What is Down syndrome?

What is Down syndrome?

Down syndrome is a genetic alteration on chromosome 21 that occurs at the time of conception.

Down syndrome is a genetic alteration caused by the presence of 3 chromosomes 21 , when normally there are two. It is the main cause of intellectual disability and the most frequent genetic alteration. It affects 1 in 700 pregnancies. It is more frequent when the mother is over 35 years of age . Pregnant women undergo specific tests during the first trimester of pregnancy to detect the risk of having a child with Down syndrome.

All human cells have 46 chromosomes arranged in 23 pairs. One of these pairs determines the sex of the individual and the other 22 are called autosomes, numbered from 1 to 22. The egg and the sperm each contain only 23 chromosomes (one chromosome from each of the 23 pairs), in such a way that when they unite they produce a new cell with the same genetic load as any other human cell, that is, 46 chromosomes divided into 23 pairs. It is during this process that most of the alterations that give rise to Down syndrome occur.

There are three types of chromosomal abnormalities that can cause Down syndrome:

  • Trisomy 21 : it is the most frequent type and is characterized by the presence of three chromosomes 21.
  • Chromosomal translocation : It is very rare. An extra piece of chromosome 21 joins chromosome pair 14.
  • Mosaicism : in the same individual there are cells with trisomy 21 and others are normal.

How is Down syndrome diagnosed?

Diagnosis can be made before delivery or after birth . When the child is born, the diagnosis is suspected by physical examination and phenotype (by facial features and physical characteristics), but the definitive diagnosis is given by analysis of the chromosomes (karyotype).

Diagnostic tests before childbirth are recommended in women over 35 years of age or if screening tests (ultrasound and laboratory tests) indicate a high risk of having a child with Down syndrome. Prenatal diagnosis consists of performing a chorionic villus biopsy (in the eighth week of pregnancy) or an amniocentesis (in the 16th week of pregnancy). Both tests must be well indicated since there is a risk of spontaneous abortion .


Children affected with Down syndrome can associate some diseases. It is important to remember that not all children with trisomy 21 will have them. The most frequent alterations are:

  • Congenital heart malformations
  • Hearing and vision problems
  • Hormonal (endocrine) problems
  • neurological alterations
  • Intestinal abnormalities

All these diseases require specific monitoring from birth, and all have treatment.


Early diagnosis is essential to correctly guide families and offer the child with Down syndrome the best stimulation . It is a multidisciplinary work that tries to alleviate, as much as possible, the effects of the syndrome on the child’s development. Early Intervention programs are essential to stimulate the child with Down syndrome at a sensory and motor level. Thanks to proper monitoring and treatment of the medical problems of children with Down syndrome, they can have an acceptable quality of life and their average life expectancy is about 60 years.

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