Introduction

Down syndrome is a condition in which a person has an extra chromosome. Chromosomes are small “packages” of genes in the body. They determine how a baby’s body forms and functions as it grows during pregnancy and after birth. Typically, a baby is born with 46 chromosomes. Babies with Down syndrome have an extra copy of one of these chromosomes, chromosome 21. A medical term for having an extra copy of a chromosome is ‘trisomy.’ Down syndrome is also referred to as Trisomy 21. This extra copy changes how the baby’s body and brain develop, which can cause both mental and physical challenges for the baby.

Even though people with Down syndrome might act and look similar, each person has different abilities. People with Down syndrome usually have an IQ (a measure of intelligence) in the mildly-to-moderately low range and are slower to speak than other children.

Some common physical features of Down syndrome include:

A flattened face, especially the bridge of the nose
Almond-shaped eyes that slant up
A short neck
Small ears
A tongue that tends to stick out of the mouth
Tiny white spots on the iris (colored part) of the eye
Small hands and feet
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

How Many Babies are Born with Down Syndrome?

Down syndrome remains the most common chromosomal condition diagnosed in the United States. Each year, about 6,000 babies born in the United States have Down syndrome. This means that Down syndrome occurs in about 1 in every 700 babies.¹

Types of Down Syndrome

There are three types of Down syndrome. People often can’t tell the difference between each type without looking at the chromosomes because the physical features and behaviors are similar.

Trisomy 21: About 95% of people with Down syndrome have Trisomy 21.² With this type of Down syndrome, each cell in the body has 3 separate copies of chromosome 21 instead of the usual 2 copies.
Translocation Down syndrome: This type accounts for a small percentage of people with Down syndrome (about 3%).² This occurs when an extra part or a whole extra chromosome 21 is present, but it is attached or “trans-located” to a different chromosome rather than being a separate chromosome 21.
Mosaic Down syndrome: This type affects about 2% of the people with Down syndrome.² Mosaic means mixture or combination. For children with mosaic Down syndrome, some of their cells have 3 copies of chromosome 21, but other cells 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 features of the condition due to the presence of some (or many) cells with a typical number of chromosomes.

Causes and Risk Factors

The extra chromosome 21 leads to the physical features and developmental challenges that can occur among people with Down syndrome. 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.
One factor that increases the risk for having a baby with Down syndrome is the mother’s age. Women who are 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.^3-5However, the majority of babies with Down syndrome are born to mothers less than 35 years old, because there are many more births among younger women.^6,7

Diagnosis

There are two basic types of tests available to detect Down syndrome during pregnancy: screening tests and diagnostic tests. A screening test can tell a woman and her healthcare provider whether her pregnancy has a lower or higher chance of having Down syndrome. Screening tests do not provide an absolute diagnosis, but they are safer for the mother and the developing baby. Diagnostic tests can typically detect whether or not a baby will have Down syndrome, but they can be more risky for the mother and developing baby. Neither screening nor diagnostic tests can predict the full impact of Down syndrome on a baby; no one can predict this.

Screening Tests

Screening tests often include a combination of a blood test, which measures the amount of various substances in the mother’s blood (e.g., MS-AFP, Triple Screen, Quad-screen), and an ultrasound, which creates a picture of the baby. During an ultrasound, one of the things the technician looks at is the fluid behind the baby’s neck. Extra fluid in this region could indicate a genetic problem. These screening tests can help determine the baby’s risk of Down syndrome. Rarely, screening tests can give an abnormal result even when there is nothing wrong with the baby. Sometimes, the test results are normal and yet they miss a problem that does exist.

Diagnostic Tests

Diagnostic tests are usually performed after a positive screening test in order to confirm a Down syndrome diagnosis. Types of diagnostic tests include:

Chorionic villus sampling (CVS)—examines material from the placenta
Amniocentesis—examines the amniotic fluid (the fluid from the sac surrounding the baby)
Percutaneous umbilical blood sampling (PUBS)—examines blood from the umbilical cord

These tests look for changes in the chromosomes that would indicate a Down syndrome diagnosis.

Other Health Problems

Many people with Down syndrome have the 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 listed below.⁸

Hearing loss
Obstructive sleep apnea, which is a condition where the person’s breathing temporarily stops while asleep
Ear infections
Eye diseases
Heart defects present at birth

Health care providers routinely monitor children with Down syndrome for these conditions.

How do health care providers diagnose Down syndrome?

Health care providers can check for Down syndrome during pregnancy or after a child is born. There are two types of tests for Down syndrome during pregnancy:

A prenatal screening test. This test can show an increased likelihood that a fetus has Down syndrome, but it cannot determine Down syndrome is definitely present. If a screening test shows an increased likelihood, a diagnostic test can be ordered.
A prenatal diagnostic test. This test can determine with certainty that Down syndrome is present. Diagnostic tests carry a slightly greater risk to the fetus than do screening tests.

The American Congress of Obstetricians and Gynecologists (ACOG) recommends that all pregnant women be offered a Down syndrome screening test.¹

Prenatal Screening for Down Syndrome

There are several options for Down syndrome prenatal screening. These include:

A blood test and an ultrasound test during the first trimester of pregnancy. This is the most accepted approach for screening during the first trimester. A blood test enables a health care provider to check for “markers,” such as certain proteins, in the mother’s blood that suggest an increased likelihood of Down syndrome.² Then the health care provider does an ultrasound test, which uses high-frequency sound waves to create images. An ultrasound can detect fluid at the back of a fetus’s neck, which sometimes indicates Down syndrome. The ultrasound test is called measurement of nuchal translucency. During the first trimester, this combined method results in more effective or comparable detection rates than methods used during the second trimester.³
A blood test during the second trimester of pregnancy. As in the first trimester, a blood test enables a health care provider to check for markers in the mother’s blood. A triple screen looks for levels of three different markers; a quadruple screen looks for levels of four different markers.³^,⁴
A combined test (sometimes called an integrated test). This approach uses both a blood test and an ultrasound during the first trimester as well as a second-trimester blood test. Health care providers then combine all these results to produce one Down syndrome risk rating.²

If a woman is pregnant with twins or triplets, a blood test will not be as reliable because the substances from a Down syndrome fetus may be harder to detect.²^,³

Prenatal Diagnostic Testing for Down Syndrome

If a screening test suggests the likelihood of Down syndrome, a diagnostic test can be performed. ACOG recommends that pregnant women of all ages be given the option of skipping the screening test and getting a diagnostic test first. Until recently, only women over age 35 and other at-risk women were offered this option because diagnostic tests carry a slight risk of miscarriage.¹ Before having diagnostic testing, a pregnant woman and her family may want to meet with a genetic counselor to discuss their family history and the risks and benefits of testing in their specific situation.

Diagnostic testing for Down syndrome involves removing a sample of genetic material. After it is removed, the sample is checked for extra material from chromosome 21, which may indicate that a fetus has Down syndrome. Parents usually get the results of the test a week or two later. The following procedures are used to extract samples.

Amniocentesis (pronounced am-nee-oh-sen-TEE-sis). A health care provider takes a sample of amniotic (pronounced am-nee-OT-ik) fluid, which is then tested for the extra chromosome. This test cannot be done until week 14 to 18 of the pregnancy.
Chorionic villus (pronounced KOHR-ee-on-ik VIL-uhs) sampling (CVS). A health care provider takes a sample of cells from a part of the placenta (pronounced pluh-SEN-tuh), which is the organ that connects a woman and her fetus, and then tests the sample for the extra chromosome. This test is done between weeks 9 and 11 of pregnancy.
Percutaneous (pronounced pur-kyoo-TEY-nee-uhs) umbilical blood sampling (PUBS). A health care provider takes a sample of fetal blood in the umbilical cord through the uterus. The blood is then tested for the extra chromosome. PUBS is the most accurate diagnostic method and can confirm the results of CVS or amniocentesis. However, PUBS cannot be performed until later in the pregnancy, during the 18th to 22nd week.⁵

Prenatal diagnostic testing does involve some risk to the mother and fetus, including a slight risk of miscarriage.⁶^,⁷^,⁸^,⁹ If you and your family are considering prenatal diagnostic testing for Down syndrome, discuss all the risks and benefits with your health care provider.

Chromosomal Testing of Maternal Blood

A pregnant woman who is at risk for having an infant with Down syndrome also can have a chromosomal test using her blood. A mother’s blood carries DNA from the fetus, which may show extra chromosome 21 material.³ A more invasive test then would usually confirm the blood test.

Testing and In Vitro Fertilization

Another approach to diagnosis is used in conjunction with in vitro fertilization. Preimplantation genetic diagnosis (PGD) allows clinicians to detect chromosome imbalances or other genetic conditions in a fertilized egg before it is implanted into the uterus.

This technique is useful mostly for couples who are at risk of passing on a variety of genetic conditions, including X-linked disorders, as well as couples who have suffered repeated spontaneous pregnancy losses, sub-fertile couples, or those at risk for single-gene disorders.

Those interested in PGD should have genetic counseling and should consider close monitoring and additional testing during their pregnancies, given some increased risk of chromosomal abnormalities arising secondary to the in vitro fertilization process.¹⁰

Diagnosis of Down Syndrome After Birth

A diagnosis of Down syndrome after birth is often based initially on physical signs of the syndrome.

But because individuals with Down syndrome may not have these symptoms, and because many of these symptoms are common in the general population, the health care provider will take a sample of the baby’s blood to confirm the diagnosis. The blood sample is analyzed to determine the number of the baby’s chromosomes.¹¹

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