Down Syndrome

Down Syndrome

Down syndrome is the most common chromosomal disorder. An estimated 1 in 800 infants are born with Down syndrome. Down syndrome is also a common cause of miscarriage. Every year, as many as 5,000 babies are born with Down syndrome in the United States.

What is Down Syndrome?

Down syndrome is a genetic disorder that results in varying degrees of physical and mental retardation. The condition varies in severity, causing developmental problems that range from mild to severe. The disorder occurs as a result of extra genetic material. In most people, the genes are contained on 23 pairs of chromosomes, for a total of 46 chromosomes. Most people with Down syndrome have an extra copy of the No. 21 chromosome, for a total of 47 chromosomes.

Down syndrome is the most common chromosomal disorder. An estimated 1 in 800 infants are born with Down syndrome. Down syndrome is also a common cause of miscarriage. Every year, as many as 5,000 babies are born with Down syndrome in the United States.

Every year, one child in every 800 to 1,000 births will be born with a condition known as Down syndrome. Down syndrome is a disorder in which the child has extra genetic material. This extra genetic material causes the baby to develop differently in the womb. This abnormal development occurs during the early stages of cell division, soon after conception.

There is no known reason for Down syndrome or a cure for it.

The disorder came by its name when an English physician, John Langdon Down, published a description of the characteristics of a person with the condition in 1886. Since Dr. Down was the first person to give the syndrome a name, the disorder is known as Down syndrome.

There are three distinct forms of Down syndrome.

The most common form is Trisomy 21. This is when a child has an extra chromosome 21. Instead of having 46 chromosomes in each cell (23 from the father and 23 from the mother), he has 47. Ninety-five percent of children born with Down syndrome have Trisomy 21.

Translocation is another form of Down syndrome and makes up 3 to 4% of the population of people with the disorder. Translocation occurs when part of chromosome 21 breaks off and attaches to another chromosome, changing the genetic makeup. In this form of Down syndrome, each cell has the normal 46 chromosomes, but there is extra genetic material from the broken off chromosome.

The third and final form is called Mosaicism, and is the rarest form of Down syndrome, occurring in only 2% of all cases. Mosaicism happens when some of the cells in the forming embryo have 46 chromosomes, and some have 47. Thus, the alternating pattern gives it the name Mosaicism. People with Mosaicism may not be as affected with the physical and/or developmentally delayed characteristics of those born with the other two forms of Down syndrome. Common Physical Traits

Babies with Down syndrome are usually diagnosed at birth or shortly thereafter. Because Down syndrome affects a child physically, there will be certain features that an alert doctor or nurse will pick up on. Most children with the syndrome will have some or all of these traits.

Common Physical Characteristics of Down Syndrome:

  • Low muscle tone
  • Small nose and flat nasal bridge
  • An upward slant to the eyes (almond shaped)
  • Small skin folds on the inner corner of the eyes
  • Tongue large for size of child's mouth
  • Small, sometimes abnormally shaped ears
  • A single crease across the center of the palm
  • Fifth finger, the pinky, has only one crease instead of two
  • Larger than normal space between the large and second toe
  • Joints are hyper flexible, able to extend greater than average

These physical traits are characteristic of a baby with Down syndrome, but in order to determine if a child does indeed have the disorder, a chromosome analysis will need to be performed. This is done by taking blood from your baby and the analysis will be done in a laboratory. Your doctor will notify you of the results.

Likelihood of Giving Birth to a Baby with Down Syndrome

Most people associate having a child with Down syndrome with older women. While it is true that women over 35 do have an increased risk of having a child with Down syndrome, 80% of these children are born to those women under age thirty-five. In the United States, approximately 5,000 babies with Down syndrome are born every year. A woman's chance of having another baby with Down syndrome is approximately 1 in 100.

Prenatal Screening for Down Syndrome

Over the last 10 years, new technology has improved the mehods of detection of Down syndrome. While there are ways to diagnose Down syndrome by obtaining fetal tissue samples by amniocentesis or chorionic villus sampling, it would not be appropriate to examine every pregnancy this way. Besides greatly increasing the cost of medical care, these methods do carry a slight amount of risk to the fetus. So screening tests have been developed to try to identify those pregnancies at "high risk." These pregnancies are then candidates for further diagnostic testing.

Screening Vs Diagnostic Test

What is the difference between a screening test and a diagnostic test?

In diagnostic tests, a positive result very likely means the patient has the disease or condition of concern. In screening tests, the goal is to estimate the risk of the patient having the disease or condition. Diagnostic tests tend to be more expensive and require an elaborate procedure; screening tests are quick and easy to do. However, screening tests have more chances of being wrong: there are "false-positives" (test states the patient has the condition when the patient really doesn't) and "false-negatives" (patient has the condition but the test states he/she doesn't). Maternal Serum Screening

The mother's blood is checked for three items:

  • alpha-fetoprotein (AFP)
  • unconjugated estriol (uE3)
  • human chorionic gonadotropin (hCG)

These three are independent measurements, and when taken along with the maternal age (discussed below), can calculate the risk of having a baby with Down syndrome. A very important consideration in the screening test is the age of the fetus (gestational age). The correct analysis of the different components depends on knowing the gestational age precisely. The best way to determine that is by ultrasound.

Test results are sometimes reported to doctors as "Multiples of the Median (MoM)." The "average" value is therefore called 1.0 MoM. Down syndrome pregnancies have lower levels of AFP and estriol, so their levels would be less than 1.0 MOM.

hCG in a Down syndrome pregnancy would be greater than 1.0 MoM.

Finally, the calculated risk is used to modify the risk already statistically calculated based on the mother's age. We already know that as the mother's age advances, the risk of having a baby with Down syndrome increases.

For example: Let's say the test results come back in the typical range for a pregnancy not associated with Down syndrome (that would be 1.0 MoM for all components). This result reduces the woman's risk of having a child with Down syndrome four-fold.