Trisomy 18

What is Trisomy 18?
What causes Trisomy 18?
Is it possible to test for Trisomy 18 during pregnancy?

What is Trisomy 18?
Trisomy 18 is a serious chromosome abnormality that affects about 1 in every 3000 to 4000 newborns. It was first described by Dr Edwards in 1960, which explains why the condition is also referred to as Edwards syndrome. A syndrome is a pattern or combination of physical, mental, and health characteristics that are usually seen together. Typically, trisomy 18 is recognizable at birth and sometimes even prior to birth due to abnormalities seen on a detailed look at the fetus with ultrasound. There are many physical signs that may be associated with trisomy 18, although no one will have all of these features. The combination of features simply helps to make a diagnosis.¹

Unfortunately, physical birth defects usually limit the life expectancy of a child with trisomy 18. Approximately 70% of fetuses with trisomy 18 will not survive to delivery. Of infants born with trisomy 18, only about 10% will survive beyond one year. Newborns are generally smaller than average and have weak cries. They may be born earlier or later than expected. Children who survive typically have profound mental retardation, feeding problems with significant growth difficulties, apnea (breathing problems), and seizures.¹

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What causes Trisomy 18 (Edwards syndrome) ?
Trisomy 18 is caused by the presence of an extra copy of chromosome 18 in the cells of the body. Typically, individuals have 46 chromosomes (23 pairs) in each cell with two copies of chromosome number 1, two copies of chromosome number 2, etc. The 23rd pair determines sex so that males usually have one X and one Y chromosome and females usually have two X chromosomes. Most individuals with trisomy 18 have three copies of this chromosome in each cell, and this form of trisomy 18 is typically not inherited.

A small percentage of people with trisomy 18 may have an extra chromosome 18 in some of their cells but not all of them (this is called mosaicism), or they may have an extra chromosome 18 attached to another chromosome (this is called translocation).¹ Trisomy 18 caused by a translocation is rare, but could be inherited. A chromosome analysis (a blood test) can identify if a person with a family history of trisomy 18 has a chromosome translocation that increases the risk of having a baby with this problem.

Is it possible to test for Trisomy 18 during pregnancy?
It is possible to diagnose trisomy 18 during pregnancy through prenatal diagnostic procedures such as amniocentesis or CVS. Because amniocentesis and CVS pose some degree of risk to the pregnancy, they are generally reserved for those situations when the chance for diagnosing a chromosome abnormality is at least as high as the complication rate.

Parents might consider amniocentesis or CVS when a mother's age is 35 years or older at delivery, there is a family history of a chromosome abnormality, or a screening test done during pregnancy identifies an increased chance for trisomy 18, or some other chromosome abnormality. Two of the most common screening tests are maternal serum screening and ultrasound. A screening test identifies those women who have an increased likelihood of having a baby with trisomy 18 by identifying certain risk factors associated with the disorder. It is important to remember that having a risk factor does not mean that a baby has trisomy 18. Screening tests do not diagnose trisomy 18, they only identify those women who may benefit from additional testing.

Maternal serum screening has many other names, such as AFP and the triple screen. Ultrasound may also be useful in screening for trisomy 18. There are some signs that may be seen on a thorough ultrasound (sometimes called level II or targeted ultrasound) that may indicate there is an increased chance for trisomy 18. Some of these are minor differences in the development of an otherwise normal baby without any serious consequences themselves. Each ultrasound sign shows up at a different stage in pregnancy and causes a different level of concern. If any of these signs is present, it should be discussed in detail with a genetic counselor, doctor or qualified health care provider.³

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The choice to have any type of screening or prenatal diagnosis procedure is very personal. It is important to fully understand the benefits and limitations of each of the options before making a decision. Genetic counselors are specially trained to guide couples through these options to a decision that seems most appropriate for them. If you would like more information about the availability of genetic counselors in your area, visit the National Society of Genetic Counselors Web Site.

You can obtain more information about trisomy 18 at:

Support Organization For Trisomy 18, 13, and Related Disorders

The Chromosome 18 Registry & Research Society

References

1. Jones, KL. Smith's Recognizable Patterns of Human Malformation. 4th ed. Philadelphia, Pa; W.B. Saunders Company; 1988.
2. Baty, BJ, Blackburn BL, Carey JC. Natural History of Trisomy 18 and Trisomy 13: I. Growth, Physical Assessment, Medical Histories, Survival, and Recurrence Risk. Am J Med Genet 1994;49:175-188.
3. Sanders RC, ed. Structural Fetal Abnormalities: The Total Picture. St. Louis, Mo: Mosby; 1996.

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