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 during pregnancy and how the baby’s body functions as it grows in the
womb 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.
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.
Ø 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.
Ø 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.
Ø 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
Ø 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.
Ø 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.
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:
Ø
Chorionic villus
sampling (CVS)—examines material from the placenta
Ø
Amniocentesis—examines
the amniotic fluid (the fluid from the sac surrounding the baby)
Ø
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
Ø Hearing
loss
Ø Obstructive sleep
apnea, which is a condition where the person’s
breathing temporarily stops while asleep
Ø Ear
infections
Ø Eye
diseases
Ø Heart
defects present at birth
Health
care providers routinely monitor children with Down syndrome for these
conditions.