Down syndrome


Down syndrome was named after John Langdon Down (1828-1896), who was the first physician to identify it, in 1866. Down syndrome occurs in about 1 out of every 700-900 live births worldwide (depending on contraception and termination attitudes in different communities), to mothers of all social, economic and racial groups, and is the most common genetic cause of developmental disability. In the UK around one in every 1000 babies will be born with Down syndrome.

Down syndrome is a chromosomal disorder caused by an error in cell division and the most common form produces an additional third chromosome 21, or "trisomy 21" (it is also known in some countries as Trisomy 21). There are currently around 30,000 people with Down syndrome in the UK, 200,000 in Western Europe, and more than 2,000,000 worldwide. Due to advances in medical technology, as many as 80% of people with Down syndrome can now expect to live to around 40-60 years of age, whereas in 1910 their life expectancy was about 9.

Is it the mother or father that is the 'carrier' of Down syndrome?

The additional genetic material which causes Down syndrome can originate from either the mother or the father, with approximately 5% of cases having been traced to the latter.

What is the probability of having more than one child with Down syndrome?

Regardless of age of the mother, the probability of having a subsequent pregnancy where a child has Down syndrome is about 1%.

The risk of having a baby with Down syndrome increases with advancing maternal age (e.g. 1 in 910 at age 30, and 1 in 28 at age 45), but where a previous pregnancy has been affected it increases further to about 1 in 200 at age 30 and 1 in 25 at age 45.

What is the link between maternal age and Down syndrome?

Since Down syndrome is a chromosomal disorder caused by an error in cell division, the likelihood of such an error occuring increases with maternal age. This means that an older mother is more likely to give birth to a child with Down sydrome than her younger counterpart. However, overall, fewer older mothers have babies than younger ones.

According to the National Association for Down syndrome, "80% of babies born with Down syndrome are born to women younger than 35. The average maternal age is 28 years old." The likelihood of a woman under 30 years of age giving birth to a child with Down syndrome is less than 1:1000, but increases the older the woman gets (see chart below), with an incidence of about 1:112 at 40 years of age.

An annual occurence of Down syndrome of about 9% is seen in women 35 years of age or older, but about 25% of babies with Down syndrome are born to women in this age group. 20-25% of babies conceived with Down syndrome survive past birth.

Down syndrome - the risks.
(Note: This chart should not be used to assess an individual's risk of having a baby with Down syndrome, it is purely a general guide).

Age:
Risk: 1 in ...
20
1529
25
1351
30
910
35
384
36
307
37
242
38
189
39
146
40
112
41
86
42
65
43
49
44
35
45
28
46
20
48
16
49
12

Screening for Down syndrome

Click here for details of the various tests available during pregnancy


What are the problems that a child born with Down syndrome might face?

Associated with Down syndrome is an increased risk of hearing and vision defects, heart abnormalities (about 50% are born with congenital heart defects, for which the majority can be surgically corrected), infection, leukaemia, thyroid disorders, and of developing Alzheimer-type dementia in later life. There can also be delayed motor skills - sitting, crawling and walking in infancy, and delayed cognitive skills such as speech and language acquisition and in short-term memory abilities.

The majority of those with Down syndrome have IQs that fall in the mild to moderate range of retardation, but some are so mildly affected that they can lead independent and relatively normal lives. However, it is important to remember that the range of abilities and thus, disabilites, interests and achievements varies greatly, just as it would for any individual and not just those with Down syndrome.

Individuals with Down syndrome can often lead fulfilling and productive lives and this is due to the range of health care, education and support that is now available to them.

Is there a cure for Down syndrome?

There is no 'cure' for Down syndrome, but researchers are working all the time to develop effective interventions and treatment strategies.

What is Trisomy 18 and Trisomy 13?

Edwards syndrome, also known as Trisomy 18, (named after Dr John Edwards), is another abnormality caused by an extra chromosome - number 18. It is less common than Down syndrome - 1 in every 3,000 live births, with three times as many girls as boys affected.

Children born with Trisomy 18 suffer severe mental retardation, heart defects and other health problems. They usually die early on: 30% by 1 month of age, 50% by 2 months of age and 90% die by 12 months of age. Advanced maternal age is a risk factor for Trisomy 18.

Patau syndrome, also known as Trisomy 13, (named after Dr Klaus Patau, who identified the chromosomes responsible for it in 1960) is caused by an extra chromosome - chromosome number 13. It affects 1 in 3000 births, with boys and girls affected in equal numbers. Trisomy 13 is the third most common autosomal anomaly after Trisomy 21 (Down Syndrome) and Trisomy 18 (Edward's Syndrome).

There are other Trisomy conditions in addition to the above, but these are not as common.

Disorders

Tests are designed principally to screen for Down syndrome and cystic fibrosis. However, in addition the Down syndrome screening tests can also detect other disorders. These are Edwards syndrome and, if testing is after 15 weeks gestation, Neural Tube Defects (NTDs) and Abdominal Wall Defects (AWDs). The table below shows that most of the additional disorders are either rare, incompatible with survival, or surgically correctable:


Disorder:
Rate at birth:
Disorder:
Down syndrome (Trisomy 21)
1 in 700
Untreatable
Edwards Syndrome (Trisomy 18)
1 in 7,000
Mostly die in first weeks
Spina Bifida
1 in 500†
Most are disabled
Anencephaly
1 in 500†
Nearly always stillborn
Exomphalos
1 in 2,000
Operable
Gastroschisis
1 in 6,000
Operable
Cystic fibrosis
1 in 2,400
Lifetime treatment

† In the 1970s: current rates unknown, but thought to be lower because of dietary changes.

There are some other disorders that are not specifically screened for, but are detected as a result of tests. Rarer chromosomal anomalies (e.g. Turner's syndrome, triploidy), rather than Down syndrome or Edwards syndrome, may be found following invasive prenatal diagnosis. Structural abnormalities (e.g. heart defects) may be found when measuring nuchal transparency.

Source: www.intellectualdisability.info