PRE-NATAL TESTING FOR
DOWN'S SYNDROME
by Mary Sawtell
Introduction
The information in this article is intended to help pregnant women and their
partners decide whether or not to undergo pre-natal testing for Down's
syndrome. It may also be useful for those who have received a positive result
following pre-natal tests, and who are trying to decide what action to take
next. It is meant to supplement information and support given by GPs and other
professional people. Pre-natal testing can be a very stressful process,
especially when waiting for results and deciding what action to take should the
result not be the reassuring outcome hoped for.
This article contains information on: Down's syndrome and what this means to
parents of children with this condition; the types of tests available; the
meaning of positive and negative results of specific tests; and the possible
options should a positive result be obtained. The aim is not to persuade people
against pre-natal testing, rather to ensure that whatever decision parents make
is based on the most up-to-date information.
Down's Syndrome
Down's syndrome is a genetic condition involving an extra chromosome, which
means that the person with Down's syndrome has forty-seven chromosomes instead
of the usual forty-six. It is the result of a genetic accident before, during,
or soon after conception. The reason for this accident is unknown.
Symptoms
It is difficult to generalise about the characteristics, since there are
more differences between people with Down's syndrome than there are
similarities, and they inherit many of their family's specific characteristics.
However, people with Down's syndrome do have distinctive physical
characteristics shared by others with the condition.
They also have learning difficulties (previously known as mental handicap).
This means that they develop and learn more slowly than other children.
Nowadays, most children with Down's syndrome will walk and talk. Many will read
and write, go to ordinary schools, and look forward to a semi-independent adult
life, away from the family home.
As with the population at large, the ability range of people with Down's
syndrome is very wide. For example, some people may acquire little speech,
others will learn to speak very well. None of the pre-natal tests available can
indicate how able or disabled a person will turn out to be. However, much more
is understood than in the past about how to help people with Down's syndrome
reach their full potential and lead rewarding lives, whatever their ability. A
range of professionals is available to offer guidance and support to
individuals and their families.
Associated problems
Down's syndrome is not a disease, and the condition cannot be treated.
People with Down's syndrome do not have pain or suffering as a direct result of
the condition. However, there are certain medical conditions to which people
with Down's syndrome are prone. For example, 40% of babies with Down's syndrome
are born with heart defects, which can be serious and require surgery. Hearing,
vision, and intestinal problems, are also more common than in the ordinary
population. However, treatment for these conditions continues to advance, and
the degree to which they are life-threatening, or limit achievement and
well-being, is reducing.
People with Down's syndrome often do realise that they are different, but
attitudes in society are becoming more positive, and parents can instil a sense
of personal value in their growing child as they do with all their children.
Family life
The early days following a diagnosis of Down's syndrome can be very
stressful, but after the initial shock there is often a period of positive
adjustment and optimism. Research shows that extra stress is not an inevitable
consequence of having a child with Down's syndrome. Nor is there any evidence
that brothers and sisters suffer.
The future
A life span of 40 to 60 years is now common for people with Down's syndrome.
Most adults with Down's syndrome have the same expectations as other adults
(e.g. to live independently of their family, to have close relationships, and
to find fulfilment through employment, further education or leisure
activities). All are realistic ambitions, provided that extra support is
available.
Prospective parents worry what will happen when they are not around to
ensure that their child's best interests are being provided for. It is
impossible to predict what will happen in the future, but at present local
authorities are duty bound to ensure that the needs of individuals with
learning difficulties are met.
Pre-natal tests
There are two basic types of tests for Down's syndrome, diagnostic tests and
screening tests. Diagnostic tests establish whether or not an unborn child has
Down's syndrome. Screening tests provide only an estimate of the chance of the
baby having Down's syndrome, and do not allow a firm diagnosis to be made in
themselves.
Diagnostic tests
The only way of being certain if an unborn baby has Down's syndrome is by
detecting the presence of the extra chromosome in the baby's cells. This
involves taking a sample of the cells and growing them in the laboratory. The
two main diagnostic tests available are amniocentesis and chorionic villus
sampling (CVS). Cordocentesis is occasionally offered.
Amniocentesis
This is now a well-established and widely used technique. An ultrasound scan
is used to check the position of the baby in the womb. A fine needle is then
inserted through the mother's abdominal wall, into the womb, and a sample of
the amniotic fluid which surrounds the baby is removed. This fluid contains
cells from the baby which can be grown and tested. Most women say there is
little pain.
Amniocentesis is generally carried out between the 16th and 18th week of
pregnancy. The results are available in 3 to 4 weeks, i.e. between the 19th and
22nd week of pregnancy. Amniocentesis is almost 100% accurate in detecting
Down's syndrome. However, there is a risk of miscarriage of around 1%, and in a
further 1% the test fails to produce a result, because the cells do not grow in
the laboratory or the results are inconclusive.
Chorionic villus sampling (CVS)
In this test, an ultrasound scan is used to guide a fine, hollow needle
either via the woman's vagina and cervix, or via her abdomen, and a tiny piece
of the developing placenta (afterbirth) is removed. This contains cells similar
to those of the baby, which are then grown and tested. The test is
uncomfortable rather than painful.
CVS can be performed after 10 weeks of pregnancy. A preliminary result may
be available after 1 to 2 days but it takes between 3 and 4 weeks to obtain an
accurate result. CVS has a reported risk of miscarriage of approximately 2%,
and failure to obtain an adequate sample or to grow the cells occasionally
occurs. Unreliable results are occasionally obtained following CVS, and
amniocentesis may be offered to clarify the result.
Cordocentesis
In cordocentesis (foetal blood sampling) a needle is passed through the
mother's abdomen and a sample of the baby's blood is taken from the cord. The
test is carried out from 18 weeks of pregnancy and the results are available in
one week. The reported miscarriage risk from the procedure is between 1% and
4%.
Target population
Routine diagnostic testing for Down's syndrome, generally by amniocentesis,
is usually offered when the mother is thirty-five or older. The main reason for
this is that until a woman is in her mid- to late-thirties, her chances of
having a baby with Down's syndrome are considerably smaller than her chance of
a miscarriage following diagnostic testing (see below). The chances of having a
baby with Down's syndrome at various maternal ages are as follows:
- age 25, 1 in 1,400
- age 30, 1 in 800
- age 35, 1 in 380
- age 38, 1 in 190
- age 40, 1 in 110
- age 45, 1 in 30
Diagnostic tests are now being offered increasingly to younger women who
have had a positive result following a screening test.
Screening tests
Although the chance of having a baby with Down's syndrome increases as a
woman gets older, more babies with Down's syndrome are born to younger women
because the overall birthrate is higher in this age group. Hence screening
tests for women, regardless of age, have been developed. These tests, which
include blood tests and ultrasound scans, do not carry a risk of miscarriage.
Unlike diagnostic tests, screening tests do not definitely tell whether or
not the unborn child has Down's syndrome. Instead they estimate the chance of
the baby having the condition.
Maternal serum screening (Blood tests)
There are several blood tests available. They work by measuring the amounts
of a number of different substances produced by the developing baby, from the
mother's blood sample. The level of these substances is used in combination
with the woman's age, weight, and length of pregnancy, to estimate the chance
of the baby having Down's syndrome, compared to the average chance for a woman
of her age. These tests are carried out between the 15th and 18th week of
pregnancy and the result is available in about one week. There is no direct
risk to the unborn child with these blood tests.
If the tests show a sufficiently increased chance, greater than a chosen
level (usually 1 in 250 or more), this is called a screen positive result.
Parents will be told what the probability of Down's syndrome is (e.g. 1 in 20,
1 in 120, 1 in 190). A screen positive result does not therefore necessarily
mean the baby is affected. In fact the vast majority of women with a screen
positive result will have an unaffected baby. However, they will be offered a
diagnostic test to confirm whether or not the baby has Down's syndrome.
Parents should not feel that they must have a diagnostic test, but if they
do and the result is positive, then the choice of continuing with the
pregnancy, or terminating it, is offered.
A screen negative result means that the chance of having a baby with Down's
syndrome is less than 1 in 250. While women with this result are unlikely to
have a child with Down's syndrome, a few will.
Types of blood test
The alpha-fetoprotein (AFP) test
This test measures levels of the substance AFP in the mother's blood.
Certain levels of AFP can be indicative of a number of conditions. For example,
four out of five babies with spina bifida can be identified because of
unusually high levels of AFP. Recently, low levels of AFP have been used in
some hospitals as a test for Down's syndrome. However, this test detects only
about 30% of babies with Down's syndrome, and has a high rate of false positive
results.
The triple test
Many NHS hospitals are now offering the Triple Test. This measures three
substances, AFP, human chorionic gonadatropin (hCG), and unconjugated oestriol
(uE3) in one blood sample, and has a detection rate for Down's syndrome of
approximately 60%. It therefore fails to detect just under half of babies with
Down's syndrome.
The double test
The Double Test measures only two of the substances listed above and has a
detection rate for Down's syndrome of approximately 55%.
The quadruple test
This test measures all the substances listed above, but splits hCG into two
sub-units. It has a detection rate of approximately 65%.
The triple plus test
This test is offered by the Screening Service in Leeds (see below). It
measures uE3, AFP, hCG, plus neutrophil alkaline phosphatase (NAP). The
producers of the test state that it has a detection rate for Down's syndrome of
approximately 85%, although there is considerable controversy about this claim.
Screening centres
If your hospital does not offer blood serum screening and you decide you
want it, you can obtain it, for a fee, from either:
Antenatal Screening Service
The Medical College of St Bartholomew's Hospital
 020-7982 6293
or:
Down's Syndrome Screening Service
Leeds
0113-234 4013
Ultrasound scans
Ultrasound scans have been used routinely for some time to monitor the
baby's growth and development. They are non-invasive, give an instant result,
and are not thought to place the baby at any risk if used sparingly. More
detailed scans (high resolution scans) have recently become available in some
large hospitals. Studies suggest that certain factors (known as markers) seen
on a scan may give an indication of Down's syndrome. These include heart
defects and an abnormal collection of fluid behind the foetal neck (nuchal
oedema). Like the blood tests, high resolution scans cannot give a definite
diagnosis.
Positive test results
If a screen positive result is obtained, some parents will decide to have a
diagnostic test in order to get a conclusive result. While awaiting the outcome
of the diagnostic test it is important that parents give consideration to what
they will do if the result is positive. Others will decide to continue with the
pregnancy without further tests, particularly where the risk of miscarriage
following a diagnostic test is higher than the chance of having a baby with
Down's syndrome.
If a positive diagnosis of Down's syndrome is obtained, some parents will
decide to continue with their pregnancy, accepting the joys and challenges of
bringing up their child with Down's syndrome as part of their family. For some,
adoption may be a suitable alternative. Finding families to adopt babies with
Down's syndrome is generally not difficult. Others, will decide to terminate
the pregnancy. By this time a woman will generally be at least 20 weeks
pregnant. The main means of terminating a pregnancy at this stage is to induce
labour and deliver the baby. For help and advice, contact:
Support Around Termination for Foetal Abnormality (SATFA)
29-30 Soho Square
London
W1V 6JB
020-7436 5881 (helpline)
Conclusion
The Down's Syndrome Association and many of the members it represents do not
believe that having a baby with Down's syndrome is a reason to terminate a
pregnancy. However, they realise that parents need to make the right decision
for themselves.
It is important to appreciate that pre-natal tests may not always provide
reassurance that all is well. There may be little time to make difficult
decisions at a time of great shock and distress. If possible, try to consider
what to do if the tests results are positive before embarking on them. It may
be that the potential stress arising from pre-natal testing is too high a price
to pay for the information it provides.
If tests have already been carried out, with a screen positive result,
remember that this only indicates the chance of Down's syndrome. If a screen
positive result or a definite diagnosis of Down's syndrome is received, try
weighing up the relative risks of the various options.
Further information
Up-to-date leaflets, paperback books and videos can be obtained from the
Down's Syndrome Association. For help, support, information, and contact with
other parents, get in touch with either:
Down's Syndrome Association
155 Mitcham Road
London
SW17 9PG
020-8682 4001
 020-8682 4012
or:
Down's Syndrome Association - Scotland
158-160 Balgreen Road
Edinburgh
EH11 3AU
0131-313 4225
This information has been compiled by the Down's Syndrome Association in
response to the many queries received on the subject of pre-natal testing. The
information is presented in good faith and is, to the best of knowledge,
correct at the time of publication.
|
