Humans have 23 pairs of chromosomes. These are DNA molecules and proteins that contain genetic information. Trisomy is a chromosomal disorder in which three copies of a chromosome are present instead of the normal two.
Trisomy 21 is caused by the presence of an extra copy of chromosome 21 and it is the most common form of chromosomal disorder in humans. This trisomy leads to Down syndrome, which is associated with generally moderate intellectual disability and other disorders, such as congenital heart disease. Trisomy 21 occurs in 1 out of every 500 pregnancies. Approximately one third of pregnancies with trisomy 21 result in miscarriage. Therefore, it is estimated that only 1 in 830 newborns is affected by trisomy 21. The probability of trisomy 21 is largely related to the mother's age. For example, the risk for 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 approximately 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. Children born with Edwards syndrome are usually severely disabled and suffer from multiple physical disorders. Their life span is generally only a few months. It is estimated that Edwards syndrome occurs in approximately 1 in 5,000 newborns. The risk of trisomy 18 also largely depends on the mother’s age.
Trisomy 13 is caused by the presence of an extra copy of chromosome 13. Trisomy 13 leads to Patau syndrome and is associated with a very high rate of miscarriage. Children born with trisomy 13 usually suffer from serious congenital heart and other disorders, rarely surviving their first year. Trisomy 13 is estimated to occur in approximately 1 in 16,000 newborns. The risk of this trisomy increases with the mother’s age.
