Trisomy 18 and 13

What are trisomies?

The term trisomy is used to describe the presence of three chromosomes, rather than the usual pair of chromosomes. For example, if a baby is born with three #21 chromosomes, rather than the usual pair, the baby would be said to have "trisomy 21." Trisomy 21 is also known as Down syndrome. Other examples of trisomy include trisomy 18 and trisomy 13. Again, trisomy 18 or trisomy 13 simply means there are three copies of the #18 chromosome (or of the #13 chromosome) present in each cell of the body, rather than the usual pair.

To answer some questions you may have about a child with Trisomy 18 & 13, visit the Frequently Asked Questions page in the Genetics Resources.

What are trisomy 18 and trisomy 13?

Trisomy 18 and trisomy 13 are genetic disorders that present a combination of birth defects including severe mental retardation, as well as health problems involving nearly every organ system in the body. Unfortunately, 90 percent of babies born with trisomy 18 or 13 die by age 1. It is important to note that 5 to 10 percent of babies with trisomy 18 or 13 do survive the first year of life. Therefore, these disorders are not universally fatal and, in the absence of any immediate life-threatening problems, accurate predictions of life expectancy are difficult to make. There are a few reports of babies with trisomy 18 or 13 surviving to their teens, however, this is unusual.

Trisomy 18 is also called "Edwards syndrome," named after the physician who first described the disorder. Trisomy 13 is called "Patau syndrome," in honor of the physician who first described it. 

What causes trisomy 18 and trisomy 13?

Usually each egg and sperm cell contains 23 chromosomes. The union of these creates 23 pairs, or 46 total chromosomes at the time of fertilization. In this manner, a person receives exactly half of their genetic material from each parent. Sometimes, an error occurs when an egg or sperm cell is forming, causing it to have an extra chromosome #18 or #13. When this cell contributes the extra chromosome 18 to the embryo, trisomy 18 results. When this cell contributes the extra chromosome 13 to the embryo, trisomy 13 results. The extra chromosome 18 or 13 can come from either the mother's egg cell or the father's sperm cell. The features of trisomy 18 and trisomy 13 result from having this extra chromosome 18 or 13 in each of the body's cells.

Occasionally, the extra chromosome 18 or 13 is attached to another chromosome in the egg or sperm; this is called a translocation. This is the only form of trisomy 18 or 13 that can be inherited in a family. Sometimes, a parent can carry a "balanced" rearrangement where chromosome 18 or 13 is attached to another chromosome. However, since the parent does not have any extra or missing chromosome material, they are said to have a "balanced translocation" and they are usually normal and healthy. Rarely, mosaic trisomy 18 or 13 may occur when the error in cell division occurs after fertilization. These persons have some cells with an extra chromosome 18 or 13 and others with the normal number.

How are trisomy 18 and trisomy 13 diagnosed?

Since trisomy 18 and trisomy 13 each have a unique group of characteristics, a physician may be able to determine simply by physical examination whether a baby has trisomy 18 or 13. To confirm the physical findings, a small blood sample can be taken and the chromosomes can be analyzed to determine the presence of an extra #18 or #13 chromosome.

Chromosomal abnormalities such as trisomy 18 and 13 can be diagnosed before birth by analyzing cells in the amniotic fluid or from the placenta. Fetal ultrasound during pregnancy can also give information about the possibility of trisomy 18 or 13, but ultrasound is not 100 percent accurate, since some babies with trisomy 18 and 13 may look the same on ultrasound as those without the syndrome. A chromosome analysis, whether performed on a blood sample, cells from the amniotic fluid, or placenta, is over 99.9 percent accurate.