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Also known as Edwards syndrome, it is a condition caused by an error in cell division, known as meiotic disjunction.
When this happens, instead of the normal pair, an extra chromosome 18 (a triple) is produced in the developing baby and the normal pattern of development is disrupted in significant ways that can be life-threatening, even before birth.
Unlike Down syndrome, which is also caused by an extra chromosome, the developmental problems caused by trissomy 18 are associated with more medical complications that are more life threatening in the first few months and years of life.
Studies have shown that only 50% of babies who are carried to term will be born alive, and babies will have higher live birth rates than babies.
At birth, intensive care in neonatal intensive care units is routine for babies with trissomy 18.
Again, boys will experience higher mortality rates in this neonatal period than girls, although those with higher birth weights perform better in all categories.
Some babies will be able to survive to be discharged from the hospital with home nursing support to help with parental care.
And although 10% or more can survive to their first birthday, there are children with Trissomy 18 who can enjoy many years of life with their families, achieving important achievements, and participating in their community.
A small number of adults (usually girls) with trissomy 18 are and are in their twenties and thirties, albeit with significant developmental delays that prevent them from living independently without full-time care.
Causes of trissomy 18
A trissomy is caused by a genetic error in which three copies of a chromosome (instead of two) are inherited from the parents.
Trissomy 18 is caused by the presence of an extra chromosome 18, and is most often maternal in origin and includes the entire chromosome in most cases (> 90%) rather than part of the chromosome.
In a small percentage of patients, an extra chromosome 18 is present in some, but not all cells, and is known as mosaicism ; these affected patients may not have the typical features of trissomy 18 detailed below.
Mosaic trissomy 18
Mosaic trissomy 18 can be a less serious form of Edwards syndrome, as only some of the cells have the extra copy of chromosome 18, rather than every single cell.
The severity of the baby’s involvement depends on the number and type of cells that have the extra chromosome. Some babies may only be mildly affected, while others may be severely disabled.
About seven out of 10 babies born with mosaic trissomy will live for at least a year and, in rare cases, can survive into early adulthood.
Partial trissomy 18
In partial trissomy 18, only an extra section of chromosome 18 is present in the cells, rather than an additional complete chromosome 18.
This type of Edwards syndrome is more likely if one of the parents has a minor alteration in their chromosomes, so blood samples are often requested from both parents to verify this and to help them understand the risks of future pregnancies.
The severity of the baby’s involvement will depend on which part of chromosome 18 is present in the cells.
Each cell in your body normally contains 23 pairs of chromosomes, which carry the genes you inherit from your parents. But a baby with Edwards syndrome has three copies of chromosome number 18, instead of two.
The presence of this extra chromosome in cells seriously affects normal development.
Edwards syndrome is rarely inherited and is not caused by anything the parents have done. The development of three copies of chromosome 18 usually occurs randomly during the formation of the egg or sperm.
Since this happens randomly, it is extremely unlikely that parents will have more than one pregnancy affected by Edwards syndrome. However, the chance of having a baby with Edwards syndrome increases as the mother grows.
Symptoms of trissomy 18
Today, most fetuses with trissomy 18 are diagnosed before birth, and research indicates that many of these fetuses do not survive to birth.
Babies with Edwards syndrome can have a wide range of different problems. For those babies who survive, they are at risk for any number of abnormalities, but typical features include:
- Intrauterine growth retardation.
- Craniofacial features such as abnormalities of the jaw, skull, ears, and neck.
- Closed fists with primordial fingers.
- Small nails.
- Short sternum.
- Heart defects
- Kidney defects.
- Neurodevelopmental delays.
- Low birth weight.
- A small head, abnormally shaped.
- A small jaw and mouth long overlapping fingers, with underdeveloped thumbs and clenched fists.
- Low ears.
- Smooth feet with rounded soles.
- A cleft lip and palate an exomphalos (where the intestines are held in a sac outside the abdomen).
Babies with Edwards syndrome or also called trissomia 18 also usually have:
- Heart and kidney problems, eating problems that lead to poor growth.
- Respiratory problems.
- Hernias in the stomach wall (where the internal tissues go through a weakness in the muscle wall).
- Bone abnormalities, such as a curved spine .
- Frequent infections of the lungs and urinary system.
- A severe learning disability.
What is the life expectancy for someone with Trissomy 18?
The average life expectancy of babies born with trissomy 18 is 3 days to 2 weeks.
Studies show that 60% to 75% of children survive for 24 hours, 20% to 60% for 1 week, 22% to 44% for 1 month, 9% to 18% for 6 months, and 5 at 10% for more than 1 year.
Therefore, there are quite a few affected children who require significant care and ongoing screening throughout their lives. Specialists in pediatric development, neurology, and genetics often help guide the ongoing care of these children.
Diagnosis of trissomy 18
During pregnancy
Pregnant women are offered an Edwards syndrome test between 10 and 14 weeks of pregnancy to assess the chances of their baby having the condition.
This screening test is known as the combined test and it also tests for Down syndrome and Patau syndrome.
During the combined test, a blood test and a special ultrasound will be done where the fluid on the back of the baby’s neck (nuchal translucency) will be measured.
At 10-14 weeks if the combined test shows that you are at increased risk of having a baby with Edwards syndrome, you will be offered a diagnostic test to determine for sure if your baby has the condition.
This involves testing a sample of your baby’s cells to see if they have an extra copy of chromosome 18.
There are two different ways to obtain this cell sample:
- Chorionic villus sampling, which collects a sample from the placenta.
- Amniocentesis, which collects a sample of the amniotic fluid that surrounds the baby.
These are invasive tests that have the risk of causing a miscarriage. Your doctor will discuss these risks with you.
Later in the pregnancy, usually when you are between 18 and 21 weeks pregnant, you will also be offered an exam that looks for physical abnormalities, known as congenital anomalies.
Make the decision
If your baby is diagnosed with Edwards syndrome during your pregnancy, your doctor will talk with you about how you want to move forward. They will discuss the options of continuing the pregnancy or ending it with an interruption, as it is such a serious condition.
This is a very difficult situation and it is normal to feel a whole range of emotions. It can help to talk with your doctor, partner, family, and friends about what you are thinking and how you feel.
If your baby is diagnosed with Edwards syndrome, either before or after birth, counseling and support will be offered.
Treatment of trissomy 18
There is no cure for Edwards syndrome, and the symptoms can be very difficult to manage.
Treatment will focus on immediately life-threatening problems, such as infections and heart problems.
If limb abnormalities affect your child’s movements as they grow, they may benefit from supportive treatment, such as physical therapy and occupational therapy.
Depending on your child’s specific problems, they may need specialized care in the hospital or hospice, or you can care for them at home with the right support.
Care tips
Caring for a child with Edwards syndrome is mentally and physically challenging. Most caregivers will need social and psychological support.
Your guide to care and support provides many tips on how you can take the time to take care of yourself, including:
- Stay fit and healthy.
- Get a break from caring.
- Get legal support and defense.
- Taking care of your well-being.
