Test Overview

Amniocentesis is a test to look at the fluid that surrounds your
baby (fetus) in the uterus. Amniotic fluid has cells and other
substances that can give clues about the health of your fetus.

For this test, a needle is put gently through your belly into your
uterus. About
2 Tbsp (30 mL) of fluid is taken out and looked at.

This test may be done
between weeks 15 and 20 to look at the baby’s genes. It can also be used later in pregnancy to see how the baby is
doing.

Amniocentesis for birth defects testing

Amniocentesis is often done around
week 16 to see if a fetus has certain types of birth defects.
It can also tell the sex of a fetus.

Amniotic fluid
has cells that have been shed by your growing fetus. The cells are checked
for the number and size of
chromosomes. This can show if there are any
problems that put the baby at risk for certain conditions, such as Down syndrome. But this test can’t find many common birth
defects. These include
cleft lip,
cleft palate, heart problems, and some types of intellectual disability.

Testing is most
often done as soon as possible so women and their families have time
to think about their options.

Amniocentesis in late pregnancy

Amniocentesis may be done in the third trimester if your doctor thinks you may have an infection of the amniotic fluid (chorioamnionitis). Or your doctor may do amniocentesis in late pregnancy to check your baby for certain types of infections or other rare problems.

In some cases, this test may be used to find out how much your baby’s lungs have developed.

Amniocentesis may also be done late in pregnancy to remove some amniotic fluid if too much has built up in the amniotic sac (polyhydramnios).

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Why It Is Done

Amniocentesis
may be done during your second trimester
to find certain birth defects.

You may choose to have this test because:

  • You are concerned about your age. As you get older, you have a greater chance of having a baby with a birth defect.
  • You want to know for sure if your baby has a certain health problem. This may help you decide early whether you want to continue your pregnancy or make plans to care for a sick child.

  • You or the baby’s father carries an abnormal gene that is known to cause a disease. These diseases include Tay-Sachs disease,
    sickle cell anemia, and
    cystic fibrosis.
  • You or the baby’s father has a family history of a genetic disorder or birth defect.
  • Screening tests suggest that your chance of having a baby with a genetic disorder or birth defect is higher than average.

Amniocentesis can tell the sex of your baby. This is
important when you or the father may pass on a disease that occurs
mainly in one sex (sex-linked). Examples include
hemophilia and
Duchenne muscular dystrophy. These both occur
mainly in males.

Amniocentesis may be done in the third trimester if your doctor thinks you may have an infection of the amniotic fluid (chorioamnionitis). Or your doctor may do amniocentesis to check your baby for certain types of infections or other rare problems.

How To Prepare

You may be asked to empty your bladder
just before the test.

You will be asked to sign a consent form that
says you understand the risks of the test and agree to have it done.

Talk to your doctor if you have any concerns about the need for the test,
its risks, how it will be done, or what the results will mean. To help you
understand the importance of this test, fill out the
medical test information form (What is a PDF document?).

How It Is Done

Amniocentesis is done by your
obstetrician in the doctor’s office or in the hospital.
You probably won’t need to stay overnight in the hospital unless problems occur
during the test.

You will
lie on your back on an exam table. Your
lower belly will be cleaned with a special soap.

Your doctor
will use a fetal ultrasound to check the position of your fetus and the
placenta. Ultrasound uses
sound waves to make a picture of the uterus, your fetus, and the placenta on a
TV screen. Your fetus’s heart rate can also be watched during the test using
ultrasound. For more information, see the topic
Fetal Ultrasound.

With the
ultrasound picture as a guide, your doctor will gently put
a thin needle through your belly and into your uterus. This will not hurt your
fetus or the placenta. If your fetus moves too close to the needle, the doctor will take the needle out and try again in another spot.

The doctor will take about 2 Tbsp (30 mL) of
amniotic fluid. Then the
needle is removed. The site is covered with a bandage.

The whole test takes
about 15 minutes. The thin needle is only in your belly for 1 to 2 minutes.
Your fetus’s heart rate and your blood pressure, pulse, and breathing will be
checked before, during, and after the test.

How It Feels

You will feel a sharp sting or burn in
your belly where the needle is put in. This lasts for only a few seconds.
When the needle is put into your uterus, you will feel a sharp cramp for
a few seconds.

When the amniotic fluid is taken out, you may get a
feeling of pulling or pressure in your belly. To keep yourself comfortable,
breathe slowly and relax your belly muscles during the test.

Risks

Amniocentesis is mostly very safe. There is a
small chance (about 1 out of 900) that this test may cause a miscarriage.footnote 1

There is also a risk of too much bleeding
(hemorrhage), infection of the amniotic fluid, or leakage of
the fluid. In very rare cases, a fetus may be poked by the needle during
the test. Your doctor does all he or she can to put the needle in a safe spot.
Most fetuses move away from the needle tip.

Amniocentesis has a
very small risk of causing bleeding that could mix your blood and
your fetus’s blood. So if you have Rh-negative blood, you will be given the
Rh immunoglobulin vaccine (such as RhoGAM). This can prevent Rh sensitization, which
could harm your fetus if he or she has Rh-positive blood.

After the test

After the test, you may have some
mild cramping. Do not do any strenuous activity for several hours after
the test. Also, do not douche, use tampons, or have sex after the test. By the
next day, you can do your normal activities, unless your doctor tells you not
to.

Call your doctor right away if:

  • You have moderate or severe belly pain or
    cramping.
  • You have a fever.
  • You become
    dizzy.
  • Fluid or blood leaks from your vagina or from the needle
    site.
  • You have redness or swelling at the needle site.

Results

Amniocentesis is a test to look at the
amniotic fluid that surrounds your
baby (fetus) in the uterus.

Normal amniotic fluid is
clear to light yellow in color and does not contain any harmful bacteria. The
cells can be tested for problems.

  • Cells from your fetus are looked at carefully. They are checked
    for the proper number and arrangement of the cell parts (chromosomes) that show genetic disease. Normally there
    are 46 chromosomes in each cell, arranged in 23 pairs. Chromosomes also tell
    the sex of your fetus.
  • The amounts of some substances in the
    amniotic fluid may be measured. These results can find some birth defects,
    genetic diseases, and the maturity of your fetus.

Normal results from amniocentesis do not
guarantee that your baby will be healthy.

What Affects the Test

You may not be able to have the test, or the results may not be helpful, if:

  • There is blood from your fetus in the amniotic fluid. This can
    falsely increase the level of the substances alpha-fetoprotein (AFP) and
    acetylcholinesterase (AChE) levels, which test for
    neural tube defects.
  • The amniotic fluid
    is exposed to light. This can falsely lower bilirubin levels.
  • There is blood or meconium in the fluid. This may cause a false result for
    the test that checks to see if your baby’s lungs are
    mature.

What To Think About

  • This test can be
    done to help you prepare if your fetus has a possible birth defect. It can also help
    you decide about ending the pregnancy if a serious problem is found.

  • In very rare cases, amniocentesis may be done before 15 weeks of pregnancy. This is rare because there may be greater risks to your fetus. Talk to your doctor about the risks and benefits of having this test done early.
  • Amniocentesis is not easy to do if the amount of
    amniotic fluid is very small or if the placenta is in front of your
    fetus.
  • Chorionic villus sampling (CVS) is another test that
    can find many fetal problems. CVS can be done earlier in pregnancy than amniocentesis, and results are ready sooner. For more
    information, see the topic
    Chorionic Villus Sampling (CVS).
  • Amniotic fluid has cells that have been shed by
    your growing fetus. The cells are checked for the number and size of
    chromosomes to see if there are any problems. For more information,
    see the topic
    Karyotype Test.
  • If you have abnormal results from amniocentesis, talk to
    your doctor or a
    genetic counselor. They can help you understand
    the problems your fetus may have and help you decide about continuing the pregnancy. They can also help you understand your possible risks with future
    pregnancies.

References

Citations

  1. Akolekar R, et al. (2015). Procedure-related
    risk of miscarriage following amniocentesis and chorionic villus sampling: a
    systematic review and meta-analysis. Ultrasound in Obstetrics and Gynecology,
    45(1): 16-26. DOI: 10.1002/uog.14636. Accessed April 5, 2017.

Other Works Consulted

  • American College of Obstetricians and Gynecologists (2016). Prenatal diagnostic testing for genetic disorders. ACOG Practice Bulletin No. 162. Obstetrics and Gynecology, 127(5): e108-e122. DOI: 10.1097/AOG.0000000000001405. Accessed April 6, 2017.
  • Wapner RJ, et al. (2009). Prenatal diagnosis of congenital disorders. In RK Creasy et al., eds., Creasy and Resnik’s Maternal-Fetal Medicine: Principles and Practice, 6th ed., pp. 221-274. Philadelphia: Saunders Elsevier.

Credits

ByHealthwise Staff
Primary Medical Reviewer Sarah Marshall, MD – Family Medicine
E. Gregory Thompson, MD – Internal Medicine
Adam Husney, MD – Family Medicine
Kathleen Romito, MD – Family Medicine
Specialist Medical Reviewer Kirtly Jones, MD – Obstetrics and Gynecology

Current as ofJune 6, 2017