Patient information: Amniocentesis

INTRODUCTION

Amniocentesis is a procedure in which a small amount of fluid is withdrawn from the amniotic sac, or "bag of water", that surrounds the growing fetus during pregnancy. The fluid is primarily made up of fetal urine, but also contains cells that have been shed from the fetus and fetal lung fluids. The cells can be analyzed in the lab to obtain genetic information and detect some genetic abnormalities in the fetus. The fluid can also be studied to provide information about fetal health.

The most common reason amniocentesis is done is to perform genetic tests on a fetus who could be at increased risk of specific genetic abnormalities. As an example, some women are at higher risk of having a baby with Down syndrome because of their age, abnormal screening test results, or a family history of Down syndrome. However, 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' red blood cells)
• Blood disorders in the fetus, such as sickle cell anemia
• Fetal infection
• Readiness of the fetus' lungs to live outside the uterus (if done late in pregnancy)

Amniocentesis that is done to analyze a fetus' genes is usually performed between the 15th and 20th week of gestation. A typical genetic amniocentesis result gives information about the number of chromosomes (as an example, there is an extra chromosome in Down syndrome and one missing in Turner syndrome), the sex of the baby, and whether there is an increased risk of some birth defects, such as open spina bifida. It does not show whether the baby has other inherited or genetic disease unless the lab is told to test for a specific disease because of a family or obstetrical history which places the baby at risk for the same problem. Also, some problems cannot be detected by this testing. Therefore, a normal test result does not guarantee that the baby will be normal.

In some cases, the procedure is done as therapy to remove excess amniotic fluid.

AMNIOCENTESIS PROCEDURE

The first step in the procedure is to perform an ultrasound of the uterine contents. During ultrasound, a wand-like instrument that transmits sound waves is passed over the outside of the woman's abdomen. The sound waves provide a picture of what is inside. Through ultrasound, the doctor can see the fetus, amniotic fluid, placenta, and some of the organs in the maternal abdomen.

After cleaning the abdomen, the doctor inserts a needle and uses the ultrasound picture to guide the needle into a pocket of amniotic fluid. Care is taken to avoid touching the fetus or passing the needle through other structures in the abdomen. The needle is directed into the amniotic sac and a small amount of fluid (about one ounce) is withdrawn.

Occasionally, more than one needle insertion is needed to get the proper amount of fluid. If there is more than one fetus, a separate needle stick for each fetus is often necessary.

There are two moments of mild discomfort:

• When the needle punctures the skin
• When the needle crosses the uterine muscle.

Local anesthesia is not effective against the discomfort associated with entering the uterus. However, this crampy discomfort is mild and is usually tolerated well.

The sample of amniotic fluid obtained at amniocentesis is sent to a laboratory that will test it for the disorder specified by the doctor. In a small number of cases, the sample obtained does not yield the needed information. This may only become apparent after the sample has been analyzed in the lab. In these rare cases, a repeat procedure is usually recommended.

AMNIOCENTESIS COMPLICATIONS

Amniocentesis involves certain risks. Complications can include:

Membrane rupture — Leakage of amniotic fluid occurs more frequently in women who have undergone amniocentesis than those who have not. Fluid loss is almost always small and usually stops on its own within one week.

In rare cases, leakage can be ongoing. If this occurs, the mother and fetus are monitored closely throughout the pregnancy for signs of problems such as infection or abnormal fetal growth. In these rare cases of ongoing fluid leak, there is an increased risk of pregnancy complications including preterm delivery, skeletal deformities, lung problems in the baby, and fetal death. The risk of these complications usually depends on how much fluid remains around the fetus.

Indirect injury to the fetus — Some studies have suggested that amniocentesis increases the risk of clubfoot, hip dislocation, and respiratory problems in the baby. These data have not been confirmed, but women should be aware of this potential risk before undergoing amniocentesis.

Infection — Studies have examined whether women who have chronic infections that could be dangerous to the fetus are more likely to transmit the infection if they have an amniocentesis procedure. Examples include HIV (the virus that causes AIDS), cytomegalovirus, hepatitis C, and toxoplasmosis. The data are not clear on whether or not there is an increased risk. Again, women should be aware that an increased risk is possible.

Infection of the amniotic fluid itself following amniocentesis is possible but extremely rare, occurring in fewer than 1 of every 1000 procedures. Infection often results in pregnancy loss.

Miscarriage — Studies have shown that women who have amniocentesis have an increased risk of miscarriage. Fortunately, this complication is rare. A number of studies have been done to estimate the exact risk. The American College of Obstetricians and Gynecologists estimates the risk of miscarriage related to amniocentesis to be 1 in 300 to 500 (0.2 to 0.3 percent) [1].

The risk may be further increased in certain circumstances, such as when there has been vaginal bleeding during the current pregnancy, if the maternal blood level of AFP (alpha-fetoprotein) is high, or if the woman has a history of three or more previous first trimester miscarriages or abortions or one or more second trimester miscarriages or abortions.

AFTER THE AMNIOCENTESIS PROCEDURE

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. If there is ongoing loss of fluid, ongoing bleeding, severe cramping lasting for several hours, or fever, the doctor should be notified. The doctor may recommend that intercourse and strenuous activity be avoided on the day of the procedure. In general, normal activities can be resumed the day after the amniocentesis.

Women who have Rh negative blood type (eg, A, B, AB, or O negative) are usually given a medicine called Rh(D) immune globulin (RhoGam®) after amniocentesis. This medicine helps protect future pregnancies against problems that can occur if an Rh negative mother is carrying a baby who is Rh positive.

AMNIOCENTESIS RESULTS

The time needed to complete the tests depends upon the nature of the testing and will vary from site to site; most sites are able to get results within 7 to 14 days. Patients should ask their provider when they can expect to have the results.

OTHER TESTING OPTIONS

There are a number of blood tests that can be done on the mother that provide information on the risk of certain problems, such as Down syndrome or neural tube defect. These screening tests (such as first trimester ultrascreen, AFP, second trimester multiple marker screen) do not provide a diagnosis, but rather indicate the probability of a problem being present. If a screening test is positive, more tests (such as amniocentesis) are then needed to determine if a problem is really present. If a screening test is negative, it does not guarantee there is no problem, but may provide reassurance to someone who does not want to undergo more invasive tests. (See "Patient information: First trimester and integrated screening for Down syndrome" and "Patient information: Second trimester screening for Down syndrome".)

Ultrasound also provides certain kinds of information about the development of the fetus, but, at present, can not make a certain diagnosis of genetic problems.

The only test that provides information comparable to amniocentesis is a procedure called chorionic villus sampling (CVS). During this test, small samples of the placenta are taken and sent for genetic analysis. CVS can be done earlier in the pregnancy than amniocentesis and is generally performed at 10 to 12 weeks. The opportunity for earlier prenatal diagnosis is attractive to many women. There is a small but significant increase in fetal loss rates following CVS compared to those following amniocentesis. The doctor can advise about other risks and benefits of CVS versus amniocentesis for an individual woman. (See "Patient information: Chorionic villus sampling".)

DECIDING TO UNDERGO AMNIOCENTESIS

Amniocentesis for prenatal diagnosis is an elective procedure. Women should discuss the risks and benefits carefully with the physician. It is important to discuss what kind of information will be gained and how the results will be used.

WHERE TO GET MORE INFORMATION

Your healthcare provider is the best source of information for questions and concerns related to your medical problem. Because no two people are exactly alike and recommendations can vary from one person to another, it is important to seek guidance from a provider who is familiar with your individual situation.

This discussion will be updated as needed every four months on our web site (www.uptodate.com/patients). Additional topics as well as selected discussions written for healthcare professionals are also available for those who would like more detailed information.

Some of the most pertinent include:

Patient Level Information:
Patient information: First trimester and integrated screening for Down syndrome
Patient information: Second trimester screening for Down syndrome
Patient information: Chorionic villus sampling

Professional Level Information:
Amniocentesis: Technique and complications
Basic principles of genetic counseling for the obstetrician
Chorionic villus sampling: Risks, complications, and techniques
Epidemiology, transmission and prevention of hepatitis B virus infection
Fetal blood sampling: Indications and invasive fetal therapy
Fetal blood sampling: Technique and complications
First trimester and integrated screening for Down syndrome and trisomy 18

A number of web sites have information about medical problems and treatments, although it can be difficult to know which sites are reputable. Information provided by the National Institutes of Health, national medical societies and some other well-established organizations are often reliable sources of information, although the frequency with which they are updated is variable.

• National Library of Medicine

      (www.nlm.nih.gov/medlineplus/healthtopics.html)

• The March of Dimes

      (www.marchofdimes.com)

• New York Online Access to Health

      (www.noah-health.org)

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