Trisomy 18
(Edwards Syndrome)

Trisomy 18 Summary
Trisomy 18 is an chromosomal condition that causes severe birth defects in newborns. Many babies born with trisomy 18 do not live past one month of age. Trisomy 18 is also known as Edwards Syndrome.
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Trisomy 18 facts

  • Trisomy 18 (Edwards syndrome) results from the presence of an extra copy of chromosome 18.
  • Trisomy 18 leads to severe intellectual and physical defects.
  • The prognosis of trisomy 18 is poor; half of those affected do not live beyond the first week of life and only 10% of infants with this condition live beyond the first year.
  • Common features of trisomy 18 include a low birth weight; a small jaw and mouth; an abnormally shaped, small head; overlapping fingers and clenched fists; and defects in the heart and other organs.
  • Trisomy 18 occurs on average in 1 out of every 6,000 births.
  • Up to 95% of fetuses with trisomy 18 die in the prenatal period (prior to delivery).
  • Girls are affected more commonly than boys.

What is trisomy 18?

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Trisomy 18, also known as Edwards Syndrome, is a severe genetic disorder associated with intellectual disability and abnormalities in many organs. Trisomy 18 was first described in 1960 and results from an additional copy of chromosome 18 (normal cells carry two copies of each chromosome; the term, "trisomy," refers to the presence of three of a given chromosome). Only about 10% of affected infants survive past one year of age, and half of affected infants do not live beyond the first week of life.

What are the chromosome basics of trisomy 18?

In most cases of trisomy 18, all the cells in the body have an extra copy of chromosome 18. Around 5% of affected people have the extra chromosome 18 in some, but not all, of the body's cells. This phenomenon is known as mosaic trisomy 18. Mosaic trisomy 18 may be very severe or hardly noticeable, depending upon the number of cells that have the extra chromosome.

In rare cases, there is no extra chromosome present; rather, a portion of the long arm of chromosome 18 becomes attached to another chromosome during the formation of egg and sperm cells or very early in development of the embryo. In this case, the individual has two copies of chromosome 18 plus the additional material from chromosome 18 that is attached to another chromosome. This is known as translocation, and the extra genetic material causes the developmental abnormalities in the same way as the presence of an entire extra chromosome. The signs and symptoms of this form of trisomy 18 are dependent upon the amount of chromosomal material that was translocated to another chromosome.

Trisomy 18 is not an inherited condition; it occurs as the result of random events during egg and sperm formation. The type of error that occurs is known as nondisjunction, and this leads to an egg or sperm cell with an abnormal number of chromosomes.

It is not known precisely why the extra genetic material causes the abnormalities specific to trisomy 18. As with Down syndrome (trisomy 21), experts believe that the presence of the extra chromosomal material interferes with the expression and interaction of various genes, resulting in impaired development and function.

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Definition of Edwards Syndrome

Edwards syndrome: Trisomy 18 syndrome. Children with the syndrome have an extra chromosome 18 with a characteristic pattern of multiple malformations and mental retardation. Features include low birth weight, small head (microcephaly), small jaw (micrognathia), malformations of the heart and kidneys, clenched fists with abnormal finger positioning, and malformed feet. The mental retardation is profound with an IQ too low to measure. Nineteen out of 20 of these children die before their first birthday. The condition is named after the British physician and geneticist John Edwards who discovered the extra chromosome in 1960.