Mosaic Down Syndrome: Symptoms, Causes, Diagnosis, and Prognosis

A variant of Down syndrome

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Mosaic Down syndrome is a variant of Down syndrome. Down syndrome is a congenital (present at birth) genetic condition that occurs when a person has three copies of chromosome 21 instead of two (the standard number). This is called trisomy 21.

In mosaic Down syndrome, some of the person’s cells have three copies of chromosome 21, but some cells have two copies of this chromosome, resulting in a mixture.

This article describes the types, symptoms, causes, and management of mosaic Down syndrome.

Fetal ultrasound image and chromosomal analysis report

Vilaiporn Chatchawal / Getty Images

Types of Mosaic Down Syndrome

There are several types of mosaic Down syndrome, defined by the percentage of cells that have three copies of chromosome 21.

With mosaic Down syndrome, some cells have two copies of chromosome 21, and some cells have three. If they have no other chromosome number anomalies, some cells have 46 total chromosomes and some have 47.

Mosaic Down Syndrome Symptoms 

The symptoms of mosaic Down syndrome are the same as symptoms of the more common type of Down syndrome, in which all of the cells have three copies of chromosome 21. 

While most people with Down syndrome have similar symptoms, there is a range in severity. With mosaic Down syndrome, there is also a range in severity. There is a tendency for people with mosaic Down syndrome to have slight differences in the effects, but this is not consistent.

Changes that occur due to Down syndrome and mosaic Down syndrome include:

  • Characteristic facial features with prominent eyelids, small eyes, and a round, full, face
  • Short height 
  • Bone structure abnormalities 
  • High body weight 
  • Heart malformations: Can cause fatigue, shortness of breath, or passing out 
  • Learning disability (can vary) 
  • Lung dysfunction: Can cause low oxygen or trouble breathing 
  • Intestinal malformations: Can cause digestive issues or blockage 
  • Gastrointestinal disease, such as celiac disease (an autoimmune disease triggered by gluten in food)
  • Immune deficiency: Causes a predisposition to infections
  • Hypothyroidism: Low thyroid hormone, can cause fatigue, weight changes, temperature intolerance, difficulty concentrating

These changes occur because the extra genes from the extra copy of chromosome 21 cause alterations in the body’s structure, which leads to the symptoms. 

A pregnancy with a fetus with Down syndrome has a higher risk of birth complications and stillbirth.


Human cells typically contain 23 pairs of chromosomes (46 total), which carry genetic material. Each sperm or egg cell typically contains only one chromosome of each pair.

Down syndrome occurs when an embryo is conceived with a third copy of chromosome 21. Throughout pregnancy, the fetus's cells are copied with the same set of chromosomes, so all of the growing fetus's cells also have an extra copy of chromosome 21. 

The extra chromosome can come either from the egg cell or from the sperm cell. Normally, the egg cell and eye sperm cell have only one copy of each chromosome, including chromosome 21. This results in a fetus having two copies of each chromosome in every one of its cells—for a total of 46 chromosomes.

Trisomy is the presence of a third chromosome for a chromosome pair. The extra copies of the genes contained in chromosome 21 cause several variations in fetal and child development, leading to Down syndrome's physical characteristics. These changes begin to develop before birth.

How Does Mosaicism Happen? 

In general, mosaicism is a rare genetic phenomenon. It means that the cells in the body do not have the exact same genetic pattern. 

In mosaic Down syndrome, an error occurs as the embryo's cells are making copies. This error occurs during the very early stages of pregnancy. 

There are two ways that mosaic Down syndrome can occur:

  • The embryo has trisomy 21 at conception. Then another error occurs, which causes some of the cells to lose the extra chromosome while the fetus grows. 
  • The embryo is conceived with two copies of chromosome 21, and an error occurs, which causes some of the cells to have an extra copy of chromosome 21. 

In either of these situations, the embryo can eventually develop a mixture of cells with three copies of chromosome 21 and two copies of chromosome 21. It is believed that the error occurs very early during pregnancy. 

It isn't known why these errors occur, but research suggests that the chromosomes in Down syndrome are more fragile than usual and that this could predispose to the chromosomal errors that cause the mosaic pattern.


Down syndrome is diagnosed based on a child’s physical features and with a chromosome test. Typically, physical features, such as facial appearance, are reliable indicators of the condition, but sometimes it isn’t completely obvious.

A definitive diagnosis is made with a chromosome test. Mosaic Down syndrome usually causes physical characteristics similar to those of Down syndrome, but the distinction between Down syndrome and mosaic Down syndrome is made based on a chromosome test.

Chromosomal Analysis 

Chromosome analysis can be done during pregnancy or after a baby is born. With mosaic Down syndrome, some cells would have three copies of chromosome 21, and some would have two copies. The percentage would be reported as a reflection of the degree of mosaicism.

Prenatal (before birth) testing is done with amniocentesis or chorionic villus sampling. Sampling involves the placement of a needle into the uterus (with ultrasound guidance). A sample of the fluid or tissue around the fetus is collected and sent to the laboratory for analysis. They are safe tests, but there is a slight risk of complications.

Chromosome testing after birth involves laboratory analysis of a blood sample from the baby. 

Medical Tests 

People with mosaic Down syndrome may have diagnostic evaluations to identify the physical effects of Down syndrome. This can include screening tests before birth, such as prenatal ultrasound and diagnostic evaluation of symptoms. 

These may include:

  • Testing of cognitive abilities 
  • Echocardiogram to examine heart structure and function 
  • Thyroid hormone blood tests 
  • X-rays of the lungs or bones
  • Blood test to measure electrolytes (charged minerals in the blood) that could be affected by intestinal and digestive issues


Many of the effects of Down syndrome can be treated with the same treatments used in all types of Down syndrome. No cure or treatment can correct the chromosome changes, and there is no way to prevent the physical effects of the condition from developing if a person has any form of Down syndrome. 

Management of effects of Down syndrome include: 

  • Individualized learning plan in school 
  • Treatment of heart malformations, if possible, which may include surgery or medication
  • Treatment of digestive issues and intestinal malformations, which may include surgical intervention or medical treatment
  • Management of lung and breathing problems
  • Physical therapy
  • Support devices to help manage the effects of skeletal changes


There is no cure for Down syndrome or mosaic Down syndrome. The chromosomal abnormalities cannot be fixed or treated at any age, even before birth. The effects on the body can be treated symptomatically, which can improve a person’s outcome, quality of life, and life expectancy.

The condition is not expected to worsen. But some effects might not substantially impact the quality of life when a child is young, and they may have a more significant impact as the child ages and enters adulthood.


Living with Down syndrome is challenging. Parents and caregivers for children with this condition need to seek special accommodations to effectively navigate schooling, socializing, sports, music, and other activities. 

Attending a school that offers accommodations suitable for children and adolescents with Down syndrome can be helpful. Additionally, certain workplaces can also provide accommodations so that a person with Down syndrome can productively work and use their skills in a way that helps improve their quality of life.

Some families may benefit from joining a support group that includes people with Down syndrome. It can be enriching to learn from others and be a source of support for others who are dealing with the condition.


Mosaic Down syndrome is a type of Down syndrome with a slightly different chromosomal pattern than the more common type of full trisomy 21. Down syndrome occurs when a person has three copies of chromosome 21 in all their cells instead of the standard of two copies. With the mosaic pattern, some cells have two copies, and some have three.

This often leads to the same symptoms and prognosis as the more common type of Down syndrome, but sometimes the effects may be milder. The diagnosis is made with chromosome testing, which can be done prenatally or after birth. Treatment is focused on the symptoms and effects, but there is no cure. 

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Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Heidi Moawad, MD
Heidi Moawad is a neurologist and expert in the field of brain health and neurological disorders. Dr. Moawad regularly writes and edits health and career content for medical books and publications.