What is trisomy 18?
Trisomy 18, also known as Edwards syndrome, is the second most common trisomy behind trisomy 21 (Down syndrome). It occurs in 1 in 7,000 live births and it is caused by the presence of an extra chromosome 18 and similar to Down syndrome. It is seen more commonly with increasing maternal age. Babies born with trisomy 18 have distinct clinical features, including developmental delay and craniofacial, limb, heart, and kidney abnormalities. Half of all babies born with Edwards syndrome die within the first week, and only 5% to 10% live beyond the first year of life.
What causes trisomy 18? What are the different kinds of trisomy 18?
A trisomy is caused by a genetic error in which three copies of a chromosome (instead of two) are inherited from the parents. Trisomy 18 is caused by the presence of an extra chromosome 18, and most frequently is of maternal origin and includes the entire chromosome in most cases (>90%) rather than a part of the chromosome (also known as partial trisomy or incomplete trisomy). In a small percentage of patients, an additional chromosome 18 is present in some, but not all cells, and is referred to as a mosaicism; these affected patients may not present with the typical features of Edwards syndrome listed below.
What are the signs and symptoms of trisomy 18?
Today the majority of fetuses with trisomy 18 are diagnosed prior to birth and research indicates that many of these fetuses do not survive to birth. For those infants who do survive, they are at risk for any number of abnormalities, but the typical features include:
- intrauterine growth retardation
- craniofacial features such as abnormalities of the jaw, skull, ears, and neck
- clenched fists with overriding fingers
- small fingernails
- short sternum
- club feet
- heart defects
- kidney defects
- neurodevelopmental delays