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Down syndrome
Clinical Genetics
www.nuh.nhs.uk
If you require a full list of references for this leaflet please email
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ext. 67184.
The Trust endeavours to ensure that the information given here
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This document can be provided in different languages and
formats. For more information please contact:
Clinical Genetics Service
City Hospital
The Gables, Gate 3
Hucknall Road
Nottingham NG5 1PB
Adapted from leaflet by Clinical Genetics at Guy’s and St Thomas’ NHS Foundation
Trust.
Tel: 0115 962 7728
Nicky Drury, Clinical Genetics © September 2015. All rights reserved. Nottingham
University Hospitals NHS Trust. Review September 2017. Ref: 1040/v2/0915/AM.
Public information
Introduction
Down syndrome is not...
This leaflet is for:
 Parents who have had a baby with Down syndrome
 Couples whose pregnancy has been diagnosed with Down
syndrome.
Some aspects of this leaflet may not apply directly to your
situation.
...caused by anything you did or did not do in your pregnancy,
as it happens by chance. Also, there are no known
environmental factors which cause Down syndrome.
What is Down syndrome?
Down syndrome is a common chromosome disorder which
causes a range of medical and developmental problems. Like
everyone, people who have Down syndrome are individual and
vary in appearance, personality, ability and health. People with
Down syndrome all have a degree of learning difficulty, but it is
impossible to tell at birth how significant this will be. The
condition is also associated with hearing and/or vision problems,
but these can often be treated. Some of those affected with
Down syndrome will need some level of long-term help and
support, but many will lead fairly independent lives.
Down syndrome with a heart defect
About 40 per cent of children with Down syndrome have an
associated heart defect. This can vary from small ‘holes’ in the
heart (atrial or ventricular septal defects) to more complex
problems that may require surgery. Early symptoms such as a
failure to thrive (slow growth) may indicate a heart problem.
Sometimes there may be no symptoms for several months, so
all babies born with Down syndrome should have an
echocardiogram to look at the heart. If the baby does have a
heart defect, the exact nature and likely treatment needed will
be explained at the time of diagnosis.
If Down syndrome is diagnosed prenatally (during pregnancy), a
fetal heart scan will identify in most cases whether a heart
problem is present. This can be performed at a specialist fetal
cardiology centre from 20-24 weeks into the pregnancy or
whenever the diagnosis is made after that time.
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Further information and support
ARC: Antenatal Results and Choices
Website: www.arc-uk.org
Helpline: 0845 077 2290 or 0207 713 7486 (Mon - Fri 10am to
5.30pm)
Email: [email protected]
Down’s Syndrome Association
Website: www.downs-syndrome.org.uk
Helpline: 0333 1212 300 (Mon - Fri 10am to 4pm)
Email: [email protected]
The Down’s Heart Group
Website: www.dhg.org.uk
Phone: 0300 102 1644
Email: [email protected]
Contact a Family
Website: www.cafamily.org.uk
Phone: 020 7608 8700
Helpline: 0808 808 3555 (Mon - Fri 9.30am to 5pm)
Mosaic Down Syndrome UK
Website: www.mosaicdownsyndrome.org
Children's Heart Federation
Website: www.chfed.org.uk
Infoline: 0808 808 5000 (Mon - Fri 9.30am to 4.30pm)
Office: 0207 422 0630
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Testing for Down syndrome in pregnancy
What causes Down syndrome?
Some people want to know if their baby will have Down
syndrome and others do not. In pregnancy there are screening
tests which are offered and these can tell you whether or not
there is an increased chance of your baby having Down
syndrome.
Screening tests often combine a number of calculations,
including your previous history. If you have already had a child
or pregnancy with Down syndrome, the result is likely to show
that you are at increased risk of having another baby with Down
syndrome, even before any other tests or measurements are
taken.
The only way to know for certain whether or not your baby will
have Down syndrome is to have a diagnostic test. Two tests
are available: chorionic villus sampling (CVS) and
amniocentesis. Both tests check the number of the baby's
chromosomes. There are leaflets available which explain these
tests in more detail.
Inside all our cells we have 23 pairs of chromosomes (making
46 chromosomes in total). The chromosomes are packages of
genes, which give our bodies the instructions for how to
develop. For each chromosome pair we normally inherit one
chromosome from our mother and one from our father (via the
sperm and egg). We therefore have two copies of every gene;
one from each parent. If a baby has too few or too many
chromosomes, this can lead to problems with their development.
Down syndrome is always caused by the presence of an extra
chromosome number 21. However there are three different
ways that the extra chromosome can be present.
What happens if Down syndrome is diagnosed
during a pregnancy?
If the result of a diagnostic test shows that your baby has Down
syndrome, then you will be able to prepare for the arrival of a
baby with the condition, or consider the option of terminating the
pregnancy.
Some people find that it is difficult to decide what to do in this
situation, whereas others are sure about what they want to do. It
is important that you get the chance to ask all the questions that
are on your mind, and that you have enough time and space to
make your decisions without feeling rushed or pressured. Many
people feel a mixture of emotions in this situation, including
anger, guilt and sadness. For some people it helps to talk things
through. Various organisations that can offer further information
and support are listed on the next page of this leaflet.
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1. A third copy of chromosome 21 (trisomy 21)
The majority of people with Down syndrome have an extra copy
of chromosome number 21 (i.e. three copies) so in total they
have 47 chromosomes instead of 46 as shown in the diagram.
This is known as primary trisomy 21.
Anyone can have a baby with this form of Down syndrome. In
most cases, the extra chromosome is present due to a error that
occurred in either the egg or the sperm that went to make that
baby. This is usually a one-off occurrence and is not inherited
from the parents, who usually have normal chromosomes
themselves. It is not known why this happens, but it is slightly
more likely to occur in babies of older mothers.
If your pregnancy or child
has Down syndrome
caused by a third copy of
chromosome 21, the
chances of it happening
again are about one in 100
(1%) depending on your
age at the time of
conception.
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2. Mosaic Down syndrome
A few individuals have the mosaic form of Down syndrome. This
means that some cells in the body have 46 chromosomes as
normal, while others have 47 (the extra chromosome is a third
copy of 21 as described above). In this form the severity and
extent of the condition depends on the proportions of these two
cell types. It is very difficult to predict which features of Down
syndrome someone who is mosaic will have, but mosaic Down
syndrome will usually be milder.
3. Chromosome rearrangement
In a small number of cases (about three per cent) the extra
chromosome 21 is attached to another chromosome (e.g.
chromosome 14). This is present in addition to the two normal
copies of chromosome 21, so that in total there are three copies
of chromosome 21. This is known as an unbalanced
chromosome translocation.
In these situations one of the parents may be a 'carrier' of a
balanced translocation where one copy of chromosome 21 is
attached to another chromosome. However the parent only has
two copies of chromosome 21 in total, so they do not have
Down syndrome themselves.
A parent who carries this balanced translocation may be
unaware of it. They will have no problems with their health as
they have the correct number of chromosomes and therefore the
correct amount of genes, just in a different place. When Down
syndrome is diagnosed, the chromosomes of the parents will be
checked if the baby has an unbalanced translocation.
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The diagram below on the left shows an example of an
unbalanced chromosome translocation that has caused Down
syndrome. The diagram below on the right is an example of a
balanced translocation.
What can happen in a pregnancy if one parent
carries a balanced translocation?
Each time a balanced translocation carrier has a pregnancy
there are several possibilities:
1. The baby may inherit a normal set of chromosomes and will
be healthy.
2. The baby may inherit the same balanced translocation as the
parent and will be healthy.
3. The baby may inherit an extra chromosome 21 resulting in
Down syndrome.
4. The baby may inherit too much of another chromosome (like
chromosome 14). In this case the pregnancy is likely to
miscarry, or the baby could be born with more severe health
problems.
If a parent is a translocation carrier then the chance of having
another baby with Down syndrome will depend on the
translocation involved.
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