Down syndrome is a congenital disorder that is believed to occur at the time of conception due to an extra, or “copied” chromosome (chromosome number 21) being present in either the egg or the sperm of the child’s parents. The child is born with 47 chromosomes total, as opposed to the 46 that healthy, non Down syndrome children possess. The 23 sets of chromosomes that make up the human body from the time of conception are responsible for all the myriad characteristics that each and every person possesses. For the child with Down syndrome, he result of this extra chromosome 21 is a set of birth defects that is predictable, but varied case by case in combination and severity.
The most common of the genetic disorders affecting the autosomes (the chromosomes that are not the “X” and “Y” sex determination chromosomes), Down syndrome occurs in approximately 1 out of ever 800 live births.
Diagnosing a child with Down syndrome can be done after birth, or during gestation. There are definitive tests that can be performed once a question of whether a child may have Down syndrome has been raised.
Down Syndrome Information
The Diagnosis of Down Syndrome in a Newborn
Once a child is born, if he or she is exhibiting any of the features of Down syndrome, it is natural for parents to become concerned and to request more information about Down syndrome. However, while the physical characteristics of Down syndrome are fairly standard, many of these characteristics are present in healthy newborns, as well, and resolve as the baby grows. Some of these physical characteristics include a high forehead and flattened facial features, and excess skin, or “baggy” areas under the eyes.
However, if concern that a child has been born with Down syndrome has been raised, the doctors will perform a basic test called a karyotype to diagnose or rule out the disorder.
A karyotype is a picture of all the chromosomes in a cell. All that is experienced by the child during the procedure is a simple blood draw. The chromosomes in the cell are then stained with a dye and placed on a microscope, through which they are photographed. The photograph is then cut up and rearranged so that the chromosomes are lined up into corresponding pairs. This process makes it fairly easy to determine any irregularities in the numbers of the chromosomes or in their relationship to their pairs. It is then a very simple process to identify whether the third chromosome 21 denoting Down syndrome in the child is present.
Down Syndrome Information
The Diagnosis of Down Syndrome in the Fetus
There are several factors that may prompt an couple to request that the fetus be tested for Down syndrome, or for a physician to recommend it.
The most common of these factors is the mother’s age. The older the expecting mother is, the higher the risk that her child will have Down syndrome. The incidence of Down syndrome in the child of a mother who is twenty years old at the time of the birth is approximately 1:1600, while the incidence in a mother who is thirty-five at the time of the child’s birth rises dramatically to approximately 1:370. (Benke, PJ, Risk and Recurrence Risk of Down syndrome)
Another increased risk factor for Down syndrome in the fetus is if the couple, or particularly the mother, already has a child with the disorder. Although the chances that the second child will also have Down syndrome are only about 1%, this is a much higher risk of occurrence than that expected in couples or mothers who do not have a child with Down syndrome.
There are two types of test available to pregnant women for the detection of Down syndrome: screening tests and diagnostic tests. Screening tests estimate the risk of the fetus having Down syndrome. Diagnostic tests are more conclusive, and tell whether or not the fetus actually has the condition.
The most commonly used screening tests are the Triple Screen and the Alpha-fetoprotein Plus. These tests are generally performed fifteen to twenty weeks into the gestation of the fetus. They measure the levels of various chemicals and substances in the blood and then hypothesize on the risk of the mother carrying a child with Down syndrome based of the outcome of those levels by factoring the mother’s age into the equation.
Screening tests are known for a high incidence of providing false positive readings, and have also been known to provide false negatives as well. They are currently considered to be approximately 60% accurate in the detection of Down Syndrome.
If there is a true concern that a woman may be carrying a child with Down Syndrome, a diagnostic test is a more accurate method of diagnosing the condition in the womb. Many mothers and physicians shy away from diagnostic testing without specific concern however, because each of the methods used to collect the required tissue, one of which is amniocentesis, carries a small risk of inducing miscarriage. However, diagnostic testing is over 98% accurate for diagnosing whether or not the child a mother is carrying had Down Syndrome.
More Information about Down Syndrome
Down Syndrome Information: What is Down Syndrome?
Down Syndrome Information: Causes of Down Syndrome
Down Syndrome Information: Characteristics of Down Syndrome
Down Syndrome Information: What Special Care do People with Down Syndrome Need?
Down Syndrome Information: Frequently Asked Questions
Down Syndrome Information: A Personal Account