Prenatal Diagnosis

Down Syndrome

Signs and Symptoms
Diagnosis
Treatment

Diagnosis

Screening can provide important information about potential risks for pregnancy. Screening tests have been designed to identify women at increased risk of having a baby with Down syndrome. These tests have no risks of miscarriage, but can't determine with certainty whether a fetus is affected. Diagnostic tests, on the other hand, are extremely accurate at identifying certain abnormalities in the fetus, but carry a small — generally less than 1 percent risk — risk of miscarriage. We offer options for both screening and diagnostic testing.

Screening

• Expanded AFP Screening — An expanded alpha fetoprotein (AFP) screening is a simple blood test. It is performed between 15 and 20 weeks of pregnancy and is recommended by the state of California for all pregnant women. The blood test result is combined with a woman's age to estimate her own personal risk for carrying a fetus with Down syndrome. The blood test also provides information about the risk of open neural tube defects, abdominal wall defects and trisomy 18. The rate of detection for women under 35 is about 85 percent for neural tube defects, 60 percent for Down syndrome and 60 percent for trisomy 18. The detection rate is higher in women over 35 years of age.

  This is a screening test, which means that if your test result is positive, you have an increased risk of a genetic abnormality. It does not diagnose birth defects, or tell you for certain if your fetus does or does not have a birth defect. Women who have an abnormal expanded AFP or who are going to be 35 at the time of delivery have the option of undergoing chorionic villus sample (CVS) or amniocentesis. These tests can diagnose chromosomal disorders, but not all birth defects, with a high degree of certainty.

  This test is typically performed by your obstetrician. You may be referred to the Prenatal Diagnosis Center for a follow-up examination if the AFP screening indicates a potential risk.

• Nuchal Translucency Screening (NT) — UCSF Medical Center is one of the few centers nationwide to offer nuchal translucency screening (NT) screening, a new, non-invasive test performed early in pregnancy to identify women at increased risk for Down syndrome and other birth defects. NT screening is performed between 11 and 14 weeks of pregnancy. It is offered to women of all ages. The screening is done via a high-resolution ultrasound exam of the nuchal area — a fold of skin at the back of the neck of the fetus. The results are combined with the mother's age to determine an adjusted risk for Down syndrome. The rate of detection for Down syndrome is about 80 percent. Based on the results, a woman has the option of undergoing CVS or amniocentesis for diagnosis.

  NT is a screening test only and cannot determine with certainty if the fetus does or does not have Down syndrome, only if your risk seems to be high or low. While this test has been developed for Down syndrome screening, other chromosomal abnormalities can sometimes be detected. In addition, fetuses with an increased area of nuchal translucency are at risk for other birth defects. An additional ultrasound and fetal echocardiogram, an ultrasound of the fetal heart, can be performed at 18 to 20 weeks of gestation to look for these abnormalities, if indicated.

  If the screening indicates that your fetus is at an increased risk of Down syndrome, a genetic counselor or physician will discuss the specific risk. The counselor also will discuss further diagnostic testing available, including chorionic villus sampling (CVS) or amniocentesis.

Diagnostic tests

• 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.

• Chorionic Villus Sampling (CVS) -- Like amniocentesis, chorionic 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 gestations. 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.

Please see our Frequently Asked Questions about choroid plexus cysts discovered during ultrasound.

Reviewed by health care specialists at UCSF Children's Hospital.
Last updated May 19, 2009