What Causes Down Syndrome?

It might surprise you to learn that “Down Syndrome” is the most commonly occurring genetic condition causing mental retardation in humans. Approximately 1 in 733 babies are born with the syndrome, and approximately 350,000 people have Down Syndrome in the United States alone. It was given its name when a physician, Dr. John Langdon Down, published an accurate description of the features and similarities of people with the syndrome in 1866. He was then considered the “father” of the disorder, and it was given his name.

Why does it occur?

Down Syndrome occurs because of an error in cell division, either at conception or before. Human cells have 46 chromosomes, which result when the sperm, which has 23 chromosomes, matches up with the egg, which also has 23. Sometimes the sperm or egg has extra genetic material on the 21st chromosome, due to cells dividing improperly. In that case, when the sperm and egg match up in conception, the 21st chromosome is a trio rather than a pair. This is why Down Syndrome is often referred to as “Trisomy 21.” The triple chromosome on what should be the 21st pair then replicates in every cell of the developing embryo. There are other less frequent scientific causes, all involving improper cell division.

Maternal age is clearly linked

This faulty cell division has been clearly linked to maternal age, however the exact mechanism for the error remains unknown. Studies have shown that a mother who is 25 years of age has a 1/1,300 chance of having a baby with Down Syndrome, at 35, the risk increases to 1/365. At age 45, a woman has a 1/30 chance of having a baby with the disorder. However, 80% of babies born with Down Syndrome are born to mothers age 35 and younger. But that statistical “twist” can be explained by the fact that women in that age group are having many more babies.

Features of Down Syndrome

As many as 120 recognized features are associated with the syndrome, but most individuals have about six or seven of them. Some of these features (or symptoms) typically recognized at birth may include:

  • Enlargement of tongue in relationship to size of mouth
  • Low muscle tone
  • An upward slant to the eyes
  • Excessive space between large and second toe
  • A depressed nasal bridge causing a flat facial profile and small nose
  • Abnormal shape of the ear, or dysplastic ear
  • Hyper-flexibility, an excessive ability to extend the joints
  • Pinky finger has one flexion furrow instead of two
  • Small skin folds on the inner corner of the eyes

Although all cases of Down Syndrome involve mental retardation, the level of severity can vary considerably. Most people with DS fall only within the mild to moderate range. Some individuals also suffer from congenital heart defects and a range of health problems, including hearing and vision problems, epilepsy, leukemia, and hypothyroidism, but others do not.

Although the diagnosis of a baby with Down Syndrome can be a crushing blow for parents, most experienced mothers and fathers of DS children describe them as loving, affectionate, happy, and tender. They also describe the experience of raising a DS child as being highly rewarding. I will write more about Down Syndrome and prenatal screening in a future blog.

For more information, visit the National Down Syndrome Society Website.