Amniocentesis is a test done during pregnancy to get information about the fetus by sampling the amniotic fluid, which contains cells and chemicals which are fetal in origin. The most common reason for amniocentesis is to determine for sure whether a fetus has certain genetic problems, such as Down syndrome. This test is usually done between 15 and 17 weeks of pregnancy.
Amniocentesis can also provide information about:
- Neural tube defects (such as spina bifida or anencephaly)
- Blood type of the fetus (which can be important if the mother's blood contains antibodies that can react with the fetus's red blood cells)
- Genetic disorders in the fetus, such as sickle cell anemia
- Infection in the fetus
- Readiness of the fetus's lungs to live outside the uterus (if done late in pregnancy)
More detailed information about amniocentesis is available by subscription. (See "Diagnostic amniocentesis".)
The first step in the procedure is to perform an ultrasound of the uterus. With ultrasound, the doctor can see the fetus, amniotic fluid, and the placenta.
After cleaning the abdomen, the doctor inserts a needle and uses ultrasound imaging to guide the needle safely into a pocket of amniotic fluid. The doctor removes a small amount of fluid (usually less than one ounce [30 mL]). The needle stick is mildly uncomfortable or crampy.
Occasionally, the doctor may need to insert the needle more than once to get enough fluid. If there is more than one fetus, the doctor may need to take a sample of fluid from around each fetus.
The sample of amniotic fluid is then sent to a laboratory for testing. In a small number of cases, the fluid does not have enough cells. In these rare cases, you may be asked to have another amniocentesis.
Amniocentesis involves certain risks. Complications can include:
Leakage of amniotic fluid — Leakage of amniotic fluid sometimes happens after amniocentesis. In most cases, there is only a small amount of fluid leakage that stops on its own within one week.
In rare cases, leakage can be ongoing. If this occurs, the mother and fetus are monitored closely for signs of problems, such as infection. In these rare cases, there is an increased risk of pregnancy complications, including preterm delivery. The risk of these complications usually depends on how much fluid is left around the fetus.
Injury to the fetus — There might be a very small increased risk of problems developing in the fetus because of amniocentesis. This might include clubfoot, hip dislocation, and lung problems. Talk to your doctor about these risks before having amniocentesis.
Infection — If the mother has a chronic infection, like HIV or hepatitis, there is a chance that the fetus could become infected because of the amniocentesis. The risk is probably very small, but you should talk to your doctor about the risks if you have one of these infections.
Miscarriage — Studies have shown that women who have amniocentesis have a small increased risk of miscarriage. Fortunately, this complication is rare. The risk of miscarriage related to amniocentesis is estimated to be 1 in 300 to 500 (0.2 to 0.3 percent) .
AFTER THE AMNIOCENTESIS
Immediately after the procedure, some women have mild cramping, a small amount of vaginal bleeding, and a small amount of amniotic fluid leaking from the vagina. This should all resolve quickly.
However, women should call their doctor after amniocentesis if:
- They continue to leak fluid or bleed
- They have severe cramping lasting several hours
- They have a fever (temperature higher than 100.4ºF or 38ºC) after the amniocentesis
Some doctors recommend that women avoid having sex and exercising on the day of the amniocentesis. Women can resume their normal activities the day after the amniocentesis.
Women who have Rh negative blood type (eg, A, B, AB, or O negative) are usually given an intramuscular shot called Rh(D) immune globulin (RhoGam) after amniocentesis. This shot helps protect future pregnancies against problems that can develop if the mother is Rh negative and is pregnant with a fetus who is Rh positive.
Most labs are able to provide results within 7 to 14 days; lung maturity results take only a few hours. Ask your doctor or nurse when you can expect to have the results.
OTHER TESTING OPTIONS
Amniocentesis is one option to get information about the fetus. It is not a required test. You should discuss the risks and benefits of this test with your doctor.
For genetic information, the chorionic villus sampling (CVS) is an alternative to amniocentesis. An advantage of CVS is that it can be done earlier in the pregnancy than amniocentesis (at 10 to 12 weeks of pregnancy). However, CVS has risks that are similar to amniocentesis (leakage of fluid, infection, miscarriage). More information about CVS is available separately. (See "Patient information: Chorionic villus sampling (Beyond the Basics)".)
There are other tests that can provide some information about the fetus' risk of having Down syndrome. However, these tests do not tell you for sure if your fetus is affected. They only provide you with information about the probability of the fetus having Down syndrome. Some women prefer to have this type of tests, called a screening test, before making up their mind about having an invasive test like amniocentesis or CVS.
More information about screening tests is available separately. (See "Patient information: Should I have a screening test for Down syndrome during pregnancy? (Beyond the Basics)".)
Fetal blood sampling, which can be performed after about 20 weeks of gestation, can also be used to perform fetal genetic tests or tests for infection, but is riskier than amniocentesis.
Amniocentesis is the only way to obtain information about fetal lung maturity.
WHERE TO GET MORE INFORMATION
Your healthcare provider is the best source of information for questions and concerns related to your medical problem.
This article will be updated as needed on our web site (www.uptodate.com/patients). Related topics for patients, as well as selected articles written for healthcare professionals, are also available. Some of the most relevant are listed below.
Patient level information — UpToDate offers two types of patient education materials.
The Basics — The Basics patient education pieces answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials.
Patient information: Amniocentesis (The Basics)
Patient information: Should I have a screening test for Down syndrome during pregnancy? (The Basics)
Patient information: C-section (cesarean delivery) (The Basics)
Patient information: Chorionic villus sampling (The Basics)
Patient information: Prenatal care (The Basics)
Patient information: Pregnancy in Rh-negative women (The Basics)
Beyond the Basics — Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are best for patients who want in-depth information and are comfortable with some medical jargon.
Patient information: Chorionic villus sampling (Beyond the Basics)
Patient information: Should I have a screening test for Down syndrome during pregnancy? (Beyond the Basics)
Professional level information — Professional level articles are designed to keep doctors and other health professionals up-to-date on the latest medical findings. These articles are thorough, long, and complex, and they contain multiple references to the research on which they are based. Professional level articles are best for people who are comfortable with a lot of medical terminology and who want to read the same materials their doctors are reading.
(See "Diagnostic amniocentesis" and "Basic principles of genetic counseling for the obstetrical provider" and "Chorionic villus sampling" and "Epidemiology, transmission, and prevention of hepatitis B virus infection" and "Fetal blood sampling" and "First trimester combined test and integrated tests for screening for Down syndrome and trisomy 18".)
The following organizations also provide reliable health information.
- National Library of Medicine