultrasound examination, amniocentesis, or both. A high-resolution ultrasound examination of the fetal skull, spine and other organs can quite accurately detect or rule out serious NTDs. If this type of ultrasound examination is not available, or if more information is needed after an ultrasound examination, amniocentesis is often recommended to measure the level of AFP (alpha fetoprotein), in the amniotic fluid and to do chromosomal analysis. AFP levels are markedly elevated in most NTDs. Chromosomal abnormalities can sometimes accompany an NTD.
- Other tests are in development to increase the detection of certain disorders prenatally. Please talk to your health care provider about prenatal testing if you would like more information about what is available.
Prenatal screening may lead to a diagnosis in a fetus, but may not determine how severely the fetus is affected. Types of prenatal diagnostic testing are:
- Chorionic villus sampling (CVS): This is a method of prenatal diagnosis that is done between 10-12 weeks of gestation and involves testing of the cells from the primitive placenta (chorion) surrounding a developing fetus. The number and structure of the fetus's chromosomes can be determined, biochemical testing or detection of specific mutations can be performed. CVS involves the insertion of a thin needle through the cervix or the abdominal wall into the placenta to remove the cells. Results take between 1 to 2 weeks. CVS is generally recommended when there is a family history of a genetic disorder or when the woman is over age 35 and therefore at a higher risk of having a baby with a chromosomal abnormality. It is a highly accurate test for certain chromosomal abnormalities, such as Trisomy 21 (Down Syndrome). The procedure is not very painful and has been described as a 'cramping' feeling. When performed at a qualified center, the risk of complications is 1% and these include bleeding, cramping, and pregnancy loss. The difference between a CVS and amniocentesis is the timing of testing during pregnancy and the risk of complications.
- Amniocentesis: This is a method of prenatal diagnosis that uses cells from the amniotic fluid surrounding the developing fetus to determine the number and structure of its chromosomes, biochemical testing and detection of specific mutations. Amniocentesis is usually performed at 15 to 18 weeks gestation. The procedure involves inserting a fine needle through the mother's abdomen and uterus into the amniotic fluid surrounding the baby while using an ultrasound scan for guidance to prevent injury to the fetus. One to 2 tablespoons of the amniotic fluid are removed for analysis. Amniocentesis is generally recommended when there is a family history of a genetic disorder or when the woman is over age 35 and therefore at a higher risk for having a baby with a chromosome abnormality. It is a highly accurate test for chromosome abnormalities, such as Trisomy 21 (Down Syndrome). The procedure is not very painful and has been described as a 'cramping' feeling. When performed at a qualified center, the risk of complications is about 0.5% and these include bleeding, cramping, and pregnancy loss. The difference between amniocentesis and chorionic villus sampling is the timing of testing during pregnancy and the risk of complications.
- PUBS: This is a method of prenatal diagnosis that uses the blood from the umbilical cord for rapid chromosome analysis or other testing. It involves the insertion of a needle through the mother's abdomen, like amniocentesis, to draw a blood sample from a vein in the umbilical cord. A physician may recommend this test if an ultrasound, amniocentesis, and chorionic villus sampling do not provide enough information about the fetus. PUBS is performed at about 18 weeks gestation or later and a karyotype (picture of the chromosomes) is ready in 48 hours, whereas the results of an amniocentesis may take up to two weeks to process. Another reason to do PUBS is to discover certain infections and blood disorders. If the baby is suspected to be anemic, blood sampling is the only way to confirm this and allows a blood transfusion while the needle is in place. The procedure is also known as umbilical vein sampling, fetal blood sampling, and cordocentesis. The miscarriage rate is about 2%.
- Preimplantation genetic diagnosis (PGD): This is a method of prenatal diagnosis that is performed on early embryos after in vitro fertilization. Eggs and sperm are harvested and mixed together in a sterile environment. The fertilized egg then divides to become 2 identical cells, these cells divide to become 4 identical cells, and these 4 cells divide to become 8 identical cells. At this 8-cells stage, it is possible to remove a single cell without damaging the future development of the embryo. In other words, the 7 remaining cells are still capable of the same development into a baby seen in a fetus fertilized in the more traditional manner. The DNA from the removed cell is then used to do genetic analysis for specific disorders. PGD provides an alternative to prenatal diagnosis and termination of affected pregnancies. However, it has a low success rate (live birth) due to the technical difficulties of in vitro fertilization. Also, the cost is very high and is usually not covered by insurance.

When is it done: Prenatal diagnostic tests are done when there is an increased risk of having a child with a genetic condition due to maternal age, family history, ethnicity, or prenatal screening.

Benefits of testing: If a condition is diagnosed prenatally, the parents may choose to continue or terminate the pregnancy. A couple might choose to have prenatal testing done even if they know they would not abort in order to prepare for a child with a disability. Also, if the testing is negative, there is some reassurance that the fetus is unlikely to have certain serious birth defects.

Other things to consider:

All prenatal diagnostic tests have an associated risk for complications, ranging from cramping and spotting to miscarriage. The risks are different for different procedures. Prenatal screening tests have no associated risk for the fetus.
In many cases the specific gene mutation occurring in the family must be identified before prenatal diagnosis can be performed.
Prenatal testing for adult-onset conditions with no preventative treatment is controversial (ACMG Policy Statement, http://www.faseb.org/genetics/acmg/pol-13.htm). This is discussed further in The Ethics of Mendelian Disorders and Multifactorial (Predispositional) Disorders.
No amount of testing reduces the risk of birth defects below the background rate of 3-5%.

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