The most common cause of trisomy 18 is nondisjunction during the formation of an egg or sperm cell, although individuals sometimes inherit the condition from a parent with a chromosomal abnormality called a balanced translocation, explains the National Institutes of Health. A specific form called mosaic trisomy 18 results from a random cell division error during an early stage in embryonic development, causing the baby to have some genetically normal cells and some with an extra copy of chromosome 18.Continue Reading
Nondisjunction is a cell division error that causes an egg or sperm cell to form with an abnormal number of chromosomes, specifically an extra copy of chromosome 18 in the case of trisomy 18, notes the NIH. Patients with trisomy 18 due to nondisjunction have an extra copy of the chromosome in all of their bodily cells. This is the case for the majority of trisomy 18 patients.
Balanced translocation occurs when there is no extra genetic material in a person's cells but chromosome 18 is attached to another chromosome, states Stanford Children's Health. Individuals with this mutation do not exhibit symptoms of trisomy 18 themselves, but they have a higher chance of conceiving children with the condition, according to the NIH.
Another name for trisomy 18 is Edwards syndrome, notes the NIH. Many affected individuals die within the first month after birth, with only 5 to 10 percent living past the age of 1.Learn more about Conditions & Diseases