What Is an Amnio or 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. This procedure involves removing a 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.

What Happens During An Amniocentesis?

During an amniocentesis, an ultrasound is used to visualize the baby. Then, a very thin needle is inserted into the lower abdomen and a 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 provide 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 three things:

  • chromosome abnormalities such as Down syndrome 
  • neural tube defects such as spina bifida and anencephaly
  • inherited genetic disorders like cystic fibrosis, sickle cell anemia, and Tay-Sachs disease

Both FISH testing and karyotyping directly analyze the fetus’s chromosomes to determine whether or not the fetus has Down syndrome or another trisomyDown syndrome, also called trisomy 21, is the most common chromosome abnormality and is caused by having an extra copy of chromosome 21.

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 within 1 to 2 days. A karyotype takes longer (two to three weeks), but gives you 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. In addition, 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.

That being said, neither amniocentesis nor chorionic villi sampling (CVS) 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 Amniocentesis

Most women are surprised by how painless an amniocentesis is. While some women have an uncomfortable pressure or cramping feeling during the procedure, most feel no pain at all. The procedure is generally quick, and 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.

When you receive the results of your amniocentesis, 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:

  • 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 13 weeks of pregnancy.
  • 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 percent 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 neural tube defects such as spina bifida or anencephaly.

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.

View Article Sources
  • American College of Obstetricians and Gynecologists (ACOG). (September 2015). Diagnostic Tests for Birth Defects.