Affected individuals may have heart defects and abnormalities of other organs that develop before birth. Other features of trisomy 18 include a small, abnormally shaped head; a small jaw and mouth; and clenched fists with overlapping fingers. Due to the presence of several life-threatening medical problems, many individuals with trisomy 18 die before birth or within their first month.
Five to 10 percent of children with this condition live past their first year, and these children often have severe intellectual disability. Trisomy 18 occurs in about 1 in 5, live-born infants; it is more common in pregnancy, but many affected fetuses do not survive to term. Although women of all ages can have a child with trisomy 18, the chance of having a child with this condition increases as a woman gets older.
Most cases of trisomy 18 result from having three copies of chromosome 18 in each cell in the body instead of the usual two copies. The extra genetic material disrupts the normal course of development, causing the characteristic features of trisomy Approximately 5 percent of people with trisomy 18 have an extra copy of chromosome 18 in only some of the body's cells.
In these people, the condition is called mosaic trisomy The severity of mosaic trisomy 18 depends on the type and number of cells that have the extra chromosome. The development of individuals with this form of trisomy 18 may range from normal to severely affected. Very rarely, part of the long q arm of chromosome 18 becomes attached translocated to another chromosome during the formation of reproductive cells eggs and sperm or very early in embryonic development. Affected individuals have two copies of chromosome 18, plus the extra material from chromosome 18 attached to another chromosome.
People with this genetic change are said to have partial trisomy If only part of the q arm is present in three copies, the physical signs of partial trisomy 18 may be less severe than those typically seen in trisomy If the entire q arm is present in three copies, individuals may be as severely affected as if they had three full copies of chromosome Most cases of 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 with an abnormal number of chromosomes. For example, an egg or sperm cell may gain an extra copy of chromosome If one of these atypical reproductive cells contributes to the genetic makeup of a child, the child will have an extra chromosome 18 in each of the body's cells. Mosaic trisomy 18 is also not inherited.
It occurs as a random event during cell division early in embryonic development. As a result, significant genetic defects, such as extremely small body size and severe physical deformities, usually result in the death of the embryo or fetus , 95 percent of developing embryos and fetuses die before birth.
Infants who are born with Trisomy 18 survive to an average age of The oldest recorded life span of a person with Trisomy 18 is twenty-seven years. Although Trisomy 18 does not affect individuals based on his or her race, 80 percent of cases occur in females. Newborns with Trisomy 18 experience severe psychomotor and growth retardation.
They usually possess head sizes that are significantly smaller than an average child of the same age and sex microcephaly. Furthermore, they also have small eyes microphthalmia , malformed ears, a small mouth microstomia , an undersized jaw micrognathia , clenched fingers, and other malformations. In addition to these visible physical abnormalities, other characteristics of Trisomy 18 include the presence of holes between the lower chambers of the heart ventricular septal defects and holes between the upper chambers of the heart atrial septal defects , malfunctioning kidneys, and the dislocation of the esophagus from the stomach esophageal atresia.
Amniocentesis and chorionic villus sampling are effective prenatal methods for diagnosing Trisomy The mortality rates for those who have Trisomy 18 are high, and there are few medical treatments. Although most medical literature that refers to Trisomy 18 discusses it as a disease linked to a full trisomy of chromosome 18, partial and mosaic trisomies of the chromosome also exist.
These trisomies, which occur when only some cells possess the extra genetic material, account for only 5 percent of Trisomy 18 cases.
This can be done while the baby is in the womb using chorionic villus sampling CVS or amniocentesis. Both these tests have a risk of miscarriage. You can talk to your obstetrician or midwife about this. Sometimes, Edwards syndrome is not suspected during pregnancy, and is only discovered when the baby is born.
Visit Pregnancy, Birth and Baby website for more information on prenatal screening. There is no effective long-term treatment for babies with Edwards syndrome. Parents in this situation face a very hard choice. They can choose to try to keep their babies alive in the neonatal intensive care unit. Or they can choose to have their baby receive palliative care until he or she dies. Visit our genetic disorders guide to learn more about genes, types of genetic disorders and where to go for help and more information.
Learn more here about the development and quality assurance of healthdirect content. Trisomy 18 or Edwards syndrome is a chromosomal disorder that causes severe disability.
Most babies with trisomy 18 die in the first weeks of life. Read more on raisingchildren. Read more on Better Health Channel website. Amniocentesis is a diagnostic test carried out during pregnancy to assess whether the baby has an abnormality or serious health condition. A nuchal translucency scan is part of the ultrasound scan that may give an indication of chromosomal abnormality.
Learn more about how and when it is performed.
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