Information on numerical chromosomal disorders

The Harmony^® Test is a very reliable test for the prenatal detection of trisomy 21 (Down syndrome) and other numerical chromosomal disorders (trisomies 18 and 13). The test can also be used to determine the sex of the child, disorders of the sex chromosomes (X, Y) and the risk of DiGeorge syndrome (Microdeletion 22q11.2).

A human has 23 pairs of chromosomes; these are DNA strands and proteins that contain the genetic information. A trisomy is a chromosomal disorder where there are three copies of a certain chromosome instead of the normal two copies.

Trisomy 21

Trisomy 21 is caused by an additional copy of chromosome 21 and is the most common form of chromosomal disorder in humans. Trisomy 21 leads to Down syndrome, which is associated with a generally moderate intellectual disability and other disorders such as congenital heart disease. Trisomy 21 occurs in approximately 1 out of every 500 pregnancies. Approximately one-third of all children with trisomy 21 die in the mother’s womb, thus it is estimated that only 1 out of every 830 newborns is affected by trisomy 21¹. The likelihood the occurrence of trisomy 21 depends largely on the age of the mother; for example, the risk of trisomy 21 for a 20-year-old pregnant woman is approximately 1 in 1050, whereas the risk for a 40-year-old woman is approximately 1 in 100 (figures at the end of the first trimester of the pregnancy).

Trisomy 18

Trisomy 18 is due to an extra copy of chromosome 18. Trisomy 18 causes Edwards syndrome and is associated with a very high miscarriage rate. Children born with Edwards syndrome are typically seriously disabled and usually suffer from multiple physical disorders. Their life expectancy is normally only a few months. It is estimated that Edwards syndrome occurs in approximately 1 out of every 5,000 newborns².

The risk of trisomy 18 also depends to a large extent on the mother’s age.

Trisomy 13

Trisomy 13 is due to an extra copy of the chromosome 13.

Ttrisomy 13 causes Patau syndrome and is associated with a very high miscarriage rate. Children born with trisomy 13 usually suffer from serious congenital heart disease and other disorders, and rarely survive their first year of life.

It is estimated that trisomy 13 occurs in approximately 1 out of every 16,000 newborns³; advanced maternal age increases the risk of this trisomy.