Edwards Syndrome, Trisomy 18 is a rare genetic disorder caused by the presence of a third copy of all or part of chromosome 18, which causes abnormal development in many of the baby’s organs. According to the Centers for Disease Control and Prevention (CDC), an estimated 1,187 babies are affected by Trisomy 18, and most are female.
There are three types of Trisomy 18:
- Full Trisomy 18. The extra chromosome is in every cell in the baby’s body. This is the most common type of Trisomy 18.
- Mosaic Trisomy 18. The extra chromosome 18 is only in some of the baby’s cells. Approximately 5% of individuals born with Trisomy 18 will carry this type. This form is also rare.
- Partial Trisomy 18. Affected individuals have two copies of chromosome 18, plus the extra material from chromosome 18 (part of the long (q) arm) attached to another chromosome. This type of Trisomy 18 is very rare.
Women of all ages can have a child with Trisomy 18, but chances increase as a woman ages. A first-trimester screening, which includes a blood test and ultrasound, offers early information about the risks of having Trisomy 18. A second-trimester blood test called a quad screen can also help with early detection. More precise methods take cells from the amniotic fluid (amniocentesis) or placenta (chorionic villus sampling) and analyze the chromosomes to confirm the diagnosis.
Individuals with Trisomy 18 often suffer from low birth weight. Some features of Trisomy 18 include having a small, abnormally shaped head, small jaw, and mouth, cleft lip and palate, clenched hands and fists with overlapping fingers, rocker bottom feet, groin hernia, birth-defect with intestinal organs outside of the body, kidney defects, heart defects, low-set ears, single line palms and drooping of the upper eyelid. There is no cure or treatment for Trisomy 18, and many babies do not survive past the second or third trimester of pregnancy. Forty percent of babies that will survive into the third trimester do not survive labor, and nearly one-third of the surviving babies deliver preterm. Almost all Trisomy 18 pregnancies end in miscarriage or stillbirth. Boys with Trisomy 18 are more likely to be stillborn than girls.
Babies who survive birth usually die within their first month of life. The survival rate varies for babies born with Trisomy 18:
- Between 60% and 75% survive to their first week.
- Between 20% and 40% survive to their first month.
Only five to 10 percent of children with Trisomy 18 live past their first year of life but with severe intellectual disabilities.
INHERITANCE
Most cases of Full and Mosaic Trisomy 18 are not inherited but occur as random events during the formation of eggs and sperm. An error in cell division called nondisjunction results in a reproductive cell having an abnormal number of chromosomes. If one of these atypical reproductive cells contribute to the genetic make up of a child, the child will have an extra chromosome 18 in each of the body’s cells. For Mosaic Trisomy 18 specifically, some of the body’s cells have the usual two copies of chromosome 18, while some cells have three copies of chromosome 18.
Partial Trisomy 18 can be inherited. An unaffected person can carry a rearrangement of genetic material between chromosome 18 and another chromosome. This rearrangement is a balanced translocation because there is no extra material from chromosome 18. Although they do not have signs of Trisomy 18, people who carry this type of balanced translocation are at an increased risk of having children with the condition.
Source: U.S. National Library of Medicine. Medline Plus. http://www.medlineplus.gov | Centers for Disease Control and Prevention. http://www.cdc.gov | Cleveland Clinic. http://www.my.clevelandclinic.org
The E.WE Foundation 