How do doctors diagnose Down syndrome?

Three types of tests check for Down syndrome during a woman’s pregnancy: screening and diagnostic tests. Screening tests identify a mother who is likely carrying a baby with Down syndrome. The most common screening tests are the Triple Screen and the Alpha-Fetoprotein Plus. These tests measure levels of certain substances in the blood.

Alternatively, ultrasounds (which use sound waves to look inside the mother’s uterus) allow the doctor to examine the fetus in the womb for the physical signs of Down syndrome. To confirm a positive result identified in a screening test, one of the following diagnostic tests can be performed: chronic villus sampling (CVS), amniocentesis. Each takes a sample from the placenta, amniotic fluid, or umbilical cord, respectively, to examine the baby’s chromosomes and determine if he or she has an extra chromosome 21.

If Down syndrome is not diagnosed in the womb, doctors can usually recognize it after the baby is born by the distinctive facial features. The diagnosis is confirmed with a karyotype – an examination of the baby’s chromosomes.

Amniocentesis, chronic villus and ultrasound are the three primary procedures for diagnostic testing, which can identify certain abnormalities in the fetus.

Amniocentesis
Amniocentesis is used most commonly to identify chromosomal problems, such as Down syndrome. When the fetus is known to be at risk, it can detect other genetic diseases like cystic fibrosis, Tay-Sachs disease and sickle cell disease. An amniocentesis procedure for genetic testing is typically performed between 15 and 20 weeks of pregnancy. Under ultrasound guidance, a needle is inserted through the abdomen to remove a small amount of amniotic fluid. The cells from the fluid are then cultured and a karyotype analysis — an analysis of the chromosomal make-up of the cells — is performed. It takes about two weeks to receive the results of the test.Amniocentesis detects most chromosomal disorders, such as Down syndrome, with a high degree of accuracy. Testing for other genetic diseases, such as Tay-Sachs disease, is not routinely performed but can be detected through specialized testing if your fetus is known to be at risk.Testing for neural tube defects, such as spina bifida, also can be performed. There is a small risk of miscarriage as a result of amniocentesis — about 1 in 100 or less. Miscarriage rates for procedures performed at UCSF Medical Center are less than 1 in 350.

Chronic Villus Sampling
Chronic Villus Sampling (CVS) Like amniocentesis, chronic villus sampling is used most commonly to identify chromosomal problems, such as Down syndrome. It can detect other genetic diseases like cystic fibrosis, Tay-Sachs disease and sickle cell disease in at-risk fetuses. The main advantage of CVS over amniocentesis is that it is done much earlier in pregnancy, at 10 to 12 weeks, rather than 15 to 20 weeks.CVS involve removing a tiny piece of tissue from the placenta. Under ultrasound guidance, the tissue is obtained either with a needle through the abdomen or a catheter inserted through the cervix. The tissue is then cultured and a karyotype analysis &mash; analysis of the chromosomal make-up of the cells — is performed. It takes about two weeks to receive the results. The advantage of CVS over amniocentesis is that the test is performed much earlier in pregnancy, so results are typically available by the end of the third month. A disadvantage is that spinal cord defects cannot be detected. Expanded alpha fetoprotein (AFP) blood testing or ultrasound can be performed later in the pregnancy to screen for spinal cord defects. There is a small risk of miscarriage as a result of CVS -- 1 in 100 or less. Miscarriage rates for procedures performed at UCSF Medical Center are less than 1 in 350.

Ultrasound
The primary purpose of ultrasound is to determine the status of a pregnancy -- the due date, size of the fetus and multiple gestation. Ultrasound also can provide some information about possible birth defects in a fetus. All patients at UCSF Medical Center undergo a comprehensive ultrasound examination before any invasive tests are performed. Results of the ultrasound are explained at the time of the visit. In some patients, an ultrasound raises concern of a possible abnormality in the fetus. We have extensive experience in performing and interpreting ultrasounds in pregnancy.