What Is
Down Syndrome?

In every cell in the human body there is a nucleus, where genetic material is stored in genes. Genes carry the codes responsible for all of our inherited traits and are grouped along rod-like structures called chromosomes. Typically, the nucleus of each cell contains 23 pairs of chromosomes, half of which are inherited from each parent. Down syndrome occurs when an individual has a full or partial extra copy of chromosome 21.

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This additional genetic material alters the course of development and causes the characteristics associated with Down syndrome. A few of the common physical traits of Down syndrome are low muscle tone, small stature, an upward slant to the eyes, and a single deep crease across the center of the palm – although each person with Down syndrome is a unique individual and may possess these characteristics to different degrees, or not at all.

1 in every 700 babies worldwide is born with Down syndrome (source: NDSS).

What Causes Down Syndrome?

01

Usually caused by a natural error in cell division at conception.

02

It’s not caused by anything a parent does.

03

Although chances increase with maternal age, most babies, almost 80%, with Down syndrome are born to women under 35.

Fun Fact: When was Down syndrome discovered?

Down syndrome was first described in 1866 by English physician John Langdon Down, who observed a set of shared physical features. The condition was later named after him.

It wasn’t until 1959 that the genetic cause was identified. French geneticist Jerome Lejeune discovered that individuals with Down syndrome have an extra 21st chromosome. This breakthrough led to the identification of other forms, including translocation and mosaicism, deepening our understanding of the condition.

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Types of Down Syndrome

There are three main types:

Since inception, KDSP has empowered thousands of individuals with Down syndrome and their families through focused interventions and strategic outreach.

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Trisomy 21

95% of the cases.

A child with Trisomy 21 has three copies of chromosome 21, rather than the normal pair.. However, the presence of cells with the normal number of chromosomes may result in fewer characteristics of Down syndrome.

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Translocation

3–4% of cases.

Translocation is what people are referring to if they say that the condition is inherited, because usually one parent is a carrier. The extra 21 chromosome is present, but attached to a different chromosome in the egg or sperm. The clinical features of people with Translocation Down syndrome are indistinguishable from those with Trisomy 21. However, the presence of cells with the typical number of chromosomes may result in fewer characteristics of Down syndrome.

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Mosaicism

<1% of cases.

Children born with Mosaic Down syndrome have some cells with three copies of chromosome 21 and some cells that have the usual pair. Clinically, babies born with Mosaic Down syndrome can have the same features and health problems seen in babies born with Trisomy 21 or Translocation Down syndrome.

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How Is It Diagnosed?

Screening Tests

Screening tests determine what the chances are that your baby will be born with Down syndrome or other medical conditions. This kind of testing does not definitively diagnose a fetus with one of these conditions.

The different types of screening tests include blood tests, which are used to measure protein and hormone levels in pregnant women. Abnormally increased or decreased levels can indicate a genetic condition. New blood tests can also detect chromosomal material from the fetus that is circulating in the mother’s blood. Another screening test is an ultrasound, which is a non-invasive imagining technique that uses sound waves to generate an image of the fetus. An ultrasound can identify congenital heart conditions and other structural changes that may indicate a diagnosis of Down syndrome. The combined results of these two tests are used to estimate the chance that your baby will be born with Down syndrome.

Diagnostic Tests

Diagnostic tests can determine whether or not your fetus has Down syndrome with nearly 100 percent accuracy. However, because these tests are performed inside the uterus, they carry a small increased risk of miscarriage and other complications.

The different types of diagnostic testing include Chorionic villus sampling (CVS), a prenatal diagnosis method in which a small biopsy of the placenta is taken for specific genetic testing. CVS is used to detect any condition that involves specific chromosomal abnormalities, like Down syndrome. This test is usually performed during the first trimester between 10-14 weeks of gestation. Amniocentesis is a prenatal diagnosis method in which a needle is inserted into the amniotic sac that surrounds the baby. Amniocentesis is most often used to detect Down syndrome and other chromosomal abnormalities. This test is usually done in the second trimester, after 15 weeks of gestation.

Down syndrome can also be diagnosed after the baby is born. Since Down syndrome involves a set of unique characteristics, a doctor can usually tell whether a baby should be tested based on a physical examination. To confirm the findings, a karyotype (a small blood or tissue sample) can be analyzed to determine the presence of extra material from chromosome 21. This information is important in determining the chance of a mother having a baby with the condition in the future.