US Pharm
. 2023;48(10):32-33.

Down syndrome is a genetic disorder caused by abnormal cell division that results in an extra full or partial copy of chromosome 21. This extra genetic material causes the developmental changes and physical features of Down syndrome.1

A normal baby has 23 pairs of chromosomes. Babies with Down syndrome have an extra copy of one of these chromosomes—chromosome 21. The medical terminology for having an extra copy of a chromosome is trisomy. This extra copy changes how the baby’s body and brain advance, which can cause both mental and physical challenges.1 Although children with Down syndrome might act and look similar, each one has different abilities. They have an IQ in the moderately low range and are slower to speak than other children.1

Down syndrome varies in severity among individuals, causing lifelong conceptual disability and developmental delays. It is the most common genetic chromosomal disorder and cause of learning disabilities in children. It also commonly causes other medical problems, including heart and gastrointestinal disorders.1

Down syndrome remains the most common chromosomal condition diagnosed in the United States. Each year, approximately 6,000 babies born in the U.S. have Down syndrome. This means that Down syndrome occurs in about one in every 700 babies.2

In 1959, the French physician Jérôme Lejeune identified Down syndrome as a chromosomal condition. Dr. Lejeune observed 47 chromosomes in the cells of individuals with Down syndrome, instead of 46. Other names for this condition are 47,XX,+21 and 47,XY,+21.1,2

Physical Features

Common physical features of Down syndrome are as follows:
• A flattened face, especially the bridge of the nose
• A short neck and small ears
• Small hands and feet
• Almond-shaped eyes
• A tongue that tends to stick out of the mouth
• Tiny white spots on the colored part of the eye
• A single line across the palm of the hand (palmar crease)
• Small pinky fingers that sometimes curve toward the thumb
• Poor muscle tone or loose joints
• Shorter in height as children and adults.1,3

Types of Down Syndrome

Down syndrome has three types. It is difficult to differentiate between each type without looking at the chromosomes, because the physical features and behaviors are similar.

Trisomy 21

Approximately 95% of people with Down syndrome have trisomy 21. With this type of Down syndrome, each cell in the body has three separate copies of chromosome 21 instead of the usual two copies.3


This type accounts for approximately 3% of people with Down syndrome.3 This happens when an extra part or a whole extra chromosome 21 is present, but it is attached or translocated to a different chromosome rather than being a separate chromosome 21.

Mosaic Down Syndrome

Mosaic means mixture or combination. This type affects approximately 2% of people with Down syndrome.3 With this type, some of the cells have three copies of chromosome 21, but the rest have the typical two copies of chromosome 21. Children with mosaic Down syndrome may have the same features as other children with Down syndrome; however, they may have fewer characteristics of the condition due to the presence of some or many cells with a typical number of chromosomes.1,3

Risk Factors

Researchers know that Down syndrome is caused by an extra chromosome, but no one knows for sure why Down syndrome occurs or how many different factors play a role.1

One factor that increases the risk for having a baby with Down syndrome is the mother’s age. Women aged 35 years or older when they become pregnant are more likely to have a pregnancy affected by Down syndrome than women who become pregnant at a younger age.4,5 However, the majority of babies with Down syndrome are born to mothers aged younger than 35 years because there are many more births among younger women.6,7


There are two basic types of tests available to detect Down syndrome during pregnancy: screening tests and diagnostic tests.1,7,8

Screening Tests

Screening tests often include a combination of a blood test, which measures the number of various substances in the mother’s blood (e.g., maternal serum alpha-fetoprotein screening), and an ultrasound, which creates an image of the baby. During an ultrasound, one of the things the clinician looks at is the fluid behind the fetus’ neck. Extra fluid in this region could indicate a genetic problem and can help determine the fetus’ risk of Down syndrome. Sometimes the test results are normal, and the clinicians can miss a problem that exists.

Diagnostic Tests

Diagnostic tests are usually performed after a positive screening test to confirm a Down syndrome diagnosis. Types of diagnostic tests include:
Chorionic villus sampling: tests material from the placenta
Amniocentesis: examines the amniotic fluid
Percutaneous umbilical blood sampling: these tests look for changes in the chromosomes that would indicate a Down syndrome diagnosis.

Other Medical Problems

Many people with Down syndrome have common facial features and no other major birth defects. However, some people with Down syndrome might have one or more major birth defects or other medical problems. Some of the more common health problems among children with Down syndrome are hearing loss, obstructive sleep apnea, ear infections, eye diseases, and heart defects present at birth. Healthcare providers routinely monitor children with Down syndrome for these conditions.8,9


Down syndrome is a lifetime condition, and there is no single, standard treatment for it. There is also no cure for Down syndrome. Medical services early in life will often help babies and children with Down syndrome to improve their physical and intellectual abilities. These services focus on helping children with Down syndrome develop to their full potential. The services include speech, occupational, and physical therapy, and these are typically offered through early intervention programs in each state. Children with Down syndrome may also need extra help or attention in school, although many children are included in regular classes.1,9


1. Antonarakis SE, Skotko BG, Rafii MS, et al. Down syndrome. Nat Rev Dis Primers. 2020;6(1):9.
2. Mai CT, Isenburg JL, Canfield MA, et al. National population-based estimates for major birth defects, 2010–2014. Birth Defects Res. 2019;111(18):1420-1435.
3. Shin M, Siffel C, Correa A. Survival of children with mosaic Down syndrome. Am J Med Genet A. 2010;152A:800-801.
4. Ghosh S, Feingold E, Dey SK. Etiology of Down syndrome: evidence for consistent association among altered meiotic recombination, nondisjunction, and maternal age across populations. Am J Med Genet A. 2009;149(7):1415-1420.
5. Allen EG, Freeman SB, Druschel C, et al. Maternal age and risk for trisomy 21 assessed by the origin of chromosome nondisjunction: a report from the Atlanta and National Down Syndrome Projects. Hum Genet. 2009;125(1):41-52.
6. Sherman SL, Allen EG, Bean LH, et al. Epidemiology of Down syndrome. Ment Retard Dev Disabil Res Rev. 2007;13(3):221-227.
7. Adams MM, Erickson JD, Layde PM, et al. Down’s syndrome: recent trends in the United States. JAMA. 1981;246(7):758-760.
8. Bull MJ; the Committee on Genetics. Health supervision for children with Down syndrome. Pediatrics. 2011;128:393-406.
9. MedlinePlus. Down syndrome. National Library of Medicine. June 1, 2020. Accessed July 2023.

The content contained in this article is for informational purposes only. The content is not intended to be a substitute for professional advice. Reliance on any information provided in this article is solely at your own risk.

To comment on this article, contact