here - Health Promotion Agency

Antenatal care and antenatal classes
For more information
about Down’s syndrome and
about testing for it, so you
can decide whether to have
the tests read the leaflet
Testing for Down’s
syndrome in pregnancy,
which is available at
www.screening.nhs.uk/
downs/home.htm
HAEMOPHILIA
AND MUSCULAR
DYSTROPHY
Some disorders, such as
haemophilia and muscular
dystrophy, are only found in
boys (although girls may
carry the disorder in their
chromosomes and pass it on
to their sons). Tell your
doctor if these or other
genetic disorders run in your
family, as it may then be
important to know your
baby’s sex.
58
SCREENING FOR DOWN’S
SYNDROME AND OTHER
GENETIC DISORDERS
All women should be offered
screening as part of their antenatal
care.
BLOOD TESTS
Serum screening is the term used for
a test of the mother’s blood which
screens for Down’s syndrome. It
combines the AFP result (and so
gives information about the risk of
spina bifida) with the measurement
of other blood chemicals to give the
relative risk of having a baby with
Down’s syndrome. Serum screening
is not helpful in twin and other
multiple pregnancies.
Some maternity units give the
result as ‘lower risk/screen negative’
or ‘higher risk/screen positive’.
A negative result means that you are
at a low risk of having a baby with
Down’s syndrome. A positive result
means that you are at a higher risk
of having a baby with Down’s
syndrome. For example, any
level higher than 1:250 is usually
said to be a high risk. This is the
recommended cut-off level.
However, a risk of 1:100 is still only
a 1% chance of the baby having
Down’s syndrome, and 99% chance
of it not. An amniocentesis or CVS
will be offered to give you a definite
diagnosis. You may compare this risk
to that for your age (about 1:900 at
30) or to the risk of miscarriage with
amniocentesis (about 1:100). Your
doctor or midwife will explain the
significance of the result to you.
AMNIOCENTESIS
After counselling, this test may be
offered from 14 weeks of pregnancy:
•
to women who have a serum
screening which indicates an
increased risk of Down’s syndrome;
• when an ultrasound scan detects
an abnormality which is associated
with a genetic disorder;
•
when a woman’s past history or
family history suggests that there
may be a risk of her baby having a
genetic or chromosomal disorder
such as Down’s syndrome, sickle
cell disorders or thalassaemia.
It should always be performed using
ultrasound to check the position of
the baby and placenta. Whilst
continuing to scan with the
ultrasound probe, a fine needle is
passed through the wall of the
abdomen into the amniotic fluid
which surrounds the baby. A small
sample of this fluid is drawn off and
sent to the laboratory for testing.
Most women feel only mild
discomfort.
Within the fluid are cells which
contain the same chromosomes as
the baby. Looking at these
chromosomes is a complex process,
which is why the results take up to
three weeks. This test will reveal
your baby’s sex. Tell your doctor
whether or not you want to know
what it is.
Amniocentesis is associated with a
0.5–1% risk of miscarriage. At most,