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 30 mL (2 Tbsp) 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:
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 intellectual disability.
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 out of 400) when it is done by highly trained providers.1 In some studies, the risk is a little higher, about 2 to 4 out of 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.
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?|
Amniocentesis may be done during your second trimester of pregnancy to find certain birth defects.
You may choose to have this test because:
Amniocentesis can tell the gender of your fetus. This is important when you or the father may be able to pass on a disease that occurs mainly in one gender (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:
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?).
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 topic 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 30 mL (2 Tbsp) 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.
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.
Amniocentesis is generally very safe. There is a chance (about 1 out of 400) that this test may cause a miscarriage.1 In some studies, the risk is a little higher, about 2 to 4 out of 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 blood. So if you have Rh-negative blood, you will be given the Rh immune globulin vaccine (such as WinRho) to prevent Rh sensitization, which could harm your fetus if he or she has Rh-positive blood.
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:
Normal amniotic fluid is clear to light yellow in colour and does not contain any harmful bacteria. The cells can be tested for problems.
Reasons you may not be able to have the test or why the results may not be helpful include:
- 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.
- 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.
- 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.
|Primary Medical Reviewer||Sarah Marshall, MD - Family Medicine|
|Primary Medical Reviewer||Adam Husney, MD, MD - Family Medicine|
|Primary Medical Reviewer||Brian D. O'Brien, MD - Internal Medicine|
|Specialist Medical Reviewer||Kirtly Jones, MD, MD - Obstetrics and Gynecology|
|Last Revised||July 8, 2010|
Last Revised: April 8, 2012
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