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Tests and diagnosis
By Mayo Clinic staffScreening for Down syndrome is offered as a routine part of prenatal care. A mother's age has traditionally been a factor in the decision to screen for Down syndrome. But now, the American College of Obstetricians and Gynecologists recommends offering various screening tests for Down syndrome to all pregnant women, regardless of age. Your health care provider can help you weigh the pros and cons of these tests for you.
Screening tests during pregnancy
Various screening tests can help identify whether you have a high risk of carrying a baby with Down syndrome. Blood tests typically have been offered around the 16th week of pregnancy to screen for Down syndrome, spina bifida and various other chromosomal disorders.
If you want an earlier risk assessment, the first trimester combined test, done in two steps from the week 11 to week 13 of pregnancy, may be your best choice. About 5 percent of women who undergo the first trimester combined test have a false-positive result, meaning they're identified incorrectly as having a high risk of delivering a baby with Down syndrome. The first trimester combined test includes:
- Ultrasound. The doctor uses ultrasound to measure a specific region on the back of a 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 tissue.
- Blood tests. Results of the ultrasound are paired with blood tests that measure levels of pregnancy-associated plasma protein-A (PAPP-A) and a hormone known as human chorionic gonadotropin (HCG). Abnormal levels of PAPP-A and HCG may indicate a problem with the baby.
If early assessment isn't your first priority, you can have full integrated testing, which is done in two parts during the first two trimesters of your pregnancy. The results of the two parts are combined to estimate your risk that your baby has Down syndrome. Only about 1 percent of women have a false-positive result with full integrated testing, so fewer women require more invasive testing.
- First trimester. Part one includes an ultrasound to measure nuchal translucency and a blood test to measure PAPP-A.
- Second trimester. Done at 15 to 20 weeks of pregnancy, the quad screen measures your blood level of four pregnancy-associated substances, alpha-fetoprotein, estriol, HCG, and inhibin A.
If a screening test indicates a high risk of Down syndrome, a more invasive test may be used to determine whether your baby actually has Down syndrome.
About one in 20 women will have a positive result with any of these screening tests — far more than those who eventually deliver a baby with a chromosomal abnormality. In fact, most women who have a positive result from a screening test deliver healthy babies.
Diagnostic tests during pregnancy
If your screening tests are positive or worrisome or you're at high risk of having a baby with Down syndrome, you might consider further testing to confirm the diagnosis. 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 after 15 weeks of gestation. The test carries a one in 200 risk of miscarriage.
- Chorionic villus sampling (CVS). Cells taken from the mother's placenta can be used to analyze the fetal chromosomes. Typically performed between the ninth and 14th week of pregnancy, this test carries a one in 100 risk of miscarriage.
- Percutaneous umbilical blood sampling (PUBS). Blood is taken from a vein in the umbilical cord and examined for chromosomal defects. Doctors generally perform this test after 18 weeks of gestation. This test carries a greater risk of miscarriage than does amniocentesis or chorionic villus sampling. Generally, this test is only done when speed of diagnosis is essential.
Each of these three tests is 98 to 99 percent accurate in diagnosing Down syndrome before birth.
Diagnostic tests for newborns
After birth, the initial diagnosis of Down syndrome is often based on the baby's appearance. If your child displays some or all of the characteristics of Down syndrome, your doctor probably will order a test called a chromosomal karyotype. This test is an analysis of your child's chromosomes. If there's an extra chromosome 21 present in all or some of the cells, the diagnosis is Down syndrome.
- Down syndrome. National Institute of Child Health and Human Development. http://www.nichd.nih.gov/health/topics/down_syndrome.cfm. Accessed Jan. 9, 2009.
- Barss V, et al. Overview of prenatal screening and diagnosis of Down syndrome. http://www.uptodate.com/home/index.html. Accessed Jan. 9, 2009.
- Down syndrome Q & A. National Down Syndrome Society. http://www.ndss.org/index.php?option=com_content&view=article&id=55:down-syndrome-q-a-a&catid=35:about-down-syndrome&Itemid=75. Accessed Jan. 20, 2009.
- What causes Down syndrome? National Down Syndrome Society. http://www.ndss.org/index.php?option=com_content&task=category§ionid=23&id=60&Itemid=234. Accessed Jan. 20, 2009.
- Genetic conditions: Down syndrome. Genetics Home Reference. http://ghr.nlm.nih.gov/condition=downsyndrome. Accessed Jan. 9, 2009.
- Facts about Down syndrome. National Institute of Child Health and Human Development. http://www.nichd.nih.gov/publications/pubs/downsyndrome.cfm. Accessed Jan. 20, 2009.
- Canick JA, et al. Second trimester maternal serum screening for Down syndrome. http://www.uptodate.com/home/index.html. Accessed Jan. 9, 2009.
