(Edwards Syndrome; Trisomy E)
(See also Overview of Chromosomal Disorders.)
Trisomy 18 is a chromosomal disorder caused by an extra chromosome 18 that results in intellectual disability and physical abnormalities.
Chromosomes are structures within cells that contain DNA and many genes. A gene is a segment of deoxyribonucleic acid (DNA) and contains the code for a specific protein that functions in one or more types of cells in the body (see Genes and Chromosomes for a discussion about genetics). Genes contain instructions that determine how the body is supposed to function.
An extra chromosome, making three of a kind, is called trisomy (see also Overview of Chromosome and Gene Disorders). Children who have trisomy 18 have a third chromosome 18. Trisomy 18 occurs in about 1 out of 6,000 live births. However, many affected fetuses spontaneously abort. The extra chromosome almost always comes from the mother. Mothers who are over age 35 are at increased risk of having a child with trisomy 18. Girls are affected more often than boys.
In the womb, affected fetuses are not typically very active, and there is often excess amniotic fluid and a small placenta.
At birth, newborns are often very small because their muscles and body fat are underdeveloped. Newborns are typically limp and have a weak cry. The mouth and jaw may be small, which gives the newborn's face a pinched appearance. Other visible deformities are common, including a small head, low-set and malformed ears, a narrow pelvis, and a short breastbone (sternum). Physical abnormalities may be obvious at birth. However, some newborns have abnormalities that are not as severe.
The hands are clenched in fists, and the index fingers overlap the middle and ring fingers. The fingernails are underdeveloped. Skinfolds, especially over the back of the neck, are common. The big toes are shortened and frequently bend upward. Clubfeet and rocker-bottom feet are common.
Before birth, trisomy 18 may be suspected based on findings detected during an ultrasound of the fetus. Doctors can sometimes find deoxyribonucleic acid (DNA) from the fetus in the mother's blood and use this DNA to detect an increased risk of trisomy 18. If doctors suspect trisomy 18 based on these tests, they often confirm the diagnosis using chorionic villus sampling, amniocentesis, or both (see Testing for chromosome and gene abnormalities).
After birth, the infant's physical appearance may suggest the diagnosis of trisomy 18. To confirm the diagnosis of trisomy 18, the infant's chromosomes are analyzed using a blood test.