Chromosomal Conditions
What are chromosomal abnormalities?
Chromosomal abnormalities mean that the set of chromosomes in a fetus differs from the normal number or that one or more chromosomes have altered structure.
The common trisomies
Down syndrome (trisomy 21) is a genetic condition that occurs when an extra chromosome (chromosome 21) originates in the development of either the sperm or the egg. When the egg and the sperm unite to form the fertilized egg, three (rather than two) chromosomes 21 are present. As the cells divide, the extra chromosome is repeated in every cell. This is why it’s referred to as “trisomy 21”. Down syndrome is the most frequently occurring chromosomal condition, occurring in about 1 in 700 births. Down syndrome can affect physical and intellectual development and may shorten lifespan.
Edwards syndrome (trisomy 18) is due to an extra copy of chromosome 18. Trisomy 18 causes Edwards syndrome and is associated with a high rate of miscarriage. Infants born with Edwards syndrome may have various medical conditions and a shortened lifespan. It is estimated that Edwards syndrome is present in approximately one out of every 5,000 newborns.
Patau syndrome (trisomy 13) is due to an extra chromosome 13. Trisomy 13, also called Patau syndrome, is associated with a high rate of miscarriage. Infants born with trisomy 13 usually have severe congenital heart defects and other medical conditions. Survival beyond the first year is rare. It is estimated that trisomy 13 is present in approximately 1 out of every 16,000 newborns.
Sex Chromosome Aneuploidies
Turner syndrome (XO), in which only one single intact X chromosome is present, affects 1 in 2,000 girls and causes short stature. Girls and women with Turner syndrome are treated with growth hormone from the age of 3-4 and later with the female sex hormone estrogen in order to undergo normal puberty development. In this way, the affected can live a completely normal life.
Klinefelter syndrome (XXY), in which an extra X chromosome is present, affects male fetuses. The syndrome occurs in 1 in 500–1000 boys, making it one of the most common sex chromosome variations. Many people with Klinefelter’s syndrome live without symptoms, while others suffer from a variety of functional impairments such as infertility, reduced sex drive and osteoporosis. Read more about what it’s like to live with Klinefelter syndrome.
Other X and Y chromosome-related conditions affect as many as 1 in 650 newborns.
*Analysis of sex chromosome abnormalities is not possible in twin pregnancies