Trisomy 18

By: Joe Salerno


Trisomy 18 is a genetic syndrome that causes specific birth defects and causes 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, but if they do their body has some restrictions to it. Trisomy 18 occurs while at the stage of embryo/fetus.


Trisomy 18 comes in 3 parts full, partial and mosaic trisomy. However they all get inherited in the same way. Full Trisomy, is where chromosomes of a single pair do not separate during meiosis, an abnormal process known as nondisjunction. The result is one gamete with 24 chromosomes and another with 22. If a gamete with 24 chromosomes results in conception with a normal counterpart, an embryo with 47 chromosomes is produced. This is full trisomy. Then there is Mosiac Trisomy. It involves nondisjunction of chromosomes 18, in which a cell undergoing mitosis in a chromosomally normal embryo produces one cell with trisomy 18 and another with monosomy 18. The cell with monosomy 18 cannot survive, but if the trisomic cell survives, all cells in the body derived from it will have trisomy 18. Finally there is Partial Trisomy, which is where an unbalanced chromosome translocation may result in an embryo that has an extra piece of chromosome 18. All of these things cause trisomy 18, all of them cause a shift in the 18th chromosome which then results in the disorder.


The Tracking Rare Incidence Syndromes (TRIS) Research Project seeks to increase knowledge about trisomy 13 and 18 and other rare conditions by surveying parents, analyzing data and sharing the results with medical professionals and families. (Note: this is a group that everyday seeks to find and help those with trisomy 13 and 18 by asking them questions and doing tests, however very little is known about 13 and 18.)


the typical features include:

  • intrauterine growth retardation
  • craniofacial features such as abnormalities of the jaw, skull, ears, and neck
  • clenched fists with overriding fingers
  • small fingernails
  • short sternum
  • club feet
  • heart defects
  • kidney defects
  • neurodevelopmental delays

Tests- First-Trimester Screening for Trisomy 18- Disorder Testing--- No treatments

Down syndrome and trisomies 13 and 18 are chromosomal disorders that cause mental retardation and birth defects. Trisomy 18 (having an extra chromosome No. 18) is more a more severe disorder causing profound mental retardation and severe birth defects in many organ systems. Few babies born with trisomy 18 survive more than a few months. Anyone can have a baby with chromosome abnormalities; however, the chance increases with the mother's age. Christiana Care is the only health system in Delaware that offers first-trimester screening and the ability for immediate prenatal diagnostic testing for Trisomy 18 (Edwards' syndrome). The screening is noninvasive and poses no risk to the mother or baby. In fact, the American College of Obstetricians and Gynecologists has endorsed first-trimester screening for chromosomal disorders.

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