Trisomy 18 is a chromosomal condition linked to severe intellectual disability and abnormalities in many parts of the body. Individuals with trisomy 18 often have low birth weight, a small, abnormally shaped head, a small jaw and mouth, clenched fists with overlapping fingers, heart defects, as well as abnormalities of other organs. Only 5 to 10% of children with this condition live past their first year of life. Trisomy 18 occurs in about 1 in 5,000 newborns and approximately 80% of newborns affected by this disorder are female. Although women of all ages could have a baby with trisomy 18, the chance of having a baby with this condition increases as a woman gets older.
Trisomy 13 is a chromosomal condition associated with severe intellectual disability and physical abnormalities in many parts of the body. Individuals with trisomy 13 often have heart defects, brain or spinal cord abnormalities, very small or poorly developed eyes, extra fingers and/or toes, an opening in the lip (cleft lip) with or without an opening in the roof of the mouth (cleft palate), and weak muscle tone (hypotonia). Due to the presence of several life-threatening medical problems, many infants with trisomy 13 die within their first days or weeks of life. Only 5 to 10% of children with this condition live past their first year. Trisomy 13 occurs in about 1 in 16,000 newborns. Although women of any age can have a baby with trisomy 13, the chance of having a baby with this condition increases as a woman gets older.
Most cases of trisomy 18 and 13 are not inherited, but occur as random events during cell division in early embryonic development. Some of the body's cells have the usual two copies of chromosome 18 or 13, and other cells have three copies of one of these chromosomes that may come from either the egg or the sperm.
The inherited form of trisomy 18 and 13 occurs when an unaffected person carries a rearrangement of genetic material between chromosome 18 and other chromosomes. There is no “extra copy” of chromosome 18, only a rearrangement or translocation of the chromosome. People who carry this type of translocation are at an increased risk of having children with the condition. The physical features of these types of abnormalities may be different than those with full trisomy 18 or 13.