Amniocentesis

Test Overview

Amniocentesis is a test to look at the fluid (amniotic fluid) that surrounds your baby (fetus). Amniotic fluid has cells and other substances that can give clues about the health of your fetus. Amniocentesis is done by gently putting a needle through your belly into your uterus. About 2 Tbsp (30 mL) of the amniotic fluid is taken out and looked at.

Amniocentesis is generally done between weeks 15 and 20 (usually around week 16) to look at genetic information. It can also be used later in pregnancy to see how the fetus is doing.

Amniocentesis is offered to women who would like a test to find problems with the fetus. These include women:

  • With a close family member who has a genetic problem.
  • Who have an abnormal screening test for birth defects in the first trimester.
  • Who have an abnormal triple or quadruple screen test (alpha-fetoprotein, estriol, human chorionic gonadotropin, hormone inhibin A).
  • Who have a fetal ultrasound that shows problems, such as a heart problem or too much amniotic fluid.

Amniocentesis

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

Amniotic fluid has cells that have been shed by your developing fetus. The cells are checked for the number and size of chromosomes (karyotype) to see if there are any problems that put the baby at risk for certain conditions. Testing is most commonly done as early as possible so that women and their families have time to consider their options. But amniocentesis cannot find many common birth defects, such as cleft lip, cleft palate, heart problems, and some types of mental retardation.

Amniocentesis is not done as a general screening test for birth defects because it has an increased risk to your fetus. One study showed the chance of miscarriage from amniocentesis was small (about 1 in 400) when it is done by highly trained providers.1 In some studies, the risk is a little higher, about 2 to 4 in 400.2 Amniocentesis can be done to help you prepare if your fetus has a possible birth defect or to help you make a decision about ending the pregnancy if a serious problem is found.

Click here to view a Decision Point. Should I have an amniocentesis?

Amniocentesis in late pregnancy

If you are at risk of having your baby early amniocentesis may be done during the third trimester to see whether your baby's lungs are developed. Your developing fetus makes substances that can be found and measured in amniotic fluid. The amounts of these substances show how mature the lungs are and if your baby will be able to breathe without help if delivered early.

Amniocentesis may also be done later in pregnancy if an infection of the amniotic fluid (chorioamnionitis) is suspected.

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  Pregnancy: Should I have amniocentesis?

Why It Is Done

Amniocentesis may be done during your second trimester of pregnancy (between weeks 15 and 20) to find some birth defects. Amniocentesis may be done when:

  • You would like to have a test that will tell you for sure if your baby has Down syndrome or another chromosome problem.
  • Either you or the father has a family history of birth defects.
  • Both you and the father are known carriers of a family disease, such as Tay-Sachs disease, sickle cell anemia, thalassemia (Mediterranean anemia), or cystic fibrosis.
  • You had a screening test done in the first trimester that showed your baby has a higher chance of a birth defect.
  • You had an integrated test that showed your baby has a higher chance of a birth defect. This test is a combination of several tests done in two stages at two different times during the pregnancy. A positive result may mean Down syndrome is present.
  • You had a triple or quadruple screen test that showed your baby has a higher chance of a birth defect. The substances measured in a triple test are alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG), and estriol. An additional substance, the hormone inhibin A, is measured in the quadruple test.

Amniocentesis can tell the sex of your fetus. This is important when you or the father may be able to pass on a disease that occurs mainly in one sex (sex-linked), such as hemophilia or Duchenne muscular dystrophy, both of which occur mainly in males.

Amniocentesis may be done during your third trimester to:

  • See if your fetus's lungs are mature. This may be done when you may need to deliver early because of a problem with the pregnancy.
  • See whether the amniotic fluid is infected (chorioamnionitis).

How To Prepare

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

You will need to sign a consent form that says you understand the risks of amniocentesis and agree to have the test done. Talk to your doctor about any concerns you have 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 his or her office or in the hospital. An overnight stay in the hospital usually is not needed unless problems occur during the test.

You will be asked to expose your belly. You will then lie on your back with it slightly raised to relax your belly muscles. Your lower belly will be cleaned with a special soap.

Your doctor checks the position of your fetus and the placenta with a fetal ultrasound. 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 medical test Fetal Ultrasound.

With the ultrasound picture as a guide, your doctor gently puts a long thin needle through your belly and into your uterus without hurting your fetus or the placenta. If your fetus moves too close to the needle, the needle will be taken out and your doctor will try again in another spot.

About 2 Tbsp (30 mL) of amniotic fluid is taken out in a syringe attached to the needle, and then the needle is taken out. The site is covered with a bandage.

See a picture of the amniocentesis test.

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 inserted. This lasts for only a few seconds. When the needle is put into your uterus, you again 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 generally very safe. There is a chance (about 1 in 400) that this test may cause a miscarriage.1 In some studies, the risk is a little higher, about 2 to 4 in 400.2 There is also a risk of too much bleeding (hemorrhage), infection of the amniotic fluid (amnionitis), or leakage of amniotic 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 float away from the needle tip.

Amniocentesis has a very small risk of causing bleeding that could lead to mixing your blood and your fetus's. Therefore, if you have Rh-negative blood, you will be given the Rh immune globulin vaccine (such as RhoGAM) to 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. You should 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 develop a fever.
  • You become dizzy.
  • Fluid or blood leaks from your vagina or from the needle site.
  • Redness or swelling develops at the needle site.

Results

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

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 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.

What Affects the Test

Reasons you may not be able to have the test or why the results may not be helpful include:

  • 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.
  • If the amniotic fluid is exposed to light. This can falsely lower bilirubin levels.
  • If there is blood or meconium in the fluid. This may cause an incorrect result for the test that checks to see whether your baby’s (fetus’s) lungs are mature.

What To Think About

  • Normal results from amniocentesis do not guarantee that your fetus will be healthy.
  • Amniocentesis is not done as a general screening test for birth defects because it has some risk to your fetus and cannot find some common birth defects. There is a chance (about 1 in 400) that this test may cause a miscarriage.1 In some studies, the risk is a little higher, about 2 to 4 in 400.2 Amniocentesis can be done to help you prepare if your fetus has a possible birth defect or to help you make a decision about ending the pregnancy if a serious problem is found. For more information about amniocentesis in early pregnancy, see:
    Click here to view a Decision Point.Should I have an amniocentesis?
  • In rare cases, amniocentesis is done between the 12th and 15th weeks of pregnancy because there may be a greater risk to your fetus. Talk to your doctor about the risks and benefits of an early amniocentesis.
  • Amniocentesis cannot be done easily 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 (at about 10 to 12 weeks) than amniocentesis, and results are ready sooner. It can find more than 100 genetic diseases. One study showed that CVS also has about a 1-in-400 chance of miscarriage when it is done by a highly trained provider.1 CVS cannot be used to find neural tube defects. For more information, see the medical test Chorionic Villus Sampling (CVS).
  • Fetoscopy is a new test that allows your doctor to look at your fetus using a long, thin tube put through a small cut in your belly. Samples of your fetus's blood and tissue also can be collected. Fetoscopy carries a higher chance of miscarriage than amniocentesis and is not widely available.
  • Amniocentesis has a very small chance of causing bleeding that could lead to mixing your blood and your fetus's. So if you have Rh-negative blood, you will be given a vaccine (RhoGAM) to prevent Rh sensitization which could harm your fetus if he or she has Rh-positive blood.
  • Amniotic fluid has cells that have been shed by your developing fetus. The cells are checked for the number and size of chromosomes (karyotype) to see if there are any problems. For more information, see the medical test Karyotype Test.
  • If you have abnormal results from amniocentesis, you should ask your doctor or a genetic counselor for help in making decisions about the problems your fetus may have and about continuing the pregnancy. It will also be helpful to understand your possible risks with future pregnancies.

References

Citations

  1. 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.
  2. Seeds JW (2004). Diagnostic mid trimester amniocentesis: How safe? American Journal of Obstetrics and Gynecology, 191: 608–616.

Other Works Consulted

  • American College of Obstetricians and Gynecologists (2007). Invasive prenatal testing for aneuploidy. ACOG Practice Bulletin No. 88. Obstetrics and Gynecology, 110(6): 1459–1467.

Credits

Author Sandy Jocoy, RN
Editor Kathleen M. Ariss, MS
Associate Editor Tracy Landauer
Primary Medical Reviewer Sarah Marshall, MD - Family Medicine
Specialist Medical Reviewer Siobhan M. Dolan, MD, MPH - Reproductive Genetics
Last Updated May 14, 2008

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