The term "trisomy" is used to describe the presence of three chromosomes, rather than the usual pair of chromosomes. For example, if a baby is born with three #21 chromosomes, rather than the usual pair, the baby would be said to have "trisomy 21." Trisomy 21 is also known as Down syndrome. Other examples of trisomy include trisomy 18 and trisomy 13. Again, trisomy 18 or trisomy 13 simply means there are three copies of the #18 chromosome (or of the #13 chromosome) present in each cell of the body, rather than the usual pair.
Down syndrome is a genetic disorder that includes a combination of birth defects, including some degree of mental retardation, characteristic facial features and, often, heart defects, visual and hearing impairment, and other health problems. The severity of all of these problems varies greatly among affected individuals. Down syndrome is one of the most common genetic birth defects, affecting approximately one in about 800 babies. In this country, there are more than 400,000 individuals with Down syndrome, according to the National Down Syndrome Society. Life expectancy among adults with Down syndrome is about 60 years, though average lifespan varies.
The name "Down syndrome" comes from the physician, Dr. Langdon Down, who first described the collection of physical symptoms in 1866. It was not until 1959 that the cause of Down syndrome (the presence of an extra #21 chromosome) was identified.
Normally in reproduction, the egg cell of the mother and the sperm cell of the father start out with the usual number of 46 chromosomes. The egg and sperm cells undergo cell division where the 46 chromosomes are divided in half and the egg and the sperm cells end up with 23 chromosomes each. When a sperm with 23 chromosomes fertilizes an egg with 23 chromosomes, the baby ends up with a complete set of 46 chromosomes, half from the father and half from the mother.
Sometimes, an error occurs when the 46 chromosomes are being divided in half and an egg or sperm cell keeps both copies of the #21 chromosome instead of just one copy. If this egg or sperm is fertilized, the baby ends up with three copies of the #21 chromosome and this is called "trisomy 21" or Down syndrome. The features of Down syndrome result from having an extra copy of chromosome 21 in every cell in the body.
Ninety-five percent of Down syndrome results from trisomy 21. Occasionally, the extra chromosome 21 or a portion of it is attached to another chromosome in the egg or sperm; this may result in what is called "translocation" Down syndrome (3 to 4 percent of cases). This is the only form of Down syndrome that may be inherited from a parent. Some parents have a rearrangement called a balanced translocation, where the #21 chromosome is attached to another chromosome, but it does not affect their own health. Rarely, a form of Down syndrome called "mosaic" Down syndrome may occur when an error in cell division occurs after fertilization (1 percent to 2 percent of cases). These individuals have some cells with an extra chromosome 21 and others with the normal number.
A child with Down syndrome may have eyes that slant upward and small ears that may fold over slightly at the top. Their mouth may be small, making the tongue appear large. Their nose also may be small, with a flattened nasal bridge. Some babies with Down syndrome have short necks and small hands with short fingers. Rather than having three "creases" in the palm of the hand, a child with Down syndrome usually has one single crease that goes straight across the palm, and a second crease that curves down by the thumb. The child or adult with Down syndrome is often short and has unusual looseness of the joints. Most children with Down syndrome will have some, but not all, of these features.
The degree of mental retardation that accompanies Down syndrome varies widely, ranging from mild to moderate to severe. However, most mental retardation falls within the mild to moderate range. There is no way to predict the mental development of a child with Down syndrome based on physical features.
Children with Down syndrome can usually do most things that any young child can do, such as walking, talking, dressing, and being toilet trained. However, they generally do these things later than other children. The exact age that these developmental milestones will be achieved cannot be predicted. However, early intervention programs, beginning in infancy, can help these children achieve their individual potential.
Yes. There are special programs beginning in the preschool years to help children with Down syndrome develop skills as fully as possible. Along with benefiting from early intervention and special education, many children can be integrated in the regular classroom, to some extent. The outlook for these children is far brighter than it once was. Many will learn to read and write and participate in diverse childhood activities both at school and in their neighborhoods. While there are special work programs designed for adults with Down syndrome, many people with the disorder can hold regular jobs. Today, an increasing number of adults with Down syndrome live semi-independently in community group homes where they take care of themselves, participate in household chores, develop friendships, partake in leisure activities, and work in their communities.
Some people with Down syndrome marry. Although there have been rare exceptions, men with Down syndrome cannot father a child. In any pregnancy, a woman with Down syndrome has a 50/50 chance of conceiving a child with Down syndrome, but many pregnancies are miscarried.
Since Down syndrome is such a unique group of characteristics, physicians can sometimes determine simply by physical examination whether a baby has Down syndrome. To confirm the physical findings, a small blood sample can be taken and the chromosomes can be analyzed to determine the presence of extra #21 chromosome material. This information is important in determining the risk in future pregnancies. (Translocation Down syndrome and mosaic Down syndrome have different recurrence risks in future pregnancies).
Chromosomal abnormalities such as Down syndrome can often be diagnosed before birth by analyzing cells in the amniotic fluid or from the placenta. Fetal ultrasound during pregnancy can also give information about the possibility of Down syndrome, but ultrasound is not 100 percent accurate, since many babies with Down syndrome may look the same on ultrasound as those without Down syndrome. A chromosome analysis, whether performed on a blood sample, cells from the amniotic fluid, or placenta, is over 99.9 percent accurate.
The mother's age at delivery is the only factor found to be linked to the risk of having a baby with Down syndrome. This risk increases with every year, especially after the mother is 35 years old. However, because younger women are more likely to have babies than older women, 80 percent of babies with Down syndrome are born to women younger than 35.
The American College of Obstetricians and Gynecologists (ACOG) recommends that all pregnant women, regardless of age, be offered screening for Down syndrome.
In general, for women under 40 (after having one child with Down syndrome), the chance of having another baby with Down syndrome is 1 percent. The chance for Down syndrome is also known to increase with the mother's age and, after age 40, a mother would simply have the risk based on her age at delivery. It is important to know that about 80 percent of babies with Down syndrome are born to women under 35. This is due to the fact that women under 35 have more babies than women over 35. Your physician may refer you to a geneticist or genetic counselor who can explain the results of chromosomal tests in detail, including what the recurrence risks may be in another pregnancy and what tests are available to diagnose chromosome problems before a baby is born.
There is no cure for Down syndrome. We are not certain how to prevent the chromosomal error that causes Down Syndrome. To date, there is no reason to believe that a parent could have done anything to cause or prevent the birth of their baby with Down syndrome. However, a recent study suggests that some women who have had a baby with Down syndrome had an abnormality in how their bodies metabolize (process) the B vitamin folic acid. If confirmed, this finding may provide yet another reason why all women who might become pregnant should take a daily multivitamin containing 400 micrograms of folic acid (which has been shown to reduce the risk of certain birth defects of the brain and spinal cord).
Some people claim that various high dose vitamins given to children with Down syndrome will improve the mental performance and lessen the degree of mental retardation. To date, however, there has not been a medical study to prove that this actually works. It is important for new families to talk to their physician, other families, and the National Down Syndrome Congress to learn what to expect with Down syndrome and to learn about things which may be helpful in raising a child with Down syndrome.
The March of Dimes is investigating why errors in chromosome division occur, in the hope of someday preventing Down syndrome and other birth defects caused by abnormalities in the number or structure of chromosomes. Other researchers are seeking to improve the outlook for children with Down syndrome. One example of this includes developing improved language intervention programs to help these children communicate more easily.
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