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.

What are trisomy 18 and trisomy 13?

Trisomy 18 and trisomy 13 are genetic disorders that include 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 (half the normal number). The union of these cells creates 23 matched pairs, or 46 total chromosomes at the time of fertilization. In this manner, a person receives exactly half of their genetic material from each biological parent. Sometimes, an error occurs when an egg or sperm cell is forming, causing it to have an extra chromosome #18 or #13 inside. If this cell contributes that extra chromosome 18 to the embryo, then trisomy 18 results. If this cell contributes that extra chromosome 13 to the embryo, then 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 copy of 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 from a parent. 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 affected persons have some cells with an extra chromosome 18 or 13 and others with the normal number.

What types of problems do children with trisomy 18 and trisomy 13 typically have?

Babies with trisomy 18

Babies with trisomy 18 appear thin and frail. They fail to thrive and have problems feeding. Trisomy 18 causes a small head size, with the back of the head (occiput) prominent. Ears are usually low-set on the head. The mouth and jaw are unusually small, and there is a shortened sternum (breastbone).

At birth, these babies are small for their age, even when delivered full-term, and have a weak cry. Their response to sound is decreased and there is often a history of infrequent fetal activity during the pregnancy. About 90 percent of babies with trisomy 18 have heart defects. They clench their fists in a characteristic manner and fully extending the fingers is difficult. Joint contractures—where the arms and legs are in a bent position, rather than relaxed—are usually present. The feet may be referred to as "rocker bottom" due to their shape.

Babies with trisomy 18 may also have spina bifida, eye problems, cleft lip and palate, and hearing loss. It is also common to see feeding problems, slow growth, seizures, high blood pressure, kidney problems, and scoliosis (curvature of the spine). In males, the testes sometimes fail to descend into the scrotum.

Most babies with trisomy 18 have problems which affect all parts of the body in some way. The majority of children with trisomy 18 will have most, but not all, of the health problems mentioned here. Heart problems, feeding difficulties, and an increased susceptibility to infection are factors which, most often, contribute to the death of these children.



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