Exam Overview
Amniocentesis involves the collection and analysis of an
amniotic fluid sample.
Before an amniocentesis (and often during the entire procedure), a
fetal
ultrasound test is done to locate the fetus, the
placenta, and a large pocket of amniotic fluid. A
long, thin needle is inserted through the abdomen into the uterus, avoiding the
fetus and placenta, and a small amount [1 Tbsp (15 mL) to 2 Tbsp (30 mL)] of amniotic
fluid is collected through the needle.
See a picture of an
amniocentesis.
Amniotic fluid contains cells that have been shed by the developing
fetus. These can be examined for chromosomal defects that cause conditions such
as
Down syndrome and
cystic fibrosis. Amniotic fluid can also be used to
identify the sex of the fetus.
Rh disease
Amniotic fluid can be tested for a fetus's
Rh factor. (Samples from the mother's and father's
blood are tested and compared.) If the fetus is Rh-negative, no further testing
or treatment is necessary during the current pregnancy.
Chemicals in the amniotic fluid can show whether an Rh-positive
fetus is being harmed by the mother's
antibodies to the Rh factor.
Bilirubin is naturally found in amniotic fluid. During
a normal pregnancy, the bilirubin level increases until 23 to 25 weeks of
pregnancy, and then decreases throughout the remainder of the pregnancy. If
fetal blood cells are being attacked by the mother's
immune system, the bilirubin level continues to
increase throughout the pregnancy. After the 20th week of pregnancy, bilirubin
measurements are accurate enough to guide further testing or treatment.
Fetal lung maturity
Amniotic fluid can also be tested for substances from the fetus's
lungs. Test results show how mature the lungs are. This is important
information if the baby has to be born early.
Why It Is Done
Amniocentesis may be done to:
- Learn the fetal blood type and Rh factor. An
Rh-negative fetus is not at risk for Rh disease, even if the mother is
Rh-sensitized.
- Check fetal health when the mother is
sensitized to the Rh factor. (Doppler ultrasound, when done by a well-trained provider, can give you the same
anemia information without the risks of
amniocentesis.2 Doppler ultrasound is not available
everywhere.)
- Learn whether and when fetal blood sampling is
needed.
- Learn whether fetal lungs are mature if the fetus needs to
be delivered earlier than 37 weeks (preterm birth).
Results
When maternal Rh antibody levels are too high, an amniocentesis may
be done. The amount of bilirubin found in the amniotic fluid is used to predict
the level of fetal harm that is occurring from Rh sensitization.
- Amniocentesis is repeated every 2 to 4 weeks if
the fetus is mildly affected. The fetus is usually delivered close to
term.
- Amniocentesis is repeated every 1 to 2 weeks if the fetus is
being moderately affected. The fetus is usually delivered earlier than 38 weeks
and may need a
blood transfusion after
birth.
- Amniocentesis may be repeated every week if the fetus is
being severely affected. The fetus may need a blood transfusion before birth
and is usually delivered early.
The results of amniocentesis may guide treatment for Rh
sensitization. The level of fetal lung maturity may help plan the timing of
delivery.
- If the bilirubin levels are very high but the
fetus is less than 32 weeks'
gestation, a fetal blood transfusion before birth may
be done to keep the fetus healthy until delivery is possible.
- If
bilirubin levels are very high, and the fetus is older than 32 weeks' gestation
but the fetus's lungs are still immature, medicine may be given to speed up
fetal lung development. Delivery is ideally done 2 days later.
- If
fetal lungs appear to be mature, then delivery does not need to be
delayed.
What To Think About
Amniocentesis carries a slight risk of injuring the fetus, starting
labor, or introducing an infection into the uterus.
When amniocentesis is done by a highly trained provider, the risk
for
miscarriage may be as low as 1 in 400, according to
one study.1 Some studies have shown higher risks,
between 2 and 4 in 400.3
For checking anemia in a fetus, Doppler ultrasound can give you the
same information as amniocentesis, without the risks.2 Doppler ultrasound can only be done by a well-trained
provider and is not available everywhere.
A simple test of the mother's blood, for information about her
fetus's blood type, may be available in the future.
Amniocentesis is less sensitive than fetal blood sampling (FBS).
However, because it is less risky than FBS, amniocentesis is the preferred test
for detecting mild to moderate Rh disease.
Normal results from amniocentesis do not guarantee that the baby
will be healthy.
Amniocentesis may cause mixing of the mother's and fetus's blood.
Therefore, unsensitized Rh-negative women are given Rh immune globulin after
amniocentesis to prevent sensitization.
Complete the medical test information form (PDF)(What is a PDF document?) to help you prepare for this test.
References
Citations
- Caughey AB, et al. (2006). Chorionic villus sampling compared with amniocentesis and the difference in the rate of pregnancy loss. Obstetrics and Gynecology, 108(3): 612–616.
- American College of Obstetricians and Gynecologists
(2006). Management of alloimunization during pregnancy. ACOG Practice Bulletin
No. 75. Obstetrics and Gynecology, 108(20):
457–464.
- Seeds JW (2004). Diagnostic mid trimester
amniocentesis: How safe? American Journal of Obstetrics and Gynecology, 191: 608–616.
Credits
| By | Healthwise Staff |
|---|
| Primary Medical Reviewer | Sarah Marshall, MD - Family Medicine |
|---|
| Specialist Medical Reviewer | William Gilbert, MD - Maternal and Fetal Medicine |
|---|
| Last Revised | January 27, 2010 |
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Caughey AB, et al. (2006). Chorionic villus sampling compared with amniocentesis and the difference in the rate of pregnancy loss. Obstetrics and Gynecology, 108(3): 612–616.
American College of Obstetricians and Gynecologists
(2006). Management of alloimunization during pregnancy. ACOG Practice Bulletin
No. 75. Obstetrics and Gynecology, 108(20):
457–464.
Seeds JW (2004). Diagnostic mid trimester
amniocentesis: How safe? American Journal of Obstetrics and Gynecology, 191: 608–616.
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