Trisomy 18 Health Article

Definition

Trisomy 18 is a genetic syndrome of multiple congenital anomalies and severe to profound mental retardation. It is caused by the presence of an extra chromosome 18 in some or all of the cells of the body. Babies with the condition usually do not survive past their first several months. Trisomy 18 in the embryo/fetus is also a common chromosomal cause of pregnancy loss.

Description

Chromosomes are the microscopic structures inside cells that carry the genes. The genetic material inside each cell contains all of the instructions the body needs to develop and function normally. Humans have 23 different pairs of chromosomes. Chromosomes 1-22 are numbered from largest to smallest, and as a group are known as the autosomes. The last pair of chromosomes are designated X and Y, and are known as the sex chromosomes—females have two X chromosomes and males have one X and one Y. Other than sperm and eggs, each cell in the body normally has 46 chromosomes—a pair of each of the autosomes plus two sex chromosomes. In order for normal development and functioning to occur, chromosomes and genes must be present in the correct quantity and in the correct proportion to each other. Too much or too little genetic material usually causes serious problems.

The term euploid means "good set," and is used to designate a full set of 46 chromosomes. A cell is aneuploid ("not a good set") if it has any number of chromosomes other than 46. A trisomy is one type of aneuploidy, and refers to a cell that contains three of the same chromosome. Trisomy 18, then, refers to three chromosomes 18. After Down syndrome (trisomy 21), trisomy 18 is the most common autosomal aneuploid condition seen in live-born babies. Trisomy 18 is also known as Edwards syndrome.

Edwards syndrome is comprised of a specific but broad pattern of multiple congenital anomalies and mental retardation. Babies with Edwards syndrome tend to have similar physical features and medical problems because they all have the same genetic imbalance—an extra copy of the genes on chromosome 18. The physical anomalies associated with Edwards syndrome involve nearly every organ and system of the body. However, some anomalies occur more often than others, such as those of the heart, kidney, brain, skeleton, and craniofacial (head and face) area. The birth defects are typically serious and, combined with the large number of anomalies possible, result in a high mortality rate. About 60% of newborns with Edwards syndrome die within the first week, and 80% do not survive past the first month. Even those with Edwards syndrome who live longer will have severe to profound mental retardation and chronic medical problems, necessitating involved care and monitoring throughout their lives.