Down Syndrome (Trisomy 21)
What are trisomies?
Most people have 23 pairs of chromosomes in their cells. Trisomy means that a person
has 3 of a certain chromosome pair. For example, if a baby is born with 3 No. 21 chromosomes,
rather than the usual pair, the child would have trisomy 21. Trisomy 21 is also known
as Down syndrome.
Other examples of trisomy include trisomy 18 and trisomy 13. Trisomy 18 means the
child has 3 copies of the No. 18 chromosome. Trisomy 13 means the child has 3 copies
of the No. 13 chromosome.
What is Down syndrome?
Down syndrome is a genetic disorder. It includes certain birth defects, learning problems,
and certain facial features. A child with Down syndrome also may have heart defects
and visual and hearing problems. The seriousness of these problems is different from
child to child.
Down syndrome is one of the most common genetic birth defects. It affects about 1
in 800 babies. Adults with Down syndrome may live about 60 years, but this lifespan
What causes Down syndrome?
When a baby is conceived, the egg cell of the mother and the sperm cell of the father
start out with the usual number of 46 chromosomes. The egg and sperm cells both divide,
so that the chromosome number is half of 46. The egg and the sperm cells then have
23 chromosomes each. When a sperm with 23 chromosomes fertilizes an egg with 23 chromosomes,
the baby will have a complete set of 46 chromosomes. Half are from the father and
half are from the mother.
Sometimes an error occurs when the 46 chromosomes are being divided in half. An egg
or sperm cell keeps both copies of the No. 21 chromosome instead of just one copy.
If this egg or sperm is fertilized, then the baby will have 3 copies of the No. 21
chromosome. This is trisomy 21, or Down syndrome. The features of Down syndrome are
caused by that extra copy of chromosome No. 21 being in every cell in the body.
Most cases of Down syndrome are caused by trisomy 21. Sometimes the extra No. 21 chromosome
or part of it is attached to another chromosome in the egg or sperm. This may cause
"translocation Down syndrome." This is the only form of Down syndrome that may be inherited
from a parent. Some parents have a rearrangement called a balanced translocation,
in which the No. 21 chromosome is attached to another chromosome, but it does not
affect their own health.
Another form called "mosaic Down syndrome" may happen when an error in cell division
happens after the egg is fertilized. This is rare. People with this syndrome have
some cells with an extra chromosome No. 21, and other cells have the usual number.
What does a child with Down syndrome look like?
A child with Down syndrome may have eyes that slant upward and small ears that may
fold over slightly at the top. The child's mouth may be small. This makes the tongue
appear large. The child's nose also may be small, with a flattened nasal bridge. Some
babies with Down syndrome have a short neck and small hands with short fingers. Instead
of having 3 "creases" in the palm of the hand, a child with Down syndrome usually
has one crease that goes straight across the palm. A second crease curves down by
the thumb. The child or adult with Down syndrome is often short and has unusual looseness
of the joints. Most children with Down syndrome will have some but not all of these
What types of health problems do children with Down syndrome typically have?
About half of babies with Down syndrome have heart defects. Some defects are minor
and can be treated with medicines. Others may need surgery. All babies with Down syndrome
should be looked at by a pediatric cardiologist. This is a doctor who specializes
in heart diseases of children. Babies with Down syndrome should also have an echocardiogram.
This is a test that looks at the structure and function of the heart by using sound
waves. This exam and test should be done in the first 2 months of life. This is so
that any heart defects can be treated.
Some babies with Down syndrome are born with intestinal malformations that need surgery.
Children with Down syndrome are likely to have eyesight problems. Common problems
include crossed eyes, near- or farsightedness, and cataracts. Most eyesight problems
can be made better eyeglasses, surgery, or other treatments. Your child should see
an eye doctor (pediatric ophthalmologist) before he or she turns 1 year old.
Children with Down syndrome may have hearing loss. This is from fluid in the middle
ear, a nerve defect, or both. Your child should get regular hearing tests so any problems
can be treated early. This will help with language development.
Other health problems
Children with Down syndrome may have thyroid problems and leukemia. They also tend
to have many colds, as well as bronchitis and pneumonia. Your child should get regular
medical care and stay up to date on vaccines.
How serious are the learning problems that happen with Down syndrome?
The learning problems and development problems that happen with Down syndrome vary
widely. They can be mild, moderate, or severe. But most learning problems are mild
to moderate. Doctors can’t predict how serious the learning problems will be based
on a child’s physical features.
What learning and development problems does a child with Down syndrome have?
Children with Down syndrome can usually do most things that any young child can do.
They can walk, talk, dress themselves, and be toilet trained. But they usually do
these things at a later age than other children. The exact age that these development
milestones will be reached is different for each child. Early intervention programs
that begin when a child is an infant can help the child reach his or her potential.
Can a child with Down syndrome go to school?
Yes. Special programs beginning in the preschool years help children with Down syndrome
develop skills as fully as possible. Many children are helped with early intervention
and special education. They can also enter a regular classroom. Many children will
learn to read and write. They can take part in childhood activities, both at school
and in their neighborhoods.
Special work programs are designed for adults with Down syndrome, and many can hold
regular jobs. More and more adults with Down syndrome live semi-independently in community
group homes. They take care of themselves, do household chores, develop friendships,
do leisure activities, and work in their communities.
Can people with Down syndrome marry?
Some people with Down syndrome marry. Most men with Down syndrome cannot father a
child. In any pregnancy, a woman with Down syndrome has a 50/50 chance of conceiving
a child with Down syndrome. Many of the pregnancies are miscarried.
How is Down syndrome diagnosed?
Doctors can sometimes diagnose a baby with Down syndrome with just a physical exam.
To confirm the physical findings, the doctor can take a blood sample. This is looked
at in the lab to find the extra No. 21 chromosome. This information is important to
figure out the risk in future pregnancies.
Chromosomal problems such as Down syndrome can often be diagnosed before birth. This
is done by looking at cells in the amniotic fluid or from the placenta. This can also
be done by looking at the amount of fetal DNA in the mother's blood (noninvasive prenatal
Fetal ultrasound during pregnancy can also show the possibility of Down syndrome.
But ultrasound is not 100% accurate. Many babies with Down syndrome may look the same
on ultrasound as those without Down syndrome.
The chromosome analysis done on a blood sample or on cells from the amniotic fluid
or placenta is very accurate.
At what age does the risk of having a child with Down syndrome increase?
Your age at delivery is the only factor linked to the risk of having a baby with Down
syndrome. This risk goes up every year, especially after you are 35. But because younger
women are more likely to have babies than older women, most babies with Down syndrome
are born to women younger than 35.
The American College of Obstetricians and Gynecologists recommends that all pregnant
women be offered screening for Down syndrome. This is true no matter what your age.
What is the risk of having a second child with Down syndrome?
For women who have had one child with Down syndrome, the chance of having another
baby with Down syndrome depends on several things. Age is one factor. It’s important
to know that most babies with Down syndrome are born to women under 35. This is because women
under 35 have more babies than women over 35.
Your healthcare provider may refer you to a geneticist or genetic counselor. This
expert can explain the results of chromosome tests in detail. He or she can talk about
risks for future pregnancies and what tests are available to diagnose chromosome problems
before a baby is born.
Can Down syndrome be cured or prevented?
There is no cure for Down syndrome. Doctors are not certain how to prevent the chromosome
error that causes Down syndrome. To date, there is no reason to believe that a parent
could have done anything to cause or prevent the birth of a baby with Down syndrome.
Some people claim that giving high-dose vitamins to children with Down syndrome will
improve their learning and development problems. But no studies have proved that this
Your child may need physical, occupational, and speech therapy to help with his or
her development. Talk with your healthcare provider, other families, and national
Down syndrome support agencies to learn what to expect with Down syndrome. You can
also learn what may be helpful in raising a child with Down syndrome.