Reasons for testing

Understanding your testing options helps you prepare for the future.

Your reasons for testing may include:

the recommendation from The American College of Obstetricians and Gynecologists that all pregnant women be offered the option of diagnostic testing (as opposed to just screening) for Down syndrome, trisomy 18 and open neural tube defects
a risk for a known inherited disorder, birth defect or mental retardation
abnormal results from a maternal screening test, such as SequentialScreen^SM, Afp4^®, or ultrasound
a family history of chromosome abnormalities

If you are not sure whether you should have amniocentesis, please talk with your doctor, a genetic counselor or other specialist in genetics. See “Questions to Ask Your Doctor”.

Testing for...

Amniocentesis tests for:

Fetal chromosome abnormalities. The most common chromosome abnormality is Down syndrome, caused by an extra #21 chromosome. The next most common chromosome abnormalities are trisomy 18 and trisomy 13. Testing detects over 99% of all chromosome abnormalities.
Inherited diseases when parents have been shown to be carriers of the specific disease. Examples include Tay-Sachs disease, cystic fibrosis and sickle cell disease.
Open neural tube defects, such as spina bifida and anencephaly, can be tested by measuring a protein called alpha-fetoprotein (AFP) to detect 96% of all open neural tube defects.

How the test works

Amniocentesis is a procedure to withdraw a small amount of amniotic fluid (the fluid surrounding a developing fetus) from the uterus. The amniotic fluid can be tested for certain kinds of birth defects.

Amniocentesis is performed in your doctor’s office or hospital clinic, usually when you are about 16 weeks pregnant, counting from the first day of your last menstrual period.

A very thin needle is inserted into the lower abdomen usually below or to the side of the belly button and about an ounce of fluid is removed. The amniotic fluid contains shed cells from the developing fetus and the sample is sent to the laboratory to test for certain genetic diseases and birth defects.

An ultrasound is used to guide the needle during the procedure. Most women report an uncomfortable feeling of pressure, a cramping feeling during the procedure (about 30 seconds to one minute). Most women do not report feeling acute pain, but every woman is different. After the sample is taken, another ultrasound check is conducted.

Understanding results

A routine amniocentesis looks for chromosome disorders, most notably, Down syndrome and trisomy 18, but there are others that are detected at the same time. In the fluid, AFP is tested to look for spina bifida and other open neural tube defects. If your results are abnormal, you should talk with your doctor or with a genetics specialist about what your results mean.

What's not detected

Neither CVS nor amniocentesis is able to test for the kinds of birth defects that do not have a known cause. A few examples are autism, cleft lip, non-specific mental retardation and most heart defects. Every pregnancy has some risk, 3-5%, for these kinds of defects.

Risks of testing

Since it was developed in the late 1960’s, genetic amniocentesis has been performed hundreds of thousands of times. Though it is now a routine procedure, it does have some risks.

Miscarriage is uncommon, but is the most serious risk of amniocentesis. Some miscarriages happen spontaneously, unrelated to the procedure. The risk for miscarriage is about 1 in 300 to 1 in 500. Early amniocentesis (before 15 weeks) has an increased risk, up to 1 in 100. An amniocentesis is not routinely performed after 22 weeks, but may be offered at this time in specific situations, such as a recently detected ultrasound abnormality. Please discuss the risks with your doctor.

Bleeding, cramping, leaking of fluid from the vagina

These symptoms, which happen to about 1% of women having amniocentesis, do not usually result in a miscarriage. But, if you have these symptoms, you should call your doctor.

Infection

Great care is taken to prevent infection. Therefore, infection following amniocentesis is very rare. However, if you have a fever or any flu-like symptoms after amniocentesis, you should call your doctor.

Harm to the fetus

Since the ultrasound picture gives the doctor exact information about the location of the fetus inside the uterus, the risk that the needle will touch the fetus is extremely low.

Rh problems

If you have Rh negative blood type, and the baby’s father has Rh positive blood type, you should have an injection of Rh immune globulin following amniocentesis. This helps prevent Rh disease in the baby.

View Test Comparison Chart (PDF)
Compare testing periods, detection rates and risks for screening and diagnostic testing options.

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