What is Trisomy 18?

You may have heard about trisomy 18 in the media, or your doctor may have told you that your unborn child has the condition.

Here are answers to common questions about trisomy 18, including its causes, diagnosis, and effects on children.

Trisomy 18 is a chromosomal abnormality. It is also called Edwards syndrome, after the doctor who first described it.

Chromosomes are thread-like structures in cells that contain genes. Genes carry the instructions needed to create each part of a child's body.

When an egg and a sperm combine to form an embryo, their chromosomes combine. Each child receives 23 chromosomes from the mother's egg and 23 from the father's sperm, for a total of 46.

Sometimes the mother's egg or father's sperm contain the wrong number of chromosomes. When the egg and sperm come together, this error is passed on to the baby.

"Trisomy" means that the child has an extra chromosome in some or all of the body's cells. In the case of trisomy 18, the child has three copies of chromosome 18. This leads to the abnormal development of many of the child's organs.

There are three types of trisomy 18:

Full trisomy 18. An extra chromosome is found in every cell of the child's body. This is by far the most common type of trisomy 18.
Partial trisomy 18. The baby only has part of an extra chromosome 18. This extra part can be attached to another chromosome in the egg or sperm (this is called a translocation). This type of trisomy 18 is very rare.
Mosaic trisomy 18. The extra chromosome 18 is present only in certain cells of the baby. This form of trisomy 18 is also rare.

How many children have trisomy 18?

Trisomy 18 is the second most common type of trisomy syndrome after trisomy 21 (Down's syndrome). Approximately 1 in 5,000 babies are born with trisomy 18, and most of them are women.

The condition is even more common, but many children with trisomy 18 do not survive the second or third trimester of pregnancy.

What are the symptoms of trisomy 18?

Babies with trisomy 18 are often born very small and fragile. They usually have many serious health problems and physical defects, including:

Cleft palate
Clenched fists with overlapping fingers that are hard to straighten
Defects of the lungs, kidneys, and stomach/intestines
Deformed feet (called "rocker-bottom feet" because they're shaped like the bottom of a rocking chair)
Feeding problems
Heart defects, including a hole between the heart's upper (atrial septal defect) or lower (ventricular septal defect) chambers
Low-set ears
Severe developmental delays
Chest deformity
Slowed growth
Small head (microcephaly)
Small jaw (micrognathia)
Weak cry

How is trisomy 18 diagnosed?

A doctor may suspect trisomy 18 during a pregnancy ultrasound, although this is not an accurate way to diagnose the condition. More precise methods remove cells from the amniotic fluid (amniocentesis) or placenta (chorionic villus sampling) and analyze their chromosomes.

After birth, a doctor may suspect trisomy 18 by looking at the baby's face and body. A blood sample may be taken to look for a chromosomal abnormality. A chromosomal blood test can also help determine the likelihood that a mother will have another child with trisomy 18.

If you are concerned that your child is at risk for trisomy 18 due to a previous pregnancy, you may want to see a genetic counselor.

Is there a cure for trisomy 18?

Trisomy 18 is incurable. Trisomy 18 treatment consists of supportive medical care to ensure the child has the best quality of life.

What is the outlook for children with trisomy 18?

Because trisomy 18 causes such severe physical defects, many children with the condition do not live to see birth. About half of full-term babies are stillborn. Boys with trisomy 18 are more likely to be stillborn than girls.

Of surviving infants, less than 10% survive to their first birthday. Children who survive to this stage often have serious health problems that require special care. Only a very small number of people with this disease survive into their twenties or thirties.

Having a baby with Down syndrome can sometimes be emotionally overwhelming, and it's important for parents to get support during this difficult time.