1. Care & Services
  2. Prenatal Diagnosis
  3. Diagnostic Tests
  4. Amniocentesis


Amniocentesis involves drawing a small amount of amniotic fluid from the bag of waters inside the uterus. Amniotic fluid can be used for a variety of analyses, including testing baby’s chromosomes, assessing for certain viral infections, and checking by means of DNA fingerprinting for a variety of important genetic conditions.

Amniocentesis is performed after mom has had a detailed ultrasound examination. During the ultrasound, we determine the location of the placenta, the quantity of the amniotic fluid, and whether an amniocentesis can safely be done or if it is too early in the pregnancy to perform the procedure.

During the procedure, an antiseptic is used to cleanse the skin, then a needle is directed, under ultrasound guidance, into the amniotic cavity. Usually, between 15 and 30 ml (1 to 2 tablespoons) of amniotic fluid will be withdrawn. The risks of amniocentesis are very low.

Decades ago, when amniocentesis tests were done without ultrasound guidance and using much larger needles than we use today, studies showed a risk of triggering miscarriage of about 1 in 200. With more recent techniques, using much smaller needles, and direct ultrasound guidance, the risk of an amniocentesis triggering miscarriage appears to be in the range of 1 in 700 to 1 in 1,600 when the procedure is done in the prenatal diagnostic center setting such as we have here. The state of California quotes a risk figure of between 1 in 300 and 1 in 500 for “serious problems” arising after an amnio, including leakage of fluid and miscarriage.

Introducing a needle into the uterus does carry a risk of direct injury to the fetus, and injury to the placenta and cord. These are very rare in our practice, because all of the physicians here have been extensively trained in both ultrasound and amniocentesis. About 1 percent of women will leak a small amount of amniotic fluid after an amniocentesis. In most of these women, the fluid leakage will stop, and the pregnancy will go on to a successful conclusion.

Results from amniocentesis for chromosome testing and AFP testing (a marker for spina bifida) are usually back within seven to 10 days.  When there is a high suspicion for a specific chromosomal abnormality, specialized DNA fingerprinting (Fluorescent In Situ Hybridization testing) can be done, and the results are usually back within two working days.  There are many other types of tests that can be run from amniocentesis specimens.  Please check with your genetic counselor at the time of the test to learn when you can expect to have your results back.