What’s mosaic Down syndrome?

Mosaic Down syndrome, or mosaicism, is a rare form of Down syndrome. Down syndrome is a genetic disorder that results in an extra copy of chromosome 21. People with mosaic Down syndrome have a mixture of cells. Some have two copies of chromosome 21, and some have three.

Mosaic Down syndrome occurs in about 2 percent of all Down syndrome cases. People with mosaic Down syndrome often, but not always, have fewer symptoms of Down syndrome because some cells are normal.

Understanding Down syndrome

Down syndrome is a genetic disorder in which some, or all, of a person’s cells have an extra chromosome.

All normal human cells have 46 chromosomes, except the egg and sperm, which normally have 23. These sex cells are formed by division (called meiosis). When an egg is fertilized, these two cells join, normally giving the embryo 23 chromosomes from each parent for a total of 46 chromosomes.

Sometimes an error occurs in this process, causing the wrong number of chromosomes to occur in the sperm or egg. A healthy baby has two copies of chromosome 21 in each cell. People with Down syndrome have three. Any cell replicated from the flawed cell will also have the wrong number of chromosomes.

People with mosaic Down syndrome have a mixture of cells. Some cells have a normal pair of chromosome 21, and other cells contain three copies. This usually happens because the division problem that causes the extra copy of chromosome 21 happens after fertilization.

Mosaic Down syndrome symptoms

Irregular chromosome copies change the genetic makeup of a baby, ultimately affecting their mental and physical development.

People with Down syndrome typically have:

Down syndrome is sometimes accompanied by a number of other health problems, including:

These symptoms are also common in people with mosaic Down syndrome. However, they may have less of these symptoms. For example, people with mosaic Down syndrome typically have a higher IQ than those with other forms of Down syndrome.

Diagnosis

Doctors can perform tests to screen for Down syndrome during pregnancy. These tests show the likelihood that the fetus will have Down syndrome and can identify health problems early.

Screening tests

Screening tests for Down syndrome are offered as routine testing during pregnancy. They’re usually provided during the first and second trimesters. These screens measure hormone levels in the blood to detect abnormalities and use an ultrasound to look for irregular fluid buildup in the baby’s neck.

Screening tests only provide the likelihood of a baby developing Down syndrome. It cannot diagnose Down syndrome. They can determine, though, whether more tests are needed to confirm the diagnosis.

Diagnostic tests

Diagnostic tests can confirm if your baby has Down syndrome before it’s born. The two most common diagnostic tests are chorionic villus sampling and amniocentesis.

Both tests take samples from the uterus to analyze chromosomes. Chorionic villus sampling uses a sample of the placenta to do this. This test can be completed in the first trimester. Amniocentesis analyzes an amniotic fluid sample surrounding the growing fetus. This test is typically performed in the second trimester.

Mosaic Down syndrome is typically described through a percentage. To confirm mosaic Down syndrome, doctors will analyze chromosomes from 20 cells.

If 5 cells have 46 chromosomes and 15 have 47 chromosomes, a baby has a positive mosaic Down syndrome diagnosis. In this case, the baby would have a 75 percent level of mosaicism.

Outlook

There’s no treatment for mosaic Down syndrome. Parents can detect the condition before birth and prepare for any associated birth defects and health complications.

Life expectancies for people with Down syndrome are much higher than in the past. They can now be expected to live to more than 60 years of age. Furthermore, early physical, speech, and occupational therapies can provide people with Down syndrome a higher quality of life and improve their intellectual abilities.