What is trisomy 18?

In healthy individuals, cells contain 23 pairs of chromosomes (for a total of 46 chromosomes per cell). These chromosomes contain a person's genetic information. Each pair of chromosomes contains one chromosome from each parent. Trisomy 18, also called Edward's syndrome, is a genetic disease that is caused by the presence of an additional copy (or part of an additional copy) of chromosome 18.

When only some of a person's cells have the extra chromosome, it is called mosaic trisomy 18. In general, people who have only part of an extra copy of chromosome 18 and people with mosaic trisomy 18 have less severe symptoms than people who have an extra copy of the complete chromosome.

About one out of 3,000 conceptions are affected with trisomy 18. Additionally, about one out of 5,000-6,000 newborn babies have trisomy 18, suggesting that many embryos with trisomy 18 die before birth. About 80% of newborns affected with trisomy 18 are female.

Only 50% of newborns with trisomy 18 live past the first week of life, and only 5%-10% of newborns with trisomy 18 live past one year of age, due to a wide range of physical and mental developmental defects, including heart defects, kidney defects, mental retardation, and feeding problems. A small number of children may survive to their teenage years, but they will likely have serious medical problems. People who have only part of an extra copy of chromosome 18 and people with mosaic trisomy 18 are more likely to live longer than people who have an extra copy of the complete chromosome.

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Trisomy 18 is a genetic condition related to the presence of an extra chromosome 18 caused by a problem that occurs when cells divide in the egg, sperm or fertilized egg. The extra chromosome causes the fetus to develop abnormally with a number of physical and mental problems.

Trisomy 18 is also called Edward's syndrome. It is the second most common trisomy condition. (Down syndrome is the most common.) A fetus with trisomy 18 has three copies of chromosome 18 in each cell.

Trisomy 18 can be identified during pregnancy. Doctors can do prenatal tests and fetal ultrasounds to screen for problems, and they can do chromosome tests to diagnose trisomy 18.

Many fetuses with trisomy 18 do not survive to birth, but some are born and live a couple of months to a couple of years. Babies born with trisomy 18 can have heart and kidney problems, a small head with low-set ears, a chest with an unusual shape and crossed legs. They also have severe intellectual disability.