What is Trisomy 18?
What is Trisomy 18? What causes it?
At conception, 23 chromosomes from the father and 23 chromosomes from the mother combine to create a baby with 46 chromosomes in each cell, two of each type. Each chromosome holds a particular "position" in the cell and carries certain genetic material. A trisomy occurs when a baby has three chromosomes in one position instead of the normal two.
The most common trisomy is Trisomy 21, also known as Down Syndrome, where a baby has three of the twenty-first chromosome. Trisomy 18 is the second most common trisomy and occurs when a baby has three of the eighteenth chromosome. This results in 47 chromosomes instead of the normal 46 in the affected cells. It is this extra genetic material that causes the problems associated with Trisomy 18.
Trisomy 18 is also called Edwards syndrome (or Edward's syndrome) and occurs in about 1 in 2500 pregnancies, resulting in 1 in 6000 live births. Many more births occur when stillbirths that occur in the 2nd and 3rd trimester are factored in that statistic. Unlike Down syndrome, Trisomy 18 is usually associated with a shorten lifespan, with most of the babies dying before birth and those who do make it to birth typically living less than a year. However, a small number of babies (<10%) live more than 1 year, with some surviving into their teens and beyond.
Most trisomies (about 95%) are full trisomies. That is, the extra chromosome occurs in every cell in the baby's body. This type of trisomy is not hereditary, and is not due to anything the parents did or did not do, and it is by far the most common type. In most literature, Trisomy 18 means full Trisomy 18.
Another type of trisomy is Trisomy 13 (Patau's Syndrome), which occurs less frequently than Trisomy 18. Although full trisomies may occur at any position, the extra genetic material usually causes such problems that the baby cannot make it to term. As a result, these three trisomies: 21, 18, and 13, are the only full trisomies
that have ever led to a live birth.
There are also partial and mosaic trisomies
, which are much more rare. Mosaic trisomies occur when the extra chromosome is present in some (not all) of the cells, and partial trisomies occur when only part of an extra chromosome is present. Some partial trisomies are translocations, usually caused by a parent's translocation, which may be hereditary. For more information, see Translocations
from the Centre for Genetics Education.
What are the characteristics of Trisomy 18?
The extra genetic material from the additional eighteenth chromosome can cause a variety of problems with varying severity. Just as children with Down Syndrome can range from mildly to severely affected, so can children with Trisomy 18. Therefore there is no hard and fast rule about what Trisomy 18 will mean for your child. However statistics show that there is a high mortality rate for children with Trisomy 18 before or shortly after birth.
Some of the typical characteristics of Trisomy 18 can include heart defects such as VSD (Ventricular Septal Defect - a hole between the lower chambers of the heart) , ASD (Atrial Septal Defect - a hole between the upper chambers of the heart), and coarctation of the aorta (a narrowing of the exit vessel from the heart), kidney abnormalities, omphalocele (a portion of the intestinal tract is located outside the stomach in a sac), esophageal atresia (the esophagus does not connect to the stomach, meaning the baby cannot eat by mouth), and polyhydramnios (excess amniotic fluid), clenched hands, choroid plexus cysts (a pocket of fluid on the brain that is not problematic in itself but may be a marker for Trisomy 18), rocker bottom feet, and delayed growth, micrognathia (small jaw), low-set ears, and strawberry-shaped head, as well as severe developmental delays.
See the Glossary
for more information about these characteristics.
More information about these characteristics can be found in these articles (note that these articles are written for physicians and are very clinical):
How is Trisomy 18 diagnosed?
While some children are diagnosed with Trisomy 18 after birth, the majority are diagnosed prenatally. Among prenatal tests, there are screening tests and diagnostic tests. Screening tests indicate an increased risk for Trisomy 18, while diagnostic tests provide an actual diagnosis of Trisomy 18.