Down syndrome is a genetic condition caused by extra genes from the 21st chromosome that result in certain characteristics, including some degree of mental retardation, or cognitive disability, and other developmental delays. The incidence of Down syndrome in the United States is about 1 in 1,000 births. There is no association between Down syndrome and culture, ethnic group, socioeconomic status or geographic region.
Generally, the chance of having a Down syndrome birth is related to the mother's age. The odds of having a child with Down syndrome at age 35 are about 1 in 350. Under age 25, the odds are about 1 in 1,400. At age 40, the odds are about 1 in 100.
There are three causes of Down syndrome:
Trisomy 21 — An estimated 95 percent of people with Down syndrome have Trisomy 21, meaning an individual has three number 21 chromosomes, instead of two. We normally have 23 pairs of chromosomes, each made up of genes. During the formation of the egg or the sperm a woman's or a man's pair of chromosomes normally split so that only one chromosome is in each egg or sperm. In Trisomy 21, the 21st chromosome pair does not split and a double-dose goes to the egg or sperm. An estimated 95 percent to 97 percent of the extra chromosome is of maternal origin.
Translocation — This occurs in about 3 to 4 percent of people with Down syndrome. In this type, an extra part of the 21st chromosome gets stuck onto another chromosome. In about half of these situations, one parent carries the extra 21st chromosome material in a "balanced" or hidden form.
Mosaicism — In mosaicism, the person with Down syndrome has an extra 21st chromosome in some of the cells but not all of them. The other cells have the usual pair of 21st chromosomes. About 1 to 2 percent of people with Down syndrome have this type.
In addition to mental retardation and other developmental delays, some common physical traits are an upward slant of the eyes; flattened bridge of the nose; single, deep crease on the palm of the hand; and decreased muscle tone. A child with Down syndrome, however, may not have all these symptoms.
Screening tests can 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.
Sequential Integrated Screening — Sequential integrated screening is noninvasive testing offered by the state of California to all pregnant women. It is completed in multiple steps. In the first step, which is performed between 10 and 14 weeks of pregnancy, a maternal blood sample is taken and an ultrasound scan is performed to measure the amount of fluid at the back of the baby's neck (nuchal translucency or NT). If the blood test is performed before the scheduled ultrasound, an instant result can be provided at the conclusion of the ultrasound appointment.
The results of the blood test, the NT measurement and the mother's age are used to estimate the risk for Down syndrome and trisomy 18. The estimated detection rate for Down syndrome is 75 percent, and based on adjusted risk, a woman has the option of undergoing chorionic villus sampling (CVS) or amniocentesis for diagnosis.
The second step is a maternal blood test between 15 to 20 weeks of pregnancy. When the results of this blood test are combined with the results from the first trimester blood test and NT ultrasound, the detection rate for Down syndrome increases to 90 percent. This test also provides a personal risk assessment for having a fetus with trisomy 18, Smith-Lemli-Opitz syndrome, an open neural tube defect or an abdominal wall defect. Based on the results, a woman has the option of undergoing amniocentesis.
Serum Integrated Screening — Serum integrated screening is a combination of the two maternal blood tests without nuchal translucency. The detection rate for Down syndrome is 85 percent. This test also estimates the probability of trisomy 18, Smith-Lemli-Opitz syndrome, and an open neural tube defect or abdominal wall defect.
Quad Screening or Second Trimester Screening — This is a maternal blood test done between 15 to 20 weeks of pregnancy. The test estimates the probability of Down syndrome, trisomy 18, Smith-Lemli-Opitz syndrome, and an open neural tube defect or abdominal wall defect. The detection rate for Down syndrome is 80 percent. A positive result on a screening test indicates an increased risk for a genetic abnormality.
Amniocentesis, chorionic 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.
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.
If you have positive results on a screening test, we recommend that you discuss this with your doctor and a genetic counselor. Options for further diagnostic testing will be explained. The decision as to whether to have invasive genetic testing is up to you.
If a diagnostic test finds a genetic abnormality, the significance of such results should be discussed with experts familiar with the condition, including a medical geneticist and a genetic counselor, as well as your own doctor.
Reviewed by health care specialists at UCSF Medical Center.
This information is for educational purposes only and is not intended to replace the advice of your doctor or health care provider. We encourage you to discuss with your doctor any questions or concerns you may have.
Prenatal Diagnostic Center
350 Parnassus Ave., Suite 810
San Francisco, CA 94117
Phone: (415) 476-4080
Fax: (415) 353-4077