Types of Down Syndrome & Incidence

Types of Down Syndrome

Trisomy 21 (nondisjunction)

This is the most commonly occurring type of Down syndrome, accounting for 95% of cases. Normally, babies are born with 23 chromosome pairs (or 46 chromosomes in total) in each cell – one inherited from the mother and one from the father. In Trisomy 21, there is an error in cell division called “nondisjunction”. This results in a baby being born with an extra copy of chromosome 21, meaning there are three copies of chromosome 21 instead of the usual two. As the embryo develops, the extra chromosome is replicated in every cell of the body.


Translocation accounts for about 4% of all cases of Down syndrome. In translocation, part of chromosome 21 breaks off during cell division and attaches to another chromosome, typically chromosome 14. While the total number of chromosomes in the cells remain 46, the presence of an extra part of chromosome 21 causes the characteristics of Down syndrome.


Mosaicism occurs when nondisjunction of chromosome 21 takes place in one - but not all – of the initial cell divisions after fertilization. In these cases, there is a mixture of two types of cells - some containing the usual 46 chromosomes and others containing 47. Those cells with 47 chromosomes contain an extra chromosome 21.

Mosaicism accounts for about 1% of all cases of Down syndrome. Research has indicated that individuals with mosaic Down syndrome may have fewer characteristics of Down syndrome than those with other types. However, broad generalizations are not possible due to the wide range of abilities people with Down syndrome possess.


One in every 700 babies is born with Down syndrome, making it the most common genetic condition on a worldwide basis.

Down syndrome can occur in people of all races and economic levels. We do know that older women have an increased chance of having a child with Down syndrome. Since many couples are postponing parenting until later in life, the incidence of Down syndrome conceptions is expected to increase. Therefore, genetic counselling for parents is becoming increasingly important.

Maternal AgeIncidence of Down Syndrome
201 in 2,000
211 in 1,700
221 in 1,500
231 in 1,400
241 in 1,300
251 in 1,200
261 in 1,100
271 in 1,050
281 in 1,000
291 in 950
301 in 900
311 in 800
321 in 720
331 in 600
341 in 450
Maternal AgeIncidence of Down Syndrome
351 in 350
361 in 300
371 in 250
381 in 200
391 in 150
401 in 100
411 in 80
421 in 70
431 in 50
441 in 40
451 in 30
461 in 25
471 in 20
481 in 15
491 in 10


The information featured in this section is reproduced via an exclusive arrangement with National Down Syndrome Society [ONLINE] Available at http://www.ndss.org