Robert Wood Johnson University Hospital



(Amniotic Fluid Analysis)

Procedure Overview

What is amniocentesis?

Amniocentesis is a procedure used to obtain a small sample of the amniotic fluid that surrounds the fetus during pregnancy. Amniotic fluid is a clear, pale yellow fluid made by the fetus. The fluid protects the fetus from injury and helps to regulate the temperature of the fetus.

In addition to various enzymes, proteins, hormones, and other substances, the amniotic fluid contains cells shed by the fetus. These cells contain genetic information that can be used to diagnose chromosomal disorders and open neural tube defects (ONTDs), such as spina bifida. Testing may be available for other genetic defects and metabolic disorders depending on the family history and availability of lab testing at the time of the procedure.

Amniotic fluid also contains other substances that provide information about certain conditions. An amniocentesis may be performed in late pregnancy to check fetal well-being and diagnose fetal conditions, such as Rh disease (complications resulting from incompatibility between the blood types of the mother and baby) and infection. An amniocentesis may be performed to check for fetal lung maturity if a baby is expected to be born prematurely. Specific substances present in the amniotic fluid can be measured or tested for these conditions.

The fluid is sent to a genetics lab so that the cells can grow and be analyzed. Results are usually available in about 10 days to two weeks, depending on the lab.

An amniocentesis is generally offered to women between the 15th and 20th weeks of pregnancy who are at increased risk for chromosome abnormalities. This includes women who are over 35 years of age at delivery, or those who have had an abnormal maternal serum (blood) screening test indicating an increased risk for a chromosomal abnormality or neural tube defect.

Amniocentesis helps confirm a tentative diagnosis of an abnormality previously found with other testing. It may also find that a fetus does not have the abnormality that was suspected.

Another related procedure that may be used to diagnose genetic and chromosomal defects is chorionic villus sampling (CVS). Please see this procedure for additional information.

Anatomy of the fetus in utero:

Illustration of fetus in utero
Click Image to Enlarge

Reasons for the Procedure

An amniocentesis may be used for genetic and chromosome testing in the second trimester of pregnancy in the presence of one or more of these conditions:

Amniocentesis may be used in the third trimester of pregnancy to assess for conditions such as:

There may be other reasons for your physician to recommend an amniocentesis.

Risks of the Procedure

As with any invasive procedure, complications may occur. Some possible complications may include, but are not limited to, the following:

The risk of miscarriage is generally considered to be less than one percent after an amniocentesis in the second trimester of pregnancy. This is only slightly higher than the normal risk of miscarriage without an amniocentesis at this time in pregnancy.

Patients who are allergic to or sensitive to medications or latex should notify their physician.

Certain factors or conditions may interfere with an amniocentesis. These factors include, but are not limited to, the following:

There may be other risks depending upon your specific medical condition. Be sure to discuss any concerns with your physician prior to the procedure.

Before the Procedure

During the Procedure

Illustration demonstrating an amniocentesis
Click Image to Enlarge

An amniocentesis may be performed on an outpatient basis or as part of your stay in a hospital. Procedures may vary depending on your condition and your physician’s practices.

Generally, an amniocentesis follows this process:

  1. You will be asked to undress completely, or from the waist down, and put on a hospital gown.
  2. You will be asked to lie down on an examination table and place your hands behind your head.
  3. Your vital signs (blood pressure, heart rate, and breathing rate) will be checked.
  4. An ultrasound will be performed to check the fetal heart rate, the position of the placenta, fetus, and umbilical cord, and to locate a pocket of amniotic fluid.
  5. Your abdomen will be cleansed with an antiseptic. You will be instructed not to touch the sterile area on your abdomen during the procedure.
  6. You will feel a needle stick if a local anesthetic is injected. This may cause a brief stinging sensation.
  7. Ultrasound will be used to help guide a long, thin, hollow needle through your abdomen, into the uterus and the amniotic sac. This may be slightly painful. You may feel some cramping as the needle enters the uterus.
  8. The physician will withdraw a small amount of amniotic fluid into a syringe. The amount depends upon the type of testing that will be performed, but usually no more than an ounce is removed. You may feel a tugging or pulling sensation as the fluid is removed.
  9. The fluid will be placed into a special light-protected container.
  10. The needle will be removed.
  11. An adhesive bandage will be placed over the needle site.
  12. The fetus’ heart rate and your vital signs will be reassessed.
  13. If you are Rh negative, you may be given Rhogam, a specially developed blood product that can prevent an Rh negative mother's antibodies from reacting to Rh positive fetal cells.
  14. The amniotic fluid will be sent to the lab for examination.

After the Procedure

You and your fetus will be monitored for a time after the procedure. Your vital signs and the fetal heart rate will be checked periodically for an hour or longer.

Amniotic fluid for genetic and chromosome testing will be sent to a specialty genetics lab for analysis. Alpha-fetoprotein, a protein made by the fetus that is present in the fluid, may also measured to rule out an open neural tube defect, such as spina bifida. Testing may also be done for other substances related to metabolic or genetic conditions. Depending on test results, counseling with a genetics specialist may be recommended.

You may feel some cramping during or after the amniocentesis. If you feel lightheaded, dizzy, or nauseated, notify the nurse. You may be instructed to rest on your left side.

You should rest at home and avoid strenuous activities for at least 24 hours, or as directed by your physician.

Notify your physician to report any of the following:

Your physician may give you additional or alternate instructions after the procedure, depending on your particular situation.

Online Resources

The content provided here is for informational purposes only, and was not designed to diagnose or treat a health problem or disease, or replace the professional medical advice you receive from your physician. Please consult your physician with any questions or concerns you may have regarding your condition.

This page contains links to other Web sites with information about this procedure and related health conditions. We hope you find these sites helpful, but please remember we do not control or endorse the information presented on these Web sites, nor do these sites endorse the information contained here

American College of Obstetricians and Gynecologists

Gene Tests

March of Dimes

National Center on Birth Defects and Developmental Disabilities

National Human Genome Research Institute

National Institutes of Health (NIH)

National Library of Medicine

National Women's Health Information Center


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