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.
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
If you are at risk of having your baby early, amniocentesis may be done during the third trimester to see if your baby's lungs are fully formed. Your growing baby makes substances that can be found in amniotic fluid and measured. The amounts of these substances show how mature the lungs are. They let you know if your baby will be able to breathe without help if born early.
This test may also be done later in pregnancy if your doctor thinks you may have an infection of the amniotic fluid (chorioamnionitis).
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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 during your third trimester to:
- Find out if your baby's lungs are mature. This is important if you need to deliver early because of a problem with the pregnancy.
- See if the amniotic fluid is infected (chorioamnionitis).
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
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.
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.
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.
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
- Normal results from amniocentesis do not guarantee that your baby will be healthy.
- 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.
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.
Other Works Consulted
American College of Obstetricians and Gynecologists (2007, reaffirmed 2009). 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 ReviewerSarah Marshall, MD - Family Medicine
E. Gregory Thompson, MD - Internal Medicine
Specialist Medical ReviewerSiobhan M. Dolan, MD, MPH - Reproductive Genetics
Current as ofAugust 11, 2015