Antenatal Screening and Diagnostic Testing for Downs
Syndrome
An information leaflet for pregnant women.
Choosing to have a screening test is an important individual decision for you and
your baby. You need to think carefully about your views of the condition and your
feelings about screening before you choose to have the screening test.
A screening test does not harm or affect a pregnancy, and is simple to perform. It is
offered to give you more information to help you to make an informed decision about
your individual chance of having a baby with Down’s syndrome. The result you
receive is only relevant to you and your current pregnancy.
Some women/couples choose to have screening for Down’s syndrome as they know
they would not be able to cope with a baby with Down’s syndrome, and they would
choose to terminate a pregnancy affected with Down’s syndrome. Others feel that
they need to know for sure so they can make plans for when their baby arrives.
Some women/couples decide not to proceed on to diagnostic testing and continue
their pregnancy not knowing for sure (this can be extremely hard to cope with
emotionally through the remainder of their pregnancy with an element of doubt
present). Some women/couple ask if it is possible to continue the pregnancy but
have the baby adopted.
Whatever your reason we will try to help, support and guide you through your
pregnancy.
Please read the rest of this information to ensure you are fully aware of what you
wish to do before embarking on the road of screening.
All women, whatever their age, have a small risk of delivering a baby with Down’s
syndrome. As maternal age increases the risk of having an affected child also
increases, this is due to the maternal eggs being older. Down’s syndrome is caused
by the presence an extra chromosome in the genetic material present in each cell of
the baby.
Down’s syndrome is the most common chromosomal abnormality and the most
common cause for severe learning disability. Having a baby with Down’s syndrome
can have a major impact on a family. For this reason all women are offered
screening for Down’s syndrome in every pregnancy. It is an optional test and you do
not have to have any screening if you do not wish too.
Please see the chart for your age related risk, but remember, these are purely age
related risks, before taking into account any biochemical markers in your blood or
nuchal translucency results from your scan.
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Risk of Down’s syndrome at term according to maternal age. (Cuckle et al 1987).
Maternal
age at
expected
date of
delivery
Risk of
having a
baby with
Down’s
syndrome
Maternal age
at expected
date of
delivery
Risk of
having a
baby with
Down’s
syndrome
Under 25
1:1500
37
1:218
25
1:1352
38
1:167
26
1:1286
39
1:128
27
1:1206
40
1:97
28
1:1113
41
1:73
29
1:1008
42
1:55
30
1:895
43
1:41
31
1:776
44
1:30
32
1:659
45
1:23
33
1:547
46
1:20
34
1:446
47
1:15
35
1:356
48
1:11
36
1:280
49
1:8
What is Down’s syndrome?
The structure of each human cell is made up of 46 chromosomes, in 23 pairs (23
from mum, and 23 from dad). On each chromosome the genetic make up of a person
is stored; this is what makes us all individuals. These chromosomes are vital for the
normal function of every part of the body.
In Down’s syndrome there is an extra chromosome making the total number present
47. This extra chromosome is the number 21, so instead of a pair of no 21
chromosome you have three, giving the medical terminology Trisomy 21 known as
Down’s syndrome.
It occurs by chance at conception and is irreversible. It is not a disease and people
with Down’s syndrome are not ill. Like all people, those with Down’s syndrome vary a
lot in appearance and ability. Most people with Down’s syndrome will walk and talk
and many will read and write and go to ordinary schools and lead fulfilling semiindependent lives.
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People with Down’s syndrome all have a certain degree of learning disability (mental
handicap). The degree of disability varies from person to person and it is impossible
to tell at birth what that degree will be.
A number of health problems are linked to Down’s syndrome such as heart problems
and reduced hearing and vision. Many of the problems can be treated, and frequent
health checks can make sure that any problems are picked up as early as possible.
Today people with Down’s syndrome can be expected to live to between 40 and 70
years of age on average. Alzheimer’s disease (a form of senile dementia) may affect
people with Down’s syndrome at an earlier age than other people.
Within Harrogate and District NHS Foundation Trust we offer two screening tests for
Down’s syndrome.
The first screening test is known as “Combined screening” and the second is
known as the “Triple Test”.
Combined Screening
This screening test is performed from 11+2 weeks to 14 weeks and 1 day of the
pregnancy. It involves a scan and a blood test. The scan allows the sonographer to
measure the length of your baby from the top of baby’s head to the bottom of its
bottom (this is called the crown rump length CRL). This gives an accurate gestation.
They will then measure the nuchal translucency (NT) which is the fluid under the skin
at the back of baby’s neck. The skin will appear as a white line, and the fluid under
the skin will look black. In some babies with Down’s syndrome there tends to be more
of this fluid than those babies not affected by Down’s syndrome.
The scan is usually done through your tummy, but occasionally it may be necessary
to have a vaginal scan, which will give better views. There is no risk to your baby and
it should not be too uncomfortable. This measurement of the nuchal translucency is
used along with the CRL and the information is added to the form used when you get
your blood taken. You will not be given a risk factor at the time of the scan.
The test can take up to an hour depending on whether the baby’s position is optimal
to allow the sonographer (the person who carries out the scan) to measure the
nuchal translucency, (the fluid under the skin on the back of the baby’s neck), so
please bear this in mind.
You are welcome to bring a partner or supportive friend with you. Due to the potential
length of the appointment and level of concentration required of the sonographers it
is not appropriate to bring children with you.
Following the scan if we successfully manage to obtain the measurements we need,
we will need to obtain a sample of your blood to send to the laboratory along with the
measurements we obtained. The blood test looks at two hormones in the blood both
produced by the placenta and unique to each pregnancy.
If the levels are favourable they will reduce the chance of your baby having Down’s
syndrome, if they are less favourable, they will increase your chance.
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Adding the blood test results to the scan results increases the overall accuracy of the
test. By sending the laboratory your scan measurements, it allows the laboratory to
generate a risk factor. Important factors we need from you to add to
this information are; they need to know if the pregnancy was an assisted conception,
and if so, was it IVF, Donor egg or other, along with the extraction/harvested date, we
need to get your weight (this is on the day the blood sample is taken) and is
important as blood serum marker levels appear to be decreased in women who
weigh more, and they are lighter in women who weigh less. We need to make sure
we have your ethnic group correct. Blood serum levels in Afro-Caribbean women
tend to be higher. We also need to know if or not you smoke. Smoking tends to
cause one of the serum levels to be lower.
The combined test will provide a detection rate of at least 75% (this mean it will pick
up around 7or 8 out of 10 babies, with Down’s syndrome. It also means it will miss 2
or 3 out of 10). The screen positive rate is <3% which means less than 3 out of 100
women in Harrogate will have a high risk result, but their baby will not have Down’s
syndrome.
An increased chance result would be one which is 1 in150 or less. (i.e.1 in 98)
anything over 1 in 150 would be classed as a low risk result. If you receive a low risk
result you will receive a first class letter in the post, we would not offer you any further
investigations Any woman who receives an increased risk will be contacted by the
screening co-ordinator and offered further investigations in the form of a diagnostic
test; chorionic villus sample (CVS). If you are offered this test it will be fully explained
to you.
If when you are scanned your baby’s length (CRL) is 84mm or more, we will be
unable to offer you the combined screening test. This is because your baby’s
measurements are too big for the calculation to take place. Therefore it is important
you book early with your midwife.
If you still wish screening for Down’s syndrome after this stage we would offer you
the triple test. You would be given a request slip of paper by the sonographer to take
to the antenatal clinic reception so an appointment can be made for the triple test.
Triple Test
This is a single blood test taken between 14 weeks and 20 weeks, however, the ideal
time is between 15-16 weeks. This test may highlight if you are at an increased
chance of having a baby with Down’s syndrome. It will pick up around 6 out of 10
babies with Down’s syndrome. This also means it will miss 4 out of 10. 5 or 7 out of
every 100 women in Harrogate will have a high risk result, but their baby will not have
Down’s syndrome. This is called the false positive rate.
It measures 3 hormones in your blood, and takes into account your age, ethnic
group, whether you have diabetes or not, whether you smoke and if you have had
any bleeding.
If your result is a risk of 1 in 200 or less, (i.e 1 in 108) this places you into the
increased chance group, and we would offer you a diagnostic test, such as an
amniocentesis. Anything over 1 in 200 would place you into the low chance group
and we would not offer you any further testing.
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You have to remember that should you receive an increased chance with either of
these screening tests, it does not say that your baby has Down’s syndrome. It is only
saying that for whatever the reason the marker used has placed you into the
increased chance group.
Multiple Pregnancies
When you are scanned if the sonographer finds that you are having a multiple
pregnancy, you will only get a dating scan. The sonographers will then give you the
telephone number for the screening co-ordinator for the maternity services for an in
depth discussion regarding screening for a multiple pregnancy. It is important that
you contact her as soon as possible so you can make a decision regarding screening
and whether you wish to continue along that road.
Combined Screening and Triple Test
Neither of these screening tests will tell you whether your baby has Down’s
syndrome they will only give you an individual risk factor as to your chance of having
a baby with Down’s syndrome.
The only tests that will confirm if your baby has Down’s syndrome are what we call
Diagnostic Tests such as an ‘amniocentesis or a chorionic villus sample’.
Amniocentesis
This diagnostic test is available at Harrogate Hospital.
The procedure is carried out in the ultrasound room in the Antenatal clinic. Firstly an
ultrasound scan is performed to ensure that the pregnancy is at the correct stage for
testing, and that we can see a clearly accessible pocket of fluid. The test can be
carried out anytime after the 15th week of your pregnancy.
The skin on your abdomen surrounding the area where the needle is to pass is
cleansed with an antiseptic solution. Following this, and with the aid of guided
ultrasound, a very fine needle is passed through your skin, into your womb, and into
the fluid that surrounds your baby. As the needle is very fine a local anaesthetic is
not used for this procedure. Indeed, a local anaesthetic probably stings more than
the procedure itself, and you cannot numb the lining of the womb. As the needle
passes through the lining of the womb you sometimes feel a sharp cramp like pain,
which settles once the needle is in position.
Approximately 15-20mls (2-3 teaspoons) of fluid is withdrawn using a syringe
attached to the end of the inserted needle. The fluid contains cells from the baby. No
harm is done to the baby with this needle. The fluid is essentially baby’s urine, which
surrounds the baby during the pregnancy. Baby swallows the fluid and then passes it
out again. The fluid that is removed will be replaced by the placenta within 24 hours.
When enough fluid is obtained the needle is removed and a plaster is placed over the
site (providing you are not allergic to elastoplast). The plaster can be removed later in
the day.
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The fluid is sent to the regional laboratory in Leeds where it will undergo analysis.
Quantitive Fluorescent Polymerase Chain Reaction (QF-PCR) allows a rapid result to
be available within 48-72 hours of the labs receiving the sample. Obviously if the
sample is taken on a weekend, or near to a Bank Holiday, this may affect the time
scale, and a result may not be available for a number of days. However, we will try
our best to avoid situations like that by choosing a suitable day for the test. The
laboratory will be able to get a result from this type of analysis in at least 99% of
cases. Very occasionally it may not be possible to get a result; this may be due to
technical problems with the sample, or if the sample is blood stained. The laboratory
will advise us of any difficulties with getting a rapid result, as it may be necessary to
grow the cells for a longer period of time.
As a QF-PCR has an extremely small chance of being wrong, the Leeds laboratory
also performs a long term culture on all the amniocentesis, which looks for more rare
chromosomal abnormalities, and will hopefully confirm the findings of the rapid result.
This result will normally be available 18 days from the date of the amniocentesis.
The passage of even a very fine needle into the womb (such as with an
amniocentesis); is associated with a small risk to the pregnancy, even if the
procedure is straight forward. The main complications are; miscarriage, or rupture of
the membranes – indicated by a loss of fluid from the vagina.
The risk of miscarriage is approximately 1 in 100 (1% nationally). This means that for
every 100 procedures carried out, 1 of the pregnancies will end in miscarriage, and
we are unable to determine which pregnancy this will be.
We advise you to rest for 24 hours following the procedure, although there is no
reason for bed rest. A simple analgesic such as paracetamol can be taken to ease
any discomfort. After 24 hours you can return to your normal routine.
It would be wise not to drive yourself home after the test, for this reason we ask you
to bring your partner, a member of your family or a friend to this appointment.
The risk of miscarriage can last 4-6 weeks following the procedure; obviously the
further through the 4-6 week period you get, the risk reduces quite dramatically.
Usually the first 7-14 days are the most crucial.
Chorionic Villus Sampling (CVS).
This diagnostic test is undertaken in Leeds at either of the Leeds Teaching Hospitals,
as it is a technically more difficult procedure. It can be carried out from 11 weeks
onwards. Please note we do not perform this test at Harrogate Hospital.
This procedure is an invasive test whereby a needle enters into the pregnancy, either
through your abdominal wall, or by use of forceps via your cervical canal. An
ultrasound scan is performed to assess where the placenta is within the womb. If you
are to have an abdominal procedure; the skin on your abdomen surrounding the area
where the needle is to pass is cleansed. Ultrasound is used throughout this
procedure so the doctor can visualise the placenta and your baby.
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As the needle used in this procedure is slightly bigger, than for the amniocentesis
you will be given a local anaesthetic into the area where the needle will be passed.
The local anaesthetic stings a little as it starts to takes effect. Once the doctor is sure
the local anaesthetic has worked, the needle is passed through your tummy and into
your womb. This time instead of going into the fluid surrounding the baby it is
directed into the placenta. Once the needle is in position, tiny villi from the placenta
are sucked out using a syringe. Once the doctor is sure they have enough of the
cells, the needle is removed. Your skin will be cleaned, and an elastoplast is applied.
The sample is sent to the regional laboratory in Leeds for analysis, and a result
should become available within 24-48 hours. The laboratory then goes on to grow the
sample to get a full karyotype (this is the baby’s genetic make up).
Chorionic Villus Sampling is associated with a small risk to the pregnancy, even if the
procedure is straight forward. The main complications are; miscarriage, or rupture of
the membranes – indicated by a loss of fluid from the vagina.
The risk of miscarriage for this procedure is, approximately 1 in 100 (1% this is
nationally). This means that for every 100 procedures carried out, 1 of the
pregnancies will end in miscarriage.
If the Doctor is unable to perform the CVS through your abdomen they may offer you
a Transvaginal CVS where the sample is obtained with fine forceps through the neck
of the womb. This procedure is associated with a 2% chance of miscarriage, and
would therefore not be offered routinely unless clinically indicated
We advise you to rest for 24 hours following the procedure, although there is no
reason for bed rest. A simple analgesic such as paracetamol can be taken to ease
any discomfort, providing you are not allergic to it. After 24 hours then you can return
to your normal routine.
It would not be wise to drive yourself home after the test, for this reason we ask you
to bring your partner, a member of your family or a friend to this appointment.
The risk of miscarriage can last 4-6 weeks following the procedure; obviously the
further through the 4-6 week period you get, the risk reduces quite dramatically.
Usually the first 7-14 days are the most crucial.
On rare occasions the result may be called a mosacism, this is where a number of
different cells have been identified, and a definitive result cannot be given this is very
rare, but has been known to occur. Under these circumstances depending upon what
the cells show, they may grow them for a longer period of time; alternatively they may
consider repeating the test.
The benefit of this test is that it can be performed earlier, and therefore should you
wish to make any decisions regarding your pregnancy, you are still at a relatively
early gestation.
It must be stressed that neither an Amniocentesis nor CVS is guaranteed to work
every time. There have been very rare occasions whereby the result given has
been wrong as the mother’s cells have been tested rather than the baby’s. Similarly
a normal result can never guarantee a normal baby as there are many other forms of
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disability that cannot be detected by ultrasound or diagnostic testing, and may not
become apparent until after the baby is born.



Please Remember
Screening will only give you a risk factor regarding the chance of you having
an affected baby. Even a low risk does not guarantee that your baby will be
normal.
Diagnostic testing will give you the information you may require but carries a
risk of miscarriage of the pregnancy.
Screening and diagnostic testing results are relevant to you only and are only
relevant to this pregnancy. They cannot be compared to a friend’s results or
pregnancy.
At the end of the day the decision you make regarding screening or diagnostic testing
is purely personal, based on how you and your partner feel regarding certain issues.
As long as you have thought about each stage of the screening or
diagnostic process and you as a couple are happy with the decision you make that is
all that matters.
Whatever you decide, you will be given help and support from the midwives working
within the Harrogate Unit.
Increased nuchal translucency when your baby does not have Down’s
syndrome.
It is important to know that there are other reasons why a baby’s nuchal may be
increased. If your diagnostic test has identified that your baby does not have Down’s
syndrome, we would automatically refer you for another scan (2-3 weeks after your
initial scan). If this demonstrates that the nuchal is still quite thick, or has begun to
getter thicker, then we would refer you too for a fetal cardiac scan at the Leeds
General Infrimary, as we know there is an association with heart problems.
If you decline screening for Down’s syndrome.
Please bear in mind that should you decline screening for Down’s syndrome and
when you are scanned, it is noted that the nuchal translucency is the equivalent to or
greater than 3.5mm, you will be told. This is due to the fact that it holds implications
for your baby. So please do not be shocked if this happens to you.
Should you wish to discuss anything in this booklet further please contact any of the
following;
Antenatal Clinic midwives; 01423 553010
Community Midwives (Hgte); 01423 553051
Ripon Midwives Answer phone; 01765 602546
Antenatal Screening Co-ordinator; 01423 555429
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Private Service
If you would prefer to go privately, there are now three private screening centres in
the region. All offer screening tests for Down’s syndrome at a cost of up to £190.
Genome (formerly the Leeds Screening Centre), they offer a wide variety of
screening tests.
The Yorkshire Obstetric Ultrasound and Diagnostic services, which are based in the
Nuffield Hospital in Leeds offer similar tests and York Pregnancy Care Service, also
offer similar tests but these two screening centres will only offer combined screening.
All of these centres offer combined screening, however, at Genome, the tests can be
performed at a later gestation, and contain the testing of more hormone levels.
Below are the contact numbers and web-site addresses.
Genome Screening Service Tel: 0113 262 1675
www.leeds.ac.uk/lass
The Yorkshire Obstetric Ultrasound and Diagnostic Services. Tel :0113 3882067
www.nuchalscans.co.uk
York Pregnancy Care service. Tel: 01904 520102
www.yorkpregnancycare.com
Other useful web-site addresses and contact numbers
Antenatal Screening Web Resource (AnSWeR)
www.antenataltesting.info
Personal Experiences of Health and Illness
www.dipex.org
Antenatal Results and Choices (ARC)Tel: 020 7631 0285
www.arc-uk.org
Down’s Syndrome Association Tel: 0845 230 0372
www.dsa-uk.com
References
Antenatal Results and Choices. Website: www.arc-uk.org
UK National Screening Committee. ‘Testing for Down’s syndrome in pregnancy’
National Down’s syndrome Screening Committee, Oxford, England.
If you require this leaflet in an alternative language or
format, such as Braille, audiotape or large print,
please contact the Maternity Department
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