Trisomy 18 is a condition in which a person has an extra chromosome. Chromosomes are small "packages" of genes in the body. Genes are pieces of code passed down from parents to children. They make us who we are.
Chromosomes determine how a fetus forms during pregnancy, functions as it grows in the womb, and after birth. Typically, a baby is born with 46 chromosomes. Babies with trisomy 18 syndrome have an extra copy of one of these chromosomes, chromosome 18. A medical term for having an extra copy of a chromosome is “trisomy.”
Trisomy 18 syndrome is a rare chromosomal disorder in which all or a large portion of chromosome 18 is present three times (trisomy), rather than twice in each cell of the body. This extra copy changes how the fetus’s body and brain develop, which can cause distinctive physical features, structural birth defects and developmental disabilities. Most babies with trisomy 18 are lost through miscarriage.
Chromosome abnormalities are usually present at conception, the result of errors in egg or sperm formation. In rare cases chromosome, abnormalities are inherited. The majority of babies born with trisomy 18 are female. Trisomy 18 syndrome can be diagnosed prenatally by amniocentesis, or chorionic villus sampling.
Trisomy 18 can affect babies born to women of any age. However, as women get older, the chances increase for having a baby with trisomy 18.
For information and references about trisomy 18, please visit: National institutes of Health, Genetic and Rare Diseases Information Center, Trisomy 18
For more information or family support on trisomy 18, please visit the following website:
Trisomy 18 Foundation
Birth Defects Monitoring Program Data Sheet on Trisomy 18