Down’s syndrome
What is it?
Down's syndrome is a condition that is present in your baby from the
time of conception and is caused by the presence of an extra chromosome.
Chromosomes are present in every cell and carry the 'blueprint' for all
our characteristics within our DNA. Each cell has 23 pairs of chromosomes,
46 in total. One from each pair comes from each parent.
However, people with Down's syndrome have an extra copy of chromosome
21 and so have 47 chromosomes in total. It is the presence of this extra
chromosome that is picked up in antenatal testing for Down's syndrome.
The different types
There are three types of Down's syndrome.
1. Standard (Regular or Primary) Trisomy 21. This is the most
common type of Down's syndrome, occurring in about 94 per cent of all
cases. There is no known reason for this type of Down's syndrome.
2. Translocation This occurs in approximately 3 per cent of babies
with Down's syndrome when part of the number 21 chromosomes becomes attached
to another chromosome so that both parts of the new large chromosome move
together as one. Half of the people with translocation have it because
one parent has one of the number 21's 'stuck on' to another chromosome.
The chances subsequent children having this type of Down's is high.
3. Mosaicism This type affects 2-3 per cent of people with Down's
syndrome and occurs when the cells with the extra 21 chromosome are mixed
with other 'normal' cells.
What causes it
Although no-one knows what causes Down's syndrome, some things are now
generally recognised:
- It can come from either the mother or the father
- It can happen entirely by chance
- Standard Trisomy 21 occurs because either the egg or the sperm that
joined together to produce the baby was carrying the extra chromosome.
When this happens, the first cell of the developing baby has 47 chromosomes
instead of 46, and all of that baby's cells will have 47 chromosomes.
- Babies born to older mothers have a higher chance of having Down's
syndrome but the reason for this is not known.
- It occurs in all races, in all social classes and in all countries
throughout the world.
- Nothing you do before or during pregnancy will cause or prevent Down's
syndrome.
How common is Down's syndrome?
Everyday in the UK, between one and two babies are born with Down's syndrome,
which means that one baby in 1000 has the condition.
The signs and symptoms
All people with Down's syndrome share some certain physical characteristics,
although not everyone will have every characteristic. However, some of
the more common signs you may notice in your baby include;
- Low birthweight and slow weight gain
- The back of your baby's head may be flatter than usual
- Many babies with Down's syndrome have eyes that slant upward and outward
and their eyelids may have an extra fold of skin (epicanthic fold)
- Your baby may have a single crease which runs right across the palm
of the hand, although some babies without Down's syndrome may also have
this
- Your baby will be slower to reach his milestones and will have a
certain degree of learning difficulty but, as with any child, there
is no way of predicting your child's level of ability.
What sort of health problems could there be?
Many people with Down's syndrome are quite fit and healthy, although
there are some physical problems associated with the condition.
- Heart problems - About one in three children born with Down's
syndrome has a heart defect. Some are mild, such as murmurs, but others
can be serious requiring surgery. Because of this, your baby will have
regular heart check-ups in his first years of life.
- Chest problems - babies and children with Down's syndrome are
more likely to suffer from chest and sinus infections
- Feeding problems - some babies have problems co-ordinating
their sucking and breathing reflexes and may choke and splutter whilst
trying to feed. However, this tends to settle down after the first couple
of weeks. Specially adapted teats are available to help babies who have
difficulty feeding.
- Dry skin - like many newborns, your baby may have very dry
skin which can be treated with emollient oils or bath washes
- Tongue control - your baby may have difficulty moving his tongue
but you can help by making faces and sticking your tongue out at him
so when he's older he can copy you.
Antenatal tests to screen for Down's syndrome
First trimester
- Nuchal translucency test (NT). 11-13 weeks. This is a special ultrasound
scan that measures the fluid under the skin at the back of the baby's
neck. Although it cannot diagnose Down's syndrome, it can be used to
determine your risk of having a baby with Down's syndrome.
- Combined Test. 11-13 weeks. This blood test measures hCG (human chorionic
gonadotrophin) and PAPP-A (pregnancy associated plasma protein). Both
of these will be raised if the baby has Down's syndrome. The blood test
can be done alone or with the Nuchal scan. Despite some disputes, these
tests are quite accurate.
- The NT scan alone probably detects about 75 per cent of babies with
Down's syndrome
- The blood test alone detects about 60 per cent
- Both tests together can detect up to 90 per cent
Second trimester
These are blood tests carried out between 15 and 20 weeks of pregnancy.
They are used to detect the levels of;
- hCG (human chorionic gonadotrophin) will be raised if baby has Down's
syndrome
- AFP (alpha fetoprotein) will be lowered if baby has Down's syndrome
- uE3 (oestriol) will be lowered if baby has Down's syndrome
- Inhibin A will be raised if baby has Down's syndrome
These tests can be done in three ways:
- The Double Test: measures hCG and AFP
- The Triple Test: measures hCG, AFP and uE3
- The Quadruple Test: measures hCG, AFP, uE3 and inhibin A.
If your risk is high you may be offered an amniocentesis test to confirm
if your baby has Down's syndrome. This is usually performed between the
15th and 18th weeks of pregnancy.
Joint first and second trimester tests
These tests, although high quality, are currently only available privately.
- The Serum Integrated Test combines a blood test for PAPP-A
in the first trimester with a further blood test for hCG, AFP, uE3 and
inhibin A in the second trimester.
- The Integrated Test combines nuchal translucency scanning and
a blood test for PAPP-A in the first trimester with a further blood
test for hCG, AFP, uE3 and inhibin A in the second. It has a detection
rate of about 94 per cent for Down's syndrome.
Further tests
In 2002 Professor Nicolaides and his team at the FMF published research
showing that if the nasal bone is absent the risk of Down's syndrome is
increased. A nasal bone assessment can be done with your nuchal scan.
- If the nasal bone is seen the risk of Down syndrome is reduced
- The absence of the nasal bone has a marked affect on the risk of Down
syndrome and it is vitally important that an absent nasal bone is diagnosed
when one is present
- The nasal bone can sometimes be very difficult to see as the baby
has to be in exactly the right position and his head needs to be at
the correct angle
- If the nasal bone is absent and the baby does not have Down syndrome
this is not linked with any other developmental problems
The main benefit of using the nasal bone is to reduce the false positive
rate which is currently about 5 per cent. Using the nasal bone assessment
reduces this to 3 per cent which means that fewer people will be given
a high risk when their baby is normal and fewer CVS & amniocentesis tests
are needed.
Looking after a child with Down's syndrome
Children with Down’s syndrome will progress through the normal stages
of development, though usually at a slower rate than normal, and they
need extra help and support from their parents and from professionals.
With this help and support, they can join fully in family and social life,
and develop their full potential. What this potential is will be different
in every case, as with any individual. Some adults with Down’s syndrome
lead almost independent lives while others need full-time care. Most adults
with Down’s syndrome live to around 55 or more.
Where to next
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