Down Syndrome Fact Sheet
Information Provided by the National Down Syndrome Society
Down syndrome occurs in one out of every 733 live births, and more than 350,000 people in the U.S. have this genetic condition. One of the most frequently occurring chromosomal abnormalities, Down syndrome affects people of all ages, races and economic levels.
What Causes Down Syndrome
The human body is made of cells. All cells contain a center, called a nucleus, in which genes are stored. Genes, which carry the codes responsible for all our inherited characteristics, are grouped along rod-like structures called chromosomes. Normally, the nucleus of each cell contains 23 pairs of chromosomes, half of which are inherited from each parent. Down syndrome occurs when some or all of a person’s cells have an extra full or partial copy of chromosome 21.
The most common form of Down syndrome is known as Trisomy 21. Individuals with Trisomy 21 have 47 chromosomes instead of the usual 46 in each of their cells. The condition results from an error in cell division called nondisjunction. Prior to or at conception, a pair of 21st chromosomes in either the sperm or the egg fails to separate. As the embryo develops, the extra chromosome is replicated in every cell of the body. This error in cell division is responsible for 95 percent of all cases of Down syndrome.
Down syndrome also encompasses two other genetic conditions: mosaicism and translocation. Mosaicism occurs when nondisjunction of chromosome 21 takes place in one of the initial cell divisions after fertilization causing a person to have 46 chromosomes in some of their cells and 47 in others. The least common form of Down syndrome, mosaicism accounts for only 1 to 2 percent of all cases. Translocation, which accounts for 3 to 4 percent of cases of Down syndrome, occurs when part of chromosome 21 breaks off during cell division and attaches to another chromosome, usually chromosome 14. While the total number of chromosomes in the cells remains 46, the presence of an extra part of chromosome 21 causes the characteristics of Down syndrome.
The cause of the extra full or partial chromosome is still unknown. What we do know that it is not caused by environmental factors or anything the mother does before or during her pregnancy. Maternal age is the only factor that has been linked to an increased chance of having a baby with Down syndrome resulting from nondisjunction. A 35-year-old woman has a one in 350 chance of conceiving a child with Down syndrome. By age 45, the incidence has increased to one in 30. However, because younger women have higher fertility rates, 80 percent of babies with Down syndrome are born to women under the age of 35. Once a woman has given birth to a baby with Down syndrome, the chance of having a second child with Down syndrome is about 1 in 100, although age may also be a factor.
Maternal age, however, is not linked to the chance of having a baby with translocation. Most cases are sporadic, chance events, but in about one third of translocation cases, one parent is a carrier of a translocated chromosome. For this reason, the chance of translocation in a second pregnancy is higher than that seen in nondisjunction.
Prenatal Testing and Diagnosis
There are two types of tests for Down syndrome that can be performed before your baby is born: screening and diagnostic tests. Prenatal screenings estimate the chance of the fetus having Down syndrome. These tests do not tell you for sure whether your baby has Down syndrome; they only provide a risk assessment. Diagnostic tests, on the other hand, can provide a definitive diagnosis with almost 100 percent accuracy.
There are two types of prenatal screening tests available: maternal serum screening and ultrasound (sonogram) screening. Maternal serum screening tests measure quantities of various substances in the blood of the mother, including alpha-fetoprotein and the hormones estriol and human chorionic gonadotropin. Together with a woman’s age, these are used to estimate her chance of having a child with Down syndrome. Typically offered between 15 and 20 weeks of gestation, maternal serum screening tests are only able to accurately detect about 60 percent of fetuses with Down syndrome. Many women who undergo these tests will be given false-positive readings, and some will be given false-negative readings.
Because maternal serum screening tests are of limited value, they are often performed in conjunction with a detailed sonogram to check for “markers” (characteristics that some researchers feel may have a significant association with Down syndrome). Recently, researchers have developed a maternal serum/ultrasound/age combination that can yield a much higher accuracy rate at an earlier stage in the pregnancy.
Prenatal screening tests are routinely offered to women over the age of 35, due to their increased chances of giving birth to a child with a disability; however, pregnant women of any age can request a test or choose not to have it done. If the estimate determined by prenatal screening is high, doctors will often advise a mother to undergo diagnostic testing.
The diagnostic procedures available for prenatal diagnosis of Down syndrome are chorionic villus sampling (CVS), amniocentesis and percutaneous umbilical blood sampling (PUBS). These procedures, which carry a small risk of miscarriage, are about 98 to 99 percent accurate in the detection of Down syndrome. Amniocentesis is usually performed between 15 and 22 weeks of gestation, CVS between 9 and 14 weeks, and PUBS after 18 weeks.
If you have any questions about these procedures, do not hesitate to ask your doctor. It is important that you receive accurate information and understand all your options. Whether or not to undergo a prenatal screening or diagnostic test is a personal decision, and expectant parents must make the choice that is best for them.
Diagnosing Down syndrome
Even though there are many prenatal tests available for Down syndrome, most cases of Down syndrome are diagnosed after the baby is born. Doctors will usually suspect Down syndrome if certain physical characteristics are present. Some of the traits common to babies with Down syndrome include:
Not all babies with Down syndrome have all these characteristics, and many of these features can be found, to some extent, in individuals who do not have the condition. Therefore, doctors must perform a special test called a karyotype before making a definitive diagnosis.
To obtain a karyotype, doctors draw a blood sample to examine your baby’s cells. They use special tools to photograph the chromosomes and then group them by size, number and shape. By examining the karyotype, they can determine accurately whether or not your baby has Down syndrome.
Hannah age 3
Parent's Perspective
When Hannah was born 5 weeks early we had no idea that she had Down syndrome. When the doctor told us we were shocked. We had no idea what that meant or what to expect. We were afraid for her future. We had a great deal of support from nurses, family, and friends who helped us to see that no matter what Down syndrome brought us we had a beautiful little girl and we should treat her as we would have treated her even is she didn't have this diagnosis. We got involved with the local Down syndrome society and an Early Intervention program. She received a great deal of love and nurturing from just about everyone she came into contact with. It was amazing.
Hannah is now 3 1/2 years old and is doing very well. She has learned to communicate by sign language and is receiving speech therapy to help with her verbal skills. She is fully included in a preschool class and is growing and learning every day. She loves being in school and we are so proud by all that she learns from her experiences. We are no longer afraid for her future because we know she has unlimited potential and is an extremely bright little girl!!
PACER Center
Parent Advocacy Coalition for Educational Rights
The following links are from the PACER Center Web site at
http://www.pacer.org/links/national/disability.htm#2
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