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Trisomy 21 (Down Syndrome)

Trisomy 21, also called Down Syndrome, is a disease caused by an abnormality in the chromosomes (the cell structures that contain the body’s genetic material). People with Down’s syndrome have three chromosomes 21 instead of just one pair. This imbalance in the functioning of the genome (all the hereditary information present in human cells) and the organism causes permanent mental retardation and developmental delay.

Down’s syndrome can vary in severity. The syndrome is increasingly well known and early intervention can make a big difference in the quality of life of children and adults with the disease. In the vast majority of cases, this disease is not hereditary, that is to say it is not transmitted from parents to their children.

Mongolism, Down Syndrome and Trisomy 21

Down Syndrome owes its name to the English physician John Langdon Down who published in 1866 the first description of people with Down’s syndrome. Before him, other French doctors had observed it. As the sufferers have features characteristic of Mongols, that is to say, a small head, a round and flattened face with slanted and splayed eyes, this disease was colloquially called “Mongoloid idiocy” or Mongolism. Today, this denomination is rather pejorative.

In 1958, the French doctor Jérôme Lejeune identified the cause of Down syndrome, namely, an additional chromosome on the 21st pair of chromosomes. For the first time, a link has been established between mental retardation and a chromosomal abnormality. This discovery opened up new possibilities for the understanding and treatment of many mental illnesses of genetic origin.

Na cúiseanna

Each human cell contains 46 chromosomes organized in 23 pairs on which the genes are found. When an egg and a sperm are fertilized, each parent transmits 23 chromosomes to their child, in other words, half of their genetic makeup. Down’s syndrome is caused by the presence of a third chromosome 21, caused by an abnormality during cell division.

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Chromosome 21 is the smallest of the chromosomes: it has around 300 genes. In 95% of cases of Down’s syndrome, this excess chromosome is found in all the cells of the body of those affected.

Rarer forms of Down syndrome are caused by other abnormalities in cell division. In about 2% of people with Down’s syndrome, excess chromosomes are found in only some of the body’s cells. This is called mosaic trisomy 21. In about 3% of people with Down’s syndrome, only part of chromosome 21 is excess. This is trisomy 21 by translocation.


In France, trisomy 21 is the leading cause of mental handicap of genetic origin. There are approximately 50 people with Down’s syndrome. This pathology affects 000 in 21 to 1 births.

According to the Quebec Ministry of Health, Down’s syndrome affects approximately 21 in 1 births. Any woman can have a baby with Down’s syndrome, but the likelihood increases with age. At age 770, a woman would have a 21 in 20 risk of having a child with Down’s syndrome. At age 1, the risk is 1500 in 30. This risk would decrease from 1 in 1000 to 1 years and from 100 in 40 to 1 years.


The diagnosis of Down syndrome is usually made after birth, when you see the baby’s features. However, to confirm the diagnosis, it is necessary to perform a karyotype (= test that allows the study of chromosomes). A sample of the baby’s blood is taken to analyze the chromosomes in the cells.

Prenatal tests

There are two types of prenatal tests that can diagnose trisomy 21 before birth.

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An tástálacha scagtha which are intended for all pregnant women, assess whether the probability or risk that the baby will have trisomy 21 is low or high. This test consists of a blood sample and then an analysis of the nuchal translucency, that is to say the space between the skin of the neck and the spine of the fetus. This test takes place during an ultrasound between 11 and 13 weeks of pregnancy. It is safe for the fetus.

An tástálacha diagnóiseacha which are intended for women at high risk, indicate whether the fetus has the disease. These tests are usually performed between the 15st agus 20st week of pregnancy. The accuracy of these techniques for screening for Down syndrome is approximately 98% to 99%. Test results are available in 2-3 weeks. Before having any of these tests, the pregnant woman and her spouse are also advised to meet with a genetic consultant to discuss the risks and benefits associated with these interventions.


THEamniocentesis can determine with certainty whether the fetus has Down’s syndrome. This test is usually done between the 21thst agus 22st week of pregnancy. A sample of amniotic fluid from the uterus of the pregnant woman is taken using a thin needle inserted into the abdomen. Amniocentesis carries certain risks of complications, which can go as far as the loss of the fetus (1 in 200 women is affected). The test is offered primarily to women who are at high risk based on screening tests.


Chorionic villus sampling.

The sampling (or biopsy) of the chorionic villi (PVC) makes it possible to determine whether the fetus has a chromosomal abnormality such as trisomy 21. The technique consists in removing fragments of the placenta called chorionic villi. The sample is taken through the abdominal wall or vaginally between the 11st agus 13st week of pregnancy. This method carries a risk of miscarriage of 0,5 to 1%.

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Current news on Passeport Santé, a promising new test to detect Down’s syndrome

Other trisomies

The term trisomy refers to the fact that an entire chromosome or a fragment of a chromosome is represented in triplicate instead of two. Among the 23 pairs of chromosomes present in human cells, others can be the subject of complete or partial trisomies. However, more than 95% of affected fetuses die before being born or after only a few weeks of life.

La trisomy 18 (or Edwards syndrome) is a chromosomal abnormality caused by the presence of an extra chromosome 18. The incidence is estimated at 1 in 6000 to 8000 births.

La trisomy 13 is a chromosomal abnormality caused by the presence of an extra 13 chromosome. It causes damage to the brain, organs and eyes, as well as deafness. Its incidence is estimated at 1 in 8000 to 15000 births.

Effects on the family

The arrival in the family of a baby with Down’s syndrome may require a period of adjustment. These children require special care and extra attention. Take the time to get to know your child and make a place for him in the family. Each child with Down’s syndrome has their own unique personality and needs as much love and support as others.

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