Patient education: Amniocentesis (Beyond the Basics)

INTRODUCTION — Amniocentesis is a test done in pregnancy that involves removing a small amount of amniotic fluid for testing. Amniotic fluid is the fluid that surrounds the fetus (developing baby) inside the uterus; it contains cells and chemicals that can give information about the health of the fetus. The most common reason for amniocentesis is to determine for sure whether a fetus has Down syndrome or other genetic syndromes. This test is usually done around 16 weeks of pregnancy. Amniocentesis is an elective procedure and most people do not need to have it performed.

Amniocentesis is most commonly used to:

●Look for genetic disorders such as Down syndrome or cystic fibrosis – This might be done if another test (like an ultrasound of the fetus or blood tests on one or both parents) suggests that there might be a genetic problem.

●Check for infection in the fetus.

It is less commonly used to diagnosis neural tube defects (such as spina bifida or anencephaly), identify the blood type of the fetus (which can be important if the mother's blood contains antibodies that react with the fetus's red blood cells), or assess the fetus's lung maturity (readiness to live outside the uterus).

BEFORE AMNIOCENTESIS — Your doctor or nurse will talk to you about the procedure and what to expect, and answer any questions you have. They will probably also refer you to a genetic counselor, who can talk to you in more detail about what the test will look for and how to interpret the results afterwards.

AMNIOCENTESIS PROCEDURE — The first step in the procedure is to perform an ultrasound of the uterus. This may be done by a doctor or a technician called a sonographer. With ultrasound, the doctor can see the fetus, amniotic fluid, placenta, and uterus.

After cleaning the abdomen, the doctor inserts a needle and uses ultrasound imaging to guide the needle safely into a pocket of amniotic fluid. Then they remove 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 (for example, twins), the doctor may need to insert two needles in order 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 contain enough cells. If this happens, you may be asked to have another amniocentesis.

AFTER AMNIOCENTESIS — Immediately after the procedure, some people may have mild cramping, a small amount of vaginal bleeding, or a small amount of amniotic fluid leaking from the vagina. This should all resolve quickly.

However, you should call your doctor if:

●You continue to leak fluid or bleed from the vagina

●You have severe cramping lasting several hours

●You have a fever (temperature higher than 100.4°F or 38°C)

You can resume your normal activities after the amniocentesis. It is usually not necessary to avoid exercise or sexual activity.

People who have Rh (A, B, AB, or O) negative blood type are usually given a shot called Rh(D) immune globulin (RhoGam) after amniocentesis. This shot helps protect future pregnancies against problems that can develop if a person who is Rh negative is pregnant with a fetus who is Rh positive.

AMNIOCENTESIS COMPLICATIONS — Amniocentesis involves certain risks, although these are rare. Possible complications include:

Leakage of amniotic fluid — Leakage of amniotic fluid from the vagina sometimes happens after amniocentesis. In most cases, there is only a small amount of fluid leakage that stops on its own soon after the procedure.

In rare cases, leakage can be ongoing. If this occurs, you and your fetus will be 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 remains 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 you have a chronic infection, like HIV or hepatitis, there is a chance that the fetus could become infected as a result of the amniocentesis. The risk is very small, but you should talk to your doctor about the risks if you have one of these infections.

Miscarriage — Amniocentesis comes with a very small (less than 3 in 1000) risk of miscarriage.

WHEN WILL I RECEIVE THE TEST RESULTS? — Most laboratories are able to provide the results of Down syndrome testing within 7 to 14 days. Other results may take less or more time, depending on the test. Ask your health care provider when you can expect to have the results.

WHAT DO MY TEST RESULTS MEAN? — Results may suggest that the fetus has the genetic problem tested for. Sometimes more tests are needed to confirm this. It may be necessary to perform genetic testing on a blood sample from the parents or do additional tests on the fetus. If amniocentesis suggests that your fetus has a genetic problem, your doctor or nurse may have you discuss the results with a genetic counselor. They can help you understand the problem, talk to you about your options, and help you decide what to do next.

Results could also suggest that the fetus does not have the specific genetic disease tested for, such as Down syndrome. However, amniocentesis does not check for all genetic problems or other abnormalities.

Alternatives to amniocentesis — 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 health care provider.

One alternative to amniocentesis is a procedure called chorionic villus sampling (CVS). An advantage of CVS is that it can be done earlier in pregnancy than amniocentesis (at 10 to 13 weeks of pregnancy). CVS has risks of miscarriage that are similar to amniocentesis. More information about CVS is available separately. (See "Patient education: Chorionic villus sampling (Beyond the Basics)".)

There are other tests that can provide some information about a fetus's risk of having Down syndrome, called screening tests. 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 people prefer to have this type of 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 education: Should I have a screening test for Down syndrome during pregnancy? (Beyond the Basics)".)

WHERE TO GET MORE INFORMATION — Your health care 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 health care 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 education: Amniocentesis (The Basics)
Patient education: Testing for Down syndrome during pregnancy (The Basics)
Patient education: Cesarean birth (The Basics)
Patient education: Chorionic villus sampling (The Basics)
Patient education: Prenatal care (The Basics)
Patient education: Pregnancy in Rh-negative people (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 education: Chorionic villus sampling (Beyond the Basics)
Patient education: 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.

Diagnostic amniocentesis
Chorionic villus sampling
Epidemiology, transmission, and prevention of hepatitis B virus infection
Fetal blood sampling
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

(https://medlineplus.gov/healthtopics.html)

●The March of Dimes

(www.marchofdimes.com)