Prenatal Diagnosis of Down Syndrome
Health care providers can check for Down syndrome during pregnancy or after a child is born.
The American Congress of Obstetricians and Gynecologists (ACOG) recommends that all pregnant women be offered a Down syndrome screening test.
There are two types of tests for Down syndrome during pregnancy:
- A prenatal screening test. This test can show an increased likelihood that a fetus has Down syndrome, but it cannot determine Down syndrome is definitely present. If a screening test shows an increased likelihood, a diagnostic test can be ordered.
- A prenatal diagnostic test. This test can determine with certainty that Down syndrome is present. Diagnostic tests carry a slightly greater risk to the fetus than do screening tests.
Prenatal Screening for Down Syndrome
There are several options for Down syndrome prenatal screening. These include:
- A blood test and an ultrasound test during the first trimester of pregnancy. This is the most accepted approach for screening during the first trimester. A blood test enables a health care provider to check for "markers," such as certain proteins, in the mother's blood that suggest an increased likelihood of Down syndrome. Then the health care provider does an ultrasound test, which uses high-frequency sound waves to create images. An ultrasound can detect fluid at the back of a fetus's neck, which sometimes indicates Down syndrome. The ultrasound test is called measurement of nuchal translucency. During the first trimester, this combined method results in more effective or comparable detection rates than methods used during the second trimester.
- A blood test during the second trimester of pregnancy. As in the first trimester, a blood test enables a health care provider to check for markers in the mother's blood. A triple screen looks for levels of three different markers; a quadruple screen looks for levels of four different markers.
- A combined test (sometimes called an integrated test). This approach uses both a blood test and an ultrasound during the first trimester as well as a second-trimester blood test. Health care providers then combine all these results to produce one Down syndrome risk rating.
If a woman is pregnant with twins or triplets, a blood test will not be as reliable because the substances from a Down syndrome fetus may be harder to detect.
Prenatal Diagnostic Testing for Down Syndrome
If a screening test suggests the likelihood of Down syndrome, a diagnostic test can be performed. ACOG recommends that pregnant women of all ages be given the option of skipping the screening test and getting a diagnostic test first. Until recently, only women over age 35 and other at-risk women were offered this option because diagnostic tests carry a slight risk of miscarriage. Before having diagnostic testing, a pregnant woman and her family may want to meet with a health care provider to discuss their family history and the risks and benefits of testing in their specific situation.
Diagnostic testing for Down syndrome involves removing a sample of genetic material. After it is removed, the sample is checked for extra material from chromosome 21, which may indicate that a fetus has Down syndrome. The following procedures are used to extract samples.
- Amniocentesis (pronounced am-nee-oh-sen-TEE-sis). A health care provider takes a sample of amniotic (pronounced am-nee-OT-ik) fluid, which is then tested for the extra chromosome.
- Chorionic villus (pronounced KOHR-ee-on-ik VIL-uhs) sampling (CVS). A health care provider takes a sample of cells from a part of the placenta (pronounced pluh-SEN-tuh), which is the organ that connects a woman and her fetus, and then tests the sample for the extra chromosome.
- Percutaneous (pronounced pur-kyoo-TEY-nee-uhs) umbilical blood sampling (PUBS). A health care provider takes a sample of fetal blood in the umbilical cord through the uterus. The blood is then tested for the extra chromosome. PUBS is the most accurate diagnostic method and can confirm the results of CVS or amniocentesis. However, PUBS cannot be performed until later in the pregnancy, during the 18th to 22nd week.5
Prenatal diagnostic testing does involve some risk to the mother and fetus, including risk of miscarriage that ranges from less than 1% to 2%.6,7,8,9 If you and your family are considering prenatal diagnostic testing for Down syndrome, discuss all the risks and benefits with your health care provider.
Chromosomal Testing of Maternal Blood
A pregnant woman who is at risk for having an infant with Down syndrome also can have a chromosomal test using her blood. A mother's blood carries DNA from the fetus, which may show extra chromosome 21 material.3 A more invasive test then would usually confirm the blood test.
Testing and In Vitro Fertilization
Another approach to diagnosis is used in conjunction with in vitro fertilization. Preimplantation genetic diagnosis (PGD) allows clinicians to detect chromosome imbalances or other genetic conditions in a fertilized egg before it is implanted into the uterus.
This technique is useful mostly for couples who are at risk of passing on a variety of genetic conditions, including X-linked disorders, as well as couples who have suffered repeated spontaneous pregnancy losses, sub-fertile couples, or those at risk for single-gene disorders.
Those interested in PGD should have genetic counseling and should consider close monitoring and additional testing during their pregnancies, given some increased risk of chromosomal abnormalities arising secondary to the in vitro fertilization process.
Reference: National Institutes of Child Health and Human Development