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Amniocentesis (Amnio)

Has your doctor ordered an amniocentesis, or amnio? Want to know more about amnio testing before agreeing to the test? Read this article to find out why amniocentesis is done, how an amnio is performed, and learn the risks of having an amniocentesis test done.

Amniocentesis, also called amnio, is a frequently administered prenatal test that helps physicians and parents understand more about the baby before it is born.  It is used to diagnose both genetic anomalies and birth defects, as well as determine how far along the pregnancy is.  According to the March of Dimes, "most" women who have amniocentesis performed learn that their baby is "fine."

Why Is Amniocentesis Done?

Amniocentesis is undertaken for several different reasons.  Maturity amniocentesis is done if an early delivery is being considered, usually to prevent complications for the mother, to ensure that the baby's lungs are developed enough for birth, which is not the case until 32 weeks gestation.  Genetic amniocentesis is done if there is some reason to think that there might be a genetic anomaly.  This could be because:

  • a prenatal screening returned abnormal results and amniocentesis can rule out or confirm a diagnosis.
  • a previous pregnancy showed a chromosome abnormality, such as Down syndrome, or a neural tube defect.
  • the mother's age is 35 or more.
  • there is a family history of a particular genetic disorder or the baby's father is known to be a carrier of a genetic disorder. 

Other reasons for amniocentesis can be either to diagnose the baby—for example, to diagnose an illness or infection or to check anemia in babies with Rh disease—or to diagnose the mother—for example, to diagnose a uterine infection—or simply to decrease the volume of the amniotic fluid.

What Does the Amniocentesis Test Consist Of?

During the amniocentesis test, usually undertaken in a health care provider's office or outpatient facility, ultrasound is used to determine the baby's position in the uterus, as the mother lies on her back.  The health care provider cleans an area of the mother's abdomen and inserts a thin needle that is used to withdraw a sample of amniotic fluid.  By watching the baby's position, the procedure can be done without disturbing the baby.

The amniotic fluid is sent to a laboratory for analysis.  Results are available in times between a few hours and a few weeks, depending on the particular analysis or analyses being performed.

Risks of Amniocentesis

There are some risks associated with amniocentesis.  The mildest of these is cramping, possibly with some vaginal bleeding that stops on its own. Other risks are more critical.  These include:

  • a needle injury to the baby if it moves into the path of the needle.  This kind of injury is rarely serious.
  • leaking amniotic fluid.  If the leak seals, things may be fine.  Sometimes, however, babies may have orthopedic problems after an amniocentesis leak.
  • Rh sensitization.  This can happen if some of the baby's blood cells enter the mother's bloodstream and if the mother has Rh negative blood, she must be given Rh immunoglobulin when the amniocentesis procedure is complete in order to prevent her body from producing antibodies against her baby's cells.
  • uterine infection.
  • transmission of a maternal infection to the baby.  This could happen with HIV, toxoplasmosis, or Hepatitis C.
  • miscarriage.  Amniocentesis is recommended after 15 weeks because the risk drops after that, but there is still a 1 in 300 to 1 in 500 chance of miscarriage in the second trimester. 


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