Trisomy 21: What is it? Causes, Symptoms, Examination During Pregnancy, Prevention and Treatment

It is the most common birth defect in the United States.

Trisomy is the presence of three (and not two, as would be normal) chromosomes of a particular type in an organism. Therefore, the presence of three chromosomes 21 is called trisomy 21.

Most trisomies result in a variable number of deficiencies at birth (generally present in most individuals with extra chromosomes).

Many trisomies result in early death. A trisomy is said to be partial when part of an extra chromosome (and not all of it) that is coupled to one of the other chromosomes. A mosaic trisomy is a condition in which not all cells contain the genetic information for the extra chromosome.

Although trisomy can occur with any chromosome, the most common type is trisomy 21, the condition that causes Down syndrome .


Free or simple trisomy

The most common or frequent form of Down syndrome is known as simple or free trisomy. 95% of people with Down syndrome are born with 47 chromosomes, of which three complete correspond to pair 21.

Humans are made up of cells with 46 chromosomes that separate during cell division, 23 follow one cell and their corresponding pairs follow another.

However, sometimes that division and separation of the chromosome pairs does not occur in the correct way, that is, one of the chromosome pairs (in our case the 21 pair) does not separate, the two 21 chromosomes remain united and are they remain in one of the two divided cells (ovum or sperm).


This occurs when two chromosomes, one of which is a number 21, join at the ends, creating two independent chromosomes from number 21, as well as a chromosome number 21 attached to another chromosome.

About 75% of translocations occur by chance. The rest are inherited from a parent known as a “balanced carrier.” Although translocation Down syndrome occurs through a different mechanism than trisomy 21, the physical appearance and characteristics that mark the condition are the same.


In this rare form of Down syndrome, which accounts for only 2-3% of cases, only some cells have an extra copy of chromosome 21. People can have all the features of complete trisomy 21. None of the features lie somewhere in between.

Previous examination in pregnancy

It is one of the situations that most frightens future mothers: trisomy 21, which causes a developmental, motor and mental delay and alterations of the body and face structure.

The prevalence of this syndrome is frequent, since every 700 babies born, one of them will have Down syndrome, and it can occur with any couple, regardless of age, being more common when mothers are over 40 years of age.

For all these reasons, it is important to carry out the so-called prenatal diagnosis that evaluates the degree of risk of the baby of being a carrier of trisomy 21.

To be able to exclude this condition definitively, it is necessary to resort to invasive diagnostic means, such as chorionic villus biopsy or amniocentesis, which allow the collection of cells from the baby but carry a risk of miscarriage (0.5 to 1%).

With these risks in mind, prenatal diagnosis is the best way for parents to have a solid foundation in deciding whether or not to move forward for invasive diagnostic methods.

With the corresponding risk calculation, the prenatal diagnosis must be made between the 10th and 13th week, plus six days of gestation, after the date of the last menstruation.

A morphological ultrasound examination, combined with maternal age, allows to detect about 75% of the cases, this percentage increases to 90 to 95% when the biochemical examination is added.


Even in newborns, trisomy 21 is externally detectable, as it changes appearance in a certain way. Consequently, all children with Down syndrome have many physical characteristics in common, typical of:

  • Below average height (children are quite small at birth for maturity).
  • A fairly small, rounded skull with a flattened back of the head.
  • A flat face.
  • Oblique eyelids.
  • A skin fold in the inner corner of the eye.
  • A wide nose.
  • Small ears.
  • A frequently open mouth due to problems with closing the mouth with increased salivation.
  • Wide hands with short fingers.
  • A continuous groove of the hand (four-finger groove) on the palms.
  • End links bent inward of the fingers.
  • A deeply seated belly button.
  • Feet that lower the folds with a large space between the big toe and the second toe (called a sandal gap).
  • A broad and flabby skin (especially in newborns, the skin on the neck is flabby).
  • Hyperextensible joints.

In their mental capacities, people with Down syndrome generally have a certain pattern, most of them have quite good social skills and visual attitudes, but they are weak in linguistic communication.

The degree to which trisomy 21 impacts mental abilities is extremely different. While some who suffer from it are almost average intelligent, others have severe mental retardation.

Can you prevent trisomy 21?

The chromosomal disorder, which triggers the so-called Down syndrome, occurs randomly. Therefore, the cause of Down syndrome cannot be prevented.

To date, there is no clear explanation as to why there is an excess of chromosome 21 in one of the parental germ cells, which ultimately leads to Down syndrome or trisomy 21. Ultimately, it is an accidental occurrence, which is observed more frequently in older mothers, but also occurs in young women.


Trisomy 21 therapy is not possible because the chromosome change is no longer reversible. However, it is important to treat the health problems that commonly occur along with Down syndrome. Because of developmental delays, children with trisomy 21 also need specific support.

Down syndrome itself cannot be treated. The chromosome change is irreversible and the excess chromosome 21 cannot be blocked. However, children with trisomy 21 need special treatment.

On the one hand, comorbidities and organ malformations, which are often associated with chromosomal disorder, must be treated. Congenital heart defects and malformations in the intestinal tract, for example, can now be treated well by surgery, according to the Professional Association of Pediatricians.