What Is Trisomy 18?

Medically Reviewed by Zilpah Sheikh, MD on February 28, 2024
Written by Stephanie Watson, Kathryn Whitbourne
6 min read

Trisomy 18 is a condition caused by a problem in your chromosomes. It's also called Edwards syndrome, after the doctor who first described it.

Chromosomes are the threadlike structures in cells that hold genes. Genes carry the instructions needed to make every part of a baby's body.

When an egg and sperm join and form an embryo, their chromosomes combine. Each baby gets 23 chromosomes from the mother's egg and 23 chromosomes from the father's sperm – 46 in total.

Sometimes the mother's egg or the father's sperm contains the wrong number of chromosomes. As the egg and sperm combine, this mistake is passed on to the baby.

A "trisomy" means that the baby has an extra chromosome in some or all of the body's cells. In the case of trisomy 18, the baby has three copies of chromosome 18. This causes many of the baby's organs to develop in an abnormal way.

Types of trisomy 18

There are three types of trisomy 18:

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

Trisomy 18 is the second-most common type of trisomy syndrome, after trisomy 21 (Down syndrome). About 1 in every 5,000 babies are born with trisomy 18, and most are female.

The condition is even more common than that, but many babies with trisomy 18 don't survive past the second or third trimester of pregnancy.

Babies with trisomy 18 are often born very small and frail. They typically 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 and mental disabilities
  • Chest deformity
  • Slowed growth
  • Small head (microcephaly)
  • Small jaw (micrognathia)
  • Weak cry

The risk of having a child with trisomy 18 increases with the age of the mother, though women of any age can have a child with trisomy 18. There isn't anything you can do to lessen your chances of having a child with the condition as it's due to a chromosome disorder.

If you've had one baby with trisomy 18, the risk of having another with it is 0.5% to 1%. If you or your partner is a carrier of a chromosome disorder that leads to partial trisomy 18, the risk of having another child with this condition could be as high as 20%.

 

The doctor usually will do a screening test using some of the mother's blood. The test can't say for sure if your baby will have trisomy 18, but it can tell if your baby is at risk for it – and for other diseases that come from chromosome errors. 

To diagnose trisomy 18, your doctor will take cells from the amniotic fluid (amniocentesis) or placenta (chorionic villus sampling) and analyze their chromosomes. These tests are done after the screening test because they carry a small risk of miscarriage or early labor.

Chorionic villus sampling (CVS) is done in the first trimester, and amniocentesis is done in the second or third trimester.

Trisomy 18 ultrasound

Around 12 weeks of pregnancy or later, your doctor can do an ultrasound, which will give them a picture of what your fetus looks like. It may tell them if your baby has trisomy 18, but it's usually confirmed by the CVS or amniocentesis. 

Some of the things a doctor may see on an ultrasound if your baby has trisomy 18 include a smaller size, problems with the intestines, and problems with the limbs, like clenched fists.

After birth, the doctor may suspect trisomy 18 based on your child's face and body. A blood sample can be taken to look for the chromosome disorder. 

The chromosome blood test can also help determine how likely you are to have another baby with trisomy 18. If you're concerned that your baby may be at risk for trisomy 18 because of a past pregnancy, you may want to see a genetic counselor.

There is no cure for trisomy 18. Treatment for trisomy 18 consists of supportive medical care to provide the child with the best quality of life possible. This might include:

  • Surgeries, especially for heart defects
  • Medicines
  • Feeding tubes
  • Breathing tubes
  • Providing comfort care (as opposed to treatments)

At one time, babies born with trisomy 18 were not resuscitated at birth because their long-term chances of survival were so low. Now, guidelines have changed, and reviving infants with trisomy 18 is more likely to be done. The Textbook of Neonatal Resuscitation has removed trisomy 18 from the list of diagnoses that it considers ethical to not resuscitate at birth.

Because trisomy 18 causes such serious physical defects, many babies with the condition don't survive to birth. About half of babies who are carried full-term are stillborn. Boys with trisomy 18 are more likely to be stillborn than girls.

Of those babies who do survive, half die within the first week of life. Fewer than 10% live to reach their first birthday. Children who do live past that milestone often have severe health problems that require a large amount of care. Only a very small number of people with this condition live into their 20s or 30s.

Having a child with trisomy 18 can sometimes be emotionally overwhelming, and it's important for parents to get support during this difficult time. Organizations such as the Chromosome 18 Registry & Research Society and the Trisomy 18 Foundation can help.

Trisomy 13 is another chromosomal condition. It's sometimes called Patau syndrome, after the doctor who first described it. 

In this condition, the person has three copies of chromosome 13, instead of two. Most of the time, all the cells in the body have three copies of chromosome 13, but as with trisomy 18, there are rarer types where an extra chromosome 13 gets attached to another chromosome (translocation) or only some of the body cells have the extra copy of chromosome 13 (mosaic trisomy 13).

Babies with trisomy 13 generally have severe medical issues and birth defects, including:

  • Heart defects
  • Spinal cord and brain disorders
  • Low-set ears
  • Cleft lip and/or cleft palate
  • Very small or poorly developed eyes (microphthalmia)
  • Extra fingers and toes
  • Weak muscle tone (hypotonia)
  • Some organs in the belly bulging through an opening near the umbilical cord 
  • Serious mental retardation

Trisomy 13 affects 1 in 16,000 babies, though most fetuses with this condition don't survive to the third trimester of pregnancy. Most babies born with this condition die within the first few days or weeks of life, as they have so many medical complications. Just 5% to 10% make it past their first year.

Like trisomy 18, no one knows why some babies get this condition. It's known that the chance increases with the mother's age, though women of any age can have a child with trisomy 13. About 80% of babies with trisomy 18 or 13 are born to mothers under 35. The condition can be diagnosed before birth with the same tests used to identify trisomy 18, or after birth by a physical examination.

 

Trisomy 18 is a condition where you have three copies of each chromosome 18 in your body's cells instead of two. This can lead to serious physical and mental disabilities. There is no cure, though treatment can include surgeries, medicines, breathing tubes, and feeding tubes. Some parents opt just for comfort care. Life expectancy is usually a year or less.

How old is the oldest living person with trisomy 18?

The oldest people were reported to be in their early 40s a few years ago. But it's unclear if they are alive today.

Are babies with trisomy 18 less active in the womb?

Yes, they are often less active.