Tests and diagnosis

By Mayo Clinic Staff

The American College of Obstetricians and Gynecologists recommends offering the option of screening tests and diagnostic tests for Down syndrome to all pregnant women, regardless of age.

  • Screening tests can indicate the likelihood a mother is carrying a baby with Down syndrome.
  • Diagnostic tests can identify whether your baby has Down syndrome.

Your health care provider can discuss the types of tests, advantages and disadvantages, benefits and risks, and the meaning of your results. If appropriate, your provider may recommend that you talk to a genetics counselor.

Screening tests during pregnancy

Screening for Down syndrome is offered as a routine part of prenatal care. Although screening tests aren't perfect, they can help you make decisions about more-specific diagnostic tests and the course of the pregnancy.

Various screening tests can help identify whether you have a high risk of carrying a baby with Down syndrome, but they can't identify whether your baby has Down syndrome. Screening tests include the first trimester combined test, the integrated screening test and the cell-free fetal DNA analysis.

The first trimester combined test

The first trimester combined test, which is done in two steps, includes:

  • Blood test. This blood test measures the levels of pregnancy-associated plasma protein-A (PAPP-A) and the pregnancy hormone known as human chorionic gonadotropin (HCG). Abnormal levels of PAPP-A and HCG may indicate a problem with the baby.
  • Ultrasound. Ultrasound is used to measure a specific area on the back of your baby's neck. This is known as a nuchal translucency screening test. When abnormalities are present, more fluid than usual tends to collect in this neck tissue.

Using your age and the results of the blood test and the ultrasound, your health care provider can estimate your risk of having a baby with Down syndrome.

Integrated screening test

The integrated screening test is done in two parts during the first and second trimesters of pregnancy. The results are combined to estimate the risk that your baby has Down syndrome. This test can achieve the same level of detection as the first trimester combined test but with a lower false-positive rate, meaning that fewer women are incorrectly identified as carrying a baby with Down syndrome.

  • First trimester. Part one includes a blood test to measure PAPP-A and an ultrasound to measure nuchal translucency.
  • Second trimester. The quad screen measures your blood level of four pregnancy-associated substances: alpha fetoprotein, estriol, HCG and inhibin A.

Cell-free fetal DNA analysis

The cell-free fetal DNA test checks for fetal DNA circulating in the mother's blood. This test is usually recommended for women who have a higher risk of having a baby with Down syndrome or in response to risk detected by one of the previous tests. The mother's blood can be tested during pregnancy after 10 weeks gestation.

This test appears to be much more specific than other screening methods for Down syndrome. If this screening test indicates a high risk of Down syndrome, a more invasive diagnostic test may be used to determine whether your baby actually has Down syndrome.

Diagnostic tests during pregnancy

If your screening test results are positive or worrisome, or you're at high risk of having a baby with Down syndrome, you might consider more testing to confirm the diagnosis. Your health care provider can help you weigh the pros and cons of these tests.

Diagnostic tests that can identify Down syndrome include:

  • Amniocentesis. A sample of the amniotic fluid surrounding the fetus is withdrawn through a needle inserted into the mother's uterus. This sample is then used to analyze the chromosomes of the fetus. Doctors usually perform this test in the second trimester, after 15 weeks of pregnancy. The test carries a slight risk of miscarriage, but risk increases if it's done before 15 weeks.
  • Chorionic villus sampling (CVS). In CVS, cells are taken from the placenta and used to analyze the fetal chromosomes. Typically performed in the first trimester, after 10 weeks of pregnancy, this test appears to carry a somewhat higher risk of miscarriage than second trimester amniocentesis.
  • Cordocentesis. In this test, also known as percutaneous umbilical blood sampling or PUBS, fetal blood is taken from a vein in the umbilical cord and examined for chromosomal defects. Doctors can perform this test between 18 and 22 weeks of pregnancy. This test carries a significantly greater risk of miscarriage than does amniocentesis or CVS, so it's only offered when results of other tests are unclear and the desired information can't be obtained any other way.

Preimplantation genetic diagnosis is one option available for couples undergoing in vitro fertilization who are at increased risk of passing along certain genetic conditions. The embryo is tested for genetic abnormalities before it's implanted in the womb.

Diagnostic tests for newborns

After birth, the initial diagnosis of Down syndrome is often based on the baby's appearance. But the features associated with Down syndrome can be found in babies without Down syndrome, so your health care provider will likely order a test called a chromosomal karyotype. Using a sample of blood, this test analyzes your child's chromosomes. If there's an extra chromosome 21 present in all or some cells, the diagnosis is Down syndrome.

April 19, 2014