What Is an Amnio (Amniocentesis)?

A type of prenatal testing that can diagnose certain conditions

doctor performing ultrasound on pregnant woman
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An amniocentesis, often shortened to amnio, is a diagnostic test usually done between 15 to 20 weeks of pregnancy. An amniocentesis involves removing small amount of amniotic fluid (the fluid surrounding a developing fetus) from the uterus. The amniotic fluid can be tested to determine if the fetus has certain kinds of chromosomal defects.

An ultrasound is used to visualize the baby and a very thin needle is inserted into the lower abdomen (not through the belly button) and small amount (about one ounce) of amniotic fluid is removed.

The amniotic fluid contains fetal skin cells that have sloughed off and are floating in the amniotic fluid. These skin cells are taken to a laboratory and a karyotype test is performed to give a picture of the baby’s chromosomes. Fluorescence in situ hybridization (FISH) testing may also be performed.

What Does an Amniocentesis Test For?

In general, an amniocentesis tests for two things –– chromosome abnormalities such as Down syndrome and neural tube defects such as spina bifida and anencephaly.

A karyotype or FISH test is done on the fetal skin cells to look at the fetal chromosomes. The most common chromosome abnormality is Down syndrome which is cause by having an extra copy of chromosome 21. Both FISH testing and karyotyping directly analyze the fetus’s chromosomes and determine whether or not the fetus has Down syndrome or another trisomy. FISH testing does not give you a complete picture of all the baby’s chromosomes, but it can give you a quick answer about trisomies.

Results of a FISH test are generally available in 3 to 4 days. A karyotype takes longer (up to two weeks), but gives you more complete information about the baby’s chromosomes.

An amniocentesis also tests for other chromosome abnormalities including trisomy 18 and trisomy 13. Testing detects over 99 percent of all chromosome abnormalities.

An amniocentesis tests for open neural tube defects, such as spina bifida and anencephaly, by measuring a protein called alpha-fetoprotein (AFP) which detects 96 percent of all open neural tube defects.

Neither amniocentesis nor chorionic villi sampling (CVS) can test for all birth defects. No prenatal test can test for all birth defects. So while normal results from an amniocentesis are reassuring, they do not guarantee your child will be healthy. 

If you are worried about a specific genetic disorder, you may want to ask your doctor or genetic counselor if prenatal testing is available for that disorder.

Getting an Amnio

Most women are surprised by how painless an amniocentesis is. Some women have an uncomfortable pressure or cramping feeling during the procedure, but most feel no pain at all. The procedure is generally quick –– about 30 seconds to one minute. An ultrasound is used throughout the procedure to monitor the position of the baby and to make sure that the needle doesn't touch the baby.

If your results are abnormal, you should talk with your doctor or with a genetics specialist about what your results mean. It is important to get accurate, up-to-date information about any diagnosis that you receive from an amniocentesis.

How does an amniocentesis differ from chorionic villi sampling (CVS)?

Both amniocentesis and chorionic villi sampling are diagnostic prenatal tests and you only need to have one of these procedures during a pregnancy. While both tests give you information about the baby’s chromosomes, there are some important differences between the procedures.

An amniocentesis differs from a chorionic villi sampling (CVS) in two ways:

1. Timing - An amniocentesis is done later in a pregnancy, during the second trimester. It is usually done between 15 to 20 weeks of pregnancy. A CVS test, on the other hand, is done in the first trimester, between 10 to 12 weeks of pregnancy.

2. Results obtained - Both amniocentesis and chorionic villi sampling will give you a picture of the fetal chromosomes which will diagnose Down syndrome with greater that 99% accuracy. However, with an amniocentesis, a substance called alpha fetoprotein (AFP) is also measured. The amount of AFP in the amniotic fluid can help determine if a fetus has an neural tub defects such as spina bifida or anencephaly.

Risk of Miscarriage

For many years, amniocentesis was thought to have a lower risk of miscarriage compared to CVS. However, it is now known that the risk of miscarriage due to these procedures is similarly low -– about 1/300 to 1/500, when performed by someone experienced with both procedures.

What are the advantages of amniocentesis?

The major advantages of an amniocentesis is that it has a lower risk of miscarriage and gives you more information. In addition to learning about the fetal chromosomes, the test also looks to see if the fetus has a neural tube defect.

What are the disadvantages?

The disadvantages of an amniocentesis is that the test is done in the later in pregnancy than a chorionic villi sampling.

Also, there is a small but real chance of miscarriage. Some people feel that if they were to get abnormal results in the second trimester of pregnancy that it would be much harder to make a decision about continuing or not continuing a pregnancy. Others feel that if they would not act on the information or results that they would get from an amniocentesis, there is no reason to have a procedure that includes a small risk of miscarriage. It can sometimes be helpful to talk to a genetic counselor about your prenatal screening options.

Bottom Line

An amniocentesis test is the most common for prenatal diagnostic testing done today. It has the lowest risk of miscarriage and give you the most complete set of results. However, if the results are abnormal, there is not cure for Down syndrome or the other birth defects detected by this procedure. The only options at this point in pregnancy are to terminate or end the pregnancy, or continue to pregnancy knowing the baby has a diagnosis of Down syndrome.

If your results are abnormal, you should talk with your doctor or with a genetics specialist about what your results mean and what your options are at this point in your pregnancy.

Sources:

Newberger, D., Down Syndrome: Prenatal Risk Assessment and Diagnosis. American Family Physician. 2001.

American College of Obstetricians and Gynecologists (ACOG). Your Pregnancy and Birth, 4th Edition. ACOG, Washington, DC, 2005.

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