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Amniocentesis is a procedure used to obtain a small sample of the amniotic fluid that surrounds the fetus. The fluid is used to diagnose chromosomal disorders and open neural tube defects (ONTDs), such as spina bifida. Testing is available for other genetic defects and disorders, depending on the family history and the availability of laboratory testing at the time of the procedure. An amniocentesis is generally offered between the 15th and 20th weeks of pregnancy to women who have an increased risk of having a child with chromosome abnormalities. Women who will be over age 35 years of age at delivery or who have had an abnormal maternal serum screening test (indicating an increased risk for a chromosomal abnormality or neural tube defect) may be considered higher risk.

There are circumstances that may make it difficult or impossible to perform amniocentesis. For example, certain positions of the baby or babies, the condition of the placenta, the amount of fluid, or the patient's anatomy could obstruct the test.

How is amniocentesis performed?

During amniocentesis, a health care provider will insert a long, thin needle through the mother's abdomen into the amniotic sac to withdraw a small sample of the amniotic fluid for examination. The amniotic fluid contains cells that were shed by the fetus. These cells contain genetic information that is tested for certain genetic abnormalities.

Most amniocentesis procedures include these steps:

  • the woman's abdomen is cleansed with an antiseptic
  • the doctor may give a local anesthetic to numb the skin
  • ultrasound is used to help guide a hollow needle into the amniotic sac
  • a small sample of fluid is withdrawn for laboratory analysis

After the test is complete, women may feel some cramping in their abdomen. Anyone who has an amniocentesis should avoid strenuous activities for at least 24 hours.

After the test, fluid is sent to a genetics laboratory where the cells are grown and analyzed. Alpha-fetoprotein, a protein made by the fetus that is present in the fluid, is also measured to rule out an open neural tube defect, such as spina bifida. Results of amniocentesis are usually available in about 10 days to two weeks, depending on the laboratory.