Amniocentesis

Alternative names
Culture - amniotic fluid; Culture - amniotic cells

Definition
Amniocentesis is a diagnostic procedure performed by inserting a hollow needle through the abdominal wall into the uterus and withdrawing a small amount of fluid from the sac surrounding the fetus.

How the test is performed
The exact location of the fetus is determined, usually by ultrasound (see pregnancy ultrasound). The skin of the abdomen is scrubbed and a topical anesthetic may be applied, or a local anesthetic may be injected into the skin. A long, thin needle is inserted through the abdomen and into the uterus. A small amount of fluid is taken from the fluid-filled sac surrounding the fetus (usually about 1 cc. per week of gestation).

How to prepare for the test
This test does not require an overnight stay in the hospital. You must sign a consent form. Your bladder must be full for the ultrasound. There are no food or fluid restrictions.

How the test will feel
If an anesthetic is used, there may be a sharp, stinging sensation for a few seconds. When the needle enters into the amniotic sac, a sharp pain lasting a few seconds may be felt. Some women have the sensation of pressure in the lower abdomen when the fluid is withdrawn. After the procedure, there may be some minor cramping.

Why the test is performed

The test can detect chromosomal disorders - such as Down’s syndrome, structural defects - such as spina bifida (open spine, where the vertebrae fail to close), anencephaly (a condition in which the brain is incomplete or missing), and many rare, inherited metabolic disorders.

Later on in a pregnancy, the test may be used to identify suspected problems such as Rh incompatibility or infection.

Late in the pregnancy, lung maturity can be determined using this test.

Normal Values
The analysis shows no defects in the chromosomes and no excess of alpha fetoprotein (a protein produced by the fetus) or bilirubin.

What abnormal results mean
Amniocentesis can be used to diagnose a large number of genetic and chromosomal abnormalities in the fetus. In addition, it is helpful in the diagnosis of the severity of Rh incompatibility, lung maturity, and neural tube defects (such as spina bifida).

DNA testing is available for many diseases. New diseases are being added to this list as genetic research advances. Ask your obstetrician or geneticist if you have a question about a specific disease.

What the risks are
There is a slight chance of infection or injury to the fetus. There is even a smaller chance of miscarriage. This test is typically performed when a problem is suspected, so the benefits outweigh the risk.

Special considerations
There may be alternatives to the amniocentesis. These should be discussed with the health care provider.

Johns Hopkins patient information

Last revised: December 5, 2012
by Potos A. Aagen, M.D.

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