Humans have 23 pairs of chromosomes. These are molecules of DNA and proteins that contain genetic information. Trisomy is a chromosomal disorder in which there are three copies of a chromosome instead of the normal two.
Trisomy 21 is caused by an extra copy of chromosome 21 and isthe most common form of chromosomal disorder in humans. This trisomy leads to Down syndrome, which is associated with usually moderate intellectual disability and other disorders, such as congenital heart disease. Trisomy 21 occurs in 1 in 500 pregnancies. About a third of pregnancies with trisomy 21 end in miscarriage. Therefore, it is estimated that only 1 in 830 newborns has trisomy 21. The likelihood of trisomy 21 is largely related to the mother's age. For example, the risk of trisomy 21 in a 20-year-old pregnant woman is on average 1 in 1,050, while the risk in a 40-year-old pregnant woman is about 1 in 100 (information from the end of the first trimester).
Trisomy 18 is caused by the presence of an extra copy of chromosome 18. Trisomy 18 leads to Edwards syndrome and is associated with a very high rate of miscarriage. Babies born with Edwards syndrome are usually severely disabled and suffer from multiple physical problems. Their life span is usually only a few months. It is estimated that Edwards syndrome occurs in about 1 in 5,000 newborns. The risk of trisomy 18 also depends largely on the mother's age.
Trisomy 13 is caused by an extra copy of chromosome 13. Trisomy 13 leads to Patau syndrome and is associated with a very high rate of miscarriage. Babies born with trisomy 13 usually suffer from severe congenital heart disease and other conditions, and rarely survive their first year of life. It is estimated that trisomy 13 occurs in about 1 in 16,000 newborns. The risk of this trisomy increases with the mother's age.
