Everyone seems to have a family member affected with Down Syndrome (DS), be it a brother, niece, first cousin or third cousin once removed. However, according to the World Health Organization, the estimated incidence of Down Syndrome is between 1 in 1,000 to 1 in 1,100 live births worldwide. Three thousand to 5,000 children are born with this chromosome disorder every year and there are approximately 250,000 families in the US alone who are affected by Down Syndrome.

From my years of experience taking family history assessments, it seems that the incidence of Down Syndrome would be higher.

I have heard patients tell me:

My partner’s cousin has DS. He’s been in a wheelchair since birth. Or He has DS because his mother had an infection during pregnancy?

I often sit on the other side of the desk and think, None of this fits with what I know to be the medical issues or cause of this condition. So I say, “There are other causes of intellectual disabilities besides DS. Are you sure this relative has DS?” Judging by the long silence that follows, I think I have my answer.

To clarify, those with DS  do have some degree of physical and intellectual delays, but there is a wide range of abilities within the population that cannot be predicted ahead of time. As for the cause, this condition is the result of an extra chromosome 21 which, the vast majority of the time, occurs as a sporadic event.

I tried to research the number of genetic conditions, think high school biology and Gregor Mendel, that are associated with intellectual disabilities. When my academic search failed, I resorted to google. Still I could not find an exact number. I could not even find an estimate let alone an educated guess. Then I considered all the genetic mechanisms that can cause intellectual disabilities, i.e. more mechanisms than Mendel ever dreamed.

Imagine your DNA like a set of encyclopedias. Each chromosome is a specific volume (A-Z), each gene is a word in a particular volume, and a mutation is a misspelling in that word.

Now, genetic causes of intellectual disabilities can include –

  • A single change in a single gene (like a misspelled word in a particular book in a set of encyclopedias)
  • Large chromosome changes (imagine an entire extra copy of volume F or K)
  • A very small missing piece of a chromosome, or microdeletion (so, a missing chunk of chapters from volume G)
  • Copy number changes in a chromosome (in which, instead of a sentence reading “the red ball” it reads “the red red red red red red red ball”)

But wait, there’s more!

Other causes of intellectual disabilities include –

  • Issues during pregnancy (exposures, malnutrition, and maternal infections)
  • Issues at birth – (prematurity, low birth weight and temporary oxygen deprivation or birth injuries)
  • Issues after birth (childhood diseases, head injuries, and exposure to environmental toxins)
  • Poverty and cultural deprivation (children in disadvantaged areas may have limited access to educational experiences, leading to under-stimulation)

Down syndrome has become a catchall for all conditions that present with intellectual or physical disabilities because all of the other conditions, independent of each other, are rare. It should be noted however that while DS is one of the more common conditions, there are many others that present with even more intellectual or physical limitations.

My advice? Know the facts.

  • Inquire about specific information regarding a relative’s diagnosis and its cause. How was a diagnosis made (what tests were performed) and by whom (what specialist)?
  • Know what the prenatal tests offered to you can (and can’t) do.
  • Control what you can (diet, exercise, exposures), knowing that there is a whole lot out there that you can’t.


Shannon Wieloch

Shannon Wieloch is a licensed board-certified genetic counselor at CooperGenomics. Her primary responsibility is to provide genetic counseling to CooperGenomics patients. Other professional roles include managing the genetic content on social media, supervising graduate students, and conducting research.

Prior to joining CooperGenomics, Shannon worked in cardiac research at The Children’s Hospital of Philadelphia and in prenatal genetic counseling at The Delaware Center for Maternal and Fetal Medicine. She received a dual B.S. in biology and psychology from The University of Pittsburgh and her M.S. in genetic counseling from Arcadia University. Her passion is to provide comprehensive genetic education to medical professionals, patients, and the general public. In her free time, she loves to travel, doodle, play board games with her girls, and take too many pictures of her cat.