here - Health Promotion Agency

Antenatal care and antenatal classes
SCREENING FOR DOWN’S
SYNDROME
58
By 2004 all women should be
offered serum screening as part of
their antenatal care. 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
This test may be offered from 16
weeks of pregnancy:
• to women who have an AFP,
serum screening or nuchal
translucency scan result which
indicates an increased risk of
Down’s syndrome or spina bifida;
• when an ultrasound scan detects
an abnormality which is associated
with a genetic disorder;
•
when a woman’s past or family
history suggests that there may be
a risk of her baby having a genetic
or chromosomal disorder such as
Down’s syndrome.
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. 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