Amniocentesis is the removal of amniotic fluid through
a needle.
The needle is inserted through the pregnant woman's
abdomen into the uterus and amniotic sac.
Advertisement
The test is performed in order to gain information
about the foetus.
The aspirated fluid contains foetal cells
which are examined for possible abnormalities.
The test is usually performed during the 14th and 18th week of pregnancy.
Alternate names
"amnio", amniocentesis test
What is amniocentesis?
Amniocentesis is a procedure whereby a small amount
of amniotic fluid from the amniotic sac is removed. The amniotic sac
contains the foetus suspended in the amniotic fluid. The test can be performed at
different times during pregnancy as from 14 weeks onwards. If done between the 15th and 18th week of
pregnancy, the test can give important information about the
chromosomes and possible developmental defects of the
baby.
If performed later in pregnancy, it can measure the
maturity of the baby's lungs and determine whether the baby may have
problems with breathing.
Amniocentesis is a fairly common (but not routine) test.
Who needs amniocentesis?
A doctor may recommend the test if a pregnant woman has already had a
previous child with a genetic problem
(such as cystic fibrosis or sickle cell disease), or if she is 35 years or
older as the risk of foetal chromosomal abnormalities increases with
age.
If there is an indication to deliver the baby earlier than the due date, the doctor may sometimes recommend
amniocentesis to check for lung maturity.
How is amniocentesis performed?
Before the
amniocentesis, a detailed ultrasound is performed first. The foetal measurements are taken, including the head
diameter and circumference, the abdominal circumference and the length of the
femur (thighbone). When combined mathematically, these measurements give an
estimate of gestational age (age of the baby from the time of conception) to
within two weeks. The previously estimated due date will not change unless the measurements indicate a
difference of at least ten days.
Next, a detailed anatomic survey of the foetus is carried
out. This includes the anatomy of the head, brain, spine, chest, heart
structure, stomach, gall bladder, kidneys, bladder, umbilical cord, arms and
legs. The location of the placenta and the amount of amniotic fluid are
assessed, and the maternal organs are examined for any uterine fibroids or
ovarian cysts. The cervix will be scanned to
identify any dilatation or where it is in relation to the placenta.
Finally, an amniocentesis site is chosen under
ultrasound guidance to avoid any contact with the placenta and
baby. This is only a preliminary site and may be
changed just before insertion of the needle if the baby has moved in the
interim. The skin area around the site is then cleaned with alcohol or
Betadine and the ultrasound probe is placed on to the
tummy again to look at the chosen path of the needle.
The doctor then inserts a needle through the abdomen and uterine wall into the fluid-filled amniotic sac. Most
doctors will not use a local anaesthetic, as the anaesthetic needle usually causes more discomfort than the
amniocentesis needle. A small amount of amniotic fluid (about 30 cc or two tablespoons) is withdrawn and sent
for testing.
Most women find amniocentesis about as uncomfortable as
having blood drawn or as experiencing mild menstrual cramps. The needle is generally in and out of the abdomen
in less than two minutes, although the entire procedure may take up to
an hour. This is to allow for monitoring of the baby's heart rate and for further
explanations.
What tests are conducted on the amniotic fluid?
Once
the fluid has been withdrawn it is sent to a laboratory for two
different types of testing:
Chromosome analysis: The
cells in the fluid consist of cells from the amniotic membrane (amniocytes)
and foetal skin cells. As they are not actively growing, the cells need to be
cultured and stimulated which takes between two and three
weeks. After that, the cells are
harvested and treated to make their chromosomes visible. The cells are then
broken to release the chromosomes, which are stained so that the
geneticist can see the bands on each chromosome and
identify them. The chromosomes are counted and matched up in 23 pairs. They
are then examined for evidence of missing or extra pieces. This test also
determines the baby's sex. The most common reason for chromosome analysis
is to look for evidence of trisomy (three copies of a chromosome instead of
the expected two). The trisomies 13 (Patau´s syndrome), 18 (Edward's syndrome)
and 21 (Down syndrome) are the most common trisomies identified. More than one
in twenty foetuses have evidence of trisomy if the mother is over 45 years
old.
Alpha-fetoprotein (AFP):
This test can help determine if there is an opening in the foetal skin. The
most common place for this to occur is the spine. This would suggest a neural
tube defect such as spina bifida (walls of spinal canal are not properly
fused) or anencephaly (absence of brain and part of the skull). A woman who is taking valproic
acid or carbamazepine for a seizure disorder has an increased risk of a neural tube defect in her
baby. The AFP test can also be performed as a blood test from the expecting
mother, however, elevated levels in the amniotic fluid are a better predictor
of possible congenital defects in the baby.
What are the advantages of amniocentesis?
Amniocentesis is a relatively safe procedure in the hands
of
an experienced doctor. In most cases, the obvious benefits of knowing about any abnormality of the foetus prior to
birth outweigh the risks of the procedure.
Before the test, both
parents should get adequate counselling to understand the significance of any potentially abnormal results.
What are the risks of amniocentesis?
Miscarriage. This happens spontaneously in 15-20% of all pregnancies, usually in early pregnancy and
before the time that amniocentesis would be performed. After amniocentesis, the risk of miscarriage is increased
by less than 1% for the remainder of the pregnancy.
Discomfort. Cramping may occur during the amniocentesis and for a short while afterwards. This is
not significant. Bleeding and leaking of amniotic fluid occur only rarely and may infrequently be followed by
miscarriage.
Infection. This is very rare. If it does happen, it usually occurs 24 to 72 hours after the
procedure.
Possible development of Rhesus sensitisation. Rh-negative women will be given an injection of Anti-D
(Rh) immunoglobulin just after the amniocentesis. This medication can prevent Rhesus factor complications with
future pregnancies.
Repeat testing required. In
less than 10% of cases, a second needle insertion is required. This does not
necessarily mean there is anything wrong with the baby. Sometimes it is better to wait a week before a second
attempt is made. The second attempt is almost always successful.
What happens when the results of the amniocentesis
become available ?
It will take two to three weeks before the results of the test become
available. Since amniocentesis is an invasive procedure, it is usually preceded
with non-invasive investigations (detailed ultrasound scan, blood tests), and counselling of the
expecting mother or both parents about the next step to be done.
If any foetal abnormalities have been detected, a
full explanation will be given by the doctor which will include all the
possibilities ranging from termination of pregnancy to neonatal surgery
depending on the degree and severity of the congenital defect.
Reviewed by Prof Bert Schaetzing, MD, FCOG(SA), FRCOG, PhD, part-time consultant, Dept of Obstetrics & Gynaecology, Faculty of Health Sciences, University of Stellenbosch.
Bookmark with:
What are social bookmarks?