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Apart from the routine blood and urine tests, certain other tests
may be done during your pregnancy.
Some of these tests can be extremely useful for women at higher
risk of carrying a child with certain birth defects. It is important
however that you know exactly what the tests involve, how they
are done and what are the risks involved.
ULTRASOUND
Using sound waves, an ultrasound
scan is done to obtain a picture of the baby in the uterus. There
are many reasons why ultrasound scans are done:
· To determine the baby's
exact age if you are unsure about the date of your last period.
· To determine certain abnormalities.
· To evaluate the health of the baby.
· To diagnose twins.
· To determine with accuracy the position of the baby.
· To assess the condition and position of the placenta,
for example if you have been bleeding.
· To locate the fetus and the placenta before amniocentesis.
As far as we know now ultrasound
is safe for the baby, although questions have been raised about
the possible effects on the baby's hearing. It makes sense therefor
to use ultrasound only when necessary and not to make fun pictures
for the photo album or just as a routine. When ultrasound is
used, the length of time you are exposed should be limited.
AMNIOCENTESIS
Amniocentesis is the withdrawal
of a sample of amniotic fluid from around the baby. Under a local
anaesthetic and guided by ultrasound a needle is inserted through
the abdominal wall into the uterus and some fluid is obtained.
This fluid is full of cells from the skin and other organs of
the baby. It can be used to detect abnormalities of the central
nervous system (spina bifida and anencephaly) or chromosome disorders
such as Down syndrome.
Because of the small but definite
risk of miscarriage, there is no point in having amniocentesis
unless there is an increased risk of producing a baby with abnormalities.
Most Western countries now offer amniocentesis to pregnant women
who are 35 and over. Amniocentesis is also often done if the
serum APF of the mother is raised, if the parents have had a
previous child with chromosomal defects or if the parents carry
certain genetic disorders.
Amniocentesis is usually done
at 16 weeks or later. One of the major disadvantages of amniocentesis
is that if you choose to have an abortion, following abnormal
results, the pregnancy will have advanced to 19 or 20 weeks (it
takes about 2-3 weeks before the test results come back). It
is always a difficult decision to have an abortion. If however
that decision must be made at 20 weeks and you are already feeling
the baby moving than it is even harder. Women in this situation
need a lot of support from their family and they should be offered
counseling.
APF SCREENING
Alpha-feto-protein (APF) is a
substance produced in the early phases of pregnancy by the yolk
sac of the embryo and later on by the fetal liver. It is normally
present in the blood of all pregnant women. It reaches its highest
level at the 18th week of pregnancy. An AFP test can be simply
performed by doing a blood test by the mother.
Abnormal high levels of AFP can
indicate a possibility of spina bifida in the baby. There are
some other reasons why the amount of AFP is abnormally high.
If your dates are wrong and the pregnancy is more advanced than
you think AFP levels may seem high. If you are expecting twins,
your AFP levels are also raised. In both cases, ultrasound can
be used to tell the real age of the baby and whether you are
expecting twins. If the AFP level appears to be high for no apparent
reason another blood test may be done to double-check. If the
level is then found still high, you will probably be offered
amniocentesis. Fortunately, more than 95% of the pregnant women
with raised levels of AFP deliver healthy babies.
CHORIONIC VILLI SAMPLING (CVS)
CVS is used to detect genetic
or chromosomal defects in the fetus early in pregnancy, at about
10 weeks. CVS is done, under ultrasound guidance, by inserting
a long thin tube into the uterus to obtain fetal tissue for chromosome
analysis. The tissue is taken from the early placenta. The test
is done either through the vagina and the cervix or the abdominal
wall.
The results from the test are
available sooner than with an amniocentesis, but the rate of
miscarriage is slightly higher and the results may not be so
accurate.
CVS is, like amniocentesis, offered
to women who are at higher risk of carrying a baby with chromosomal
disorders like Down syndrome. Furthermore, it is offered to parents
who carry certain genetic disorders and to women who previously
had a baby with a chromosomal abnormality. CVS can not be used
to detect neural tube defects.
The advantage of CVS over amniocentesis
is that it can be performed earlier in pregnancy and the test
results are available earlier. If there is an abnormality detected
and you decide to have an abortion, it is still early enough
to have a curettage operation. In addition, at this early stage
you have not felt the baby moving yet. The disadvantage of CVS
is that the miscarriage rate is slightly higher than with an
amniocentesis.
UMBILICAL VEIN SAMPLING
During an umbilical vein sampling
a needle is passed through the skin and uterus of the pregnant
woman and, under guidance of ultrasound, into the blood vessel
of the baby, usually in it's umbilical cord, to withdraw blood.
Fetal blood sampling can be done from 18 weeks of pregnancy onwards.
The results of the blood test are available in a few days.
Reasons for having an umbilical
vein sampling include:
· To check for sickle-cell
anaemia or haemophilia.
· An abnormality found on ultrasound that may be the result
of an underlying chromosomal disorder.
· Some intra-uterine infections, e.g. toxoplasmosis.
· A baby with reduced growth, to test for underlying chromosomal
abnormality or for oxygen and carbon dioxide levels in the baby's
blood.
· A major increase or decrease in the amniotic fluid volume.
The complications that can occur
are potentially the same as for amniocentesis. Other complication
is bleeding from the cord after withdrawal of the needle.
Whatever test you need to undergo
during your pregnancy, make sure the whole procedure and the
risks involved are explained to you. Write down questions that
you want to ask and do not be afraid that you take up too much
time. Remember you are asking information about procedures done
on your body and concerning your baby.
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