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Amniocentesis: What it tests for and how it’s performed

According to the March of Dimes, amniocentesis is a type of prenatal test that has the ability to diagnose some birth defects and genetic conditions in your baby.1 Amniocentesis is a diagnostic option to explore if you learn that you’re at increased risk of having a baby with a birth defect or genetic condition.1

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.
  • Inherited diseases when parents have been shown to be carriers of the specific disease. Examples include Tay-Sachs disease, cystic fibrosis and spinal muscular atrophy.
  • Open neural tube defects, such as spina bifida and anencephaly, which can be tested by measuring a protein called alpha-fetoprotein (AFP) to detect 96% of all open neural tube defects.


How the test is performed

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 as early as 15 weeks into your pregnancy,1 counting from the first day of your last menstrual period.

An ultrasound is used to guide the physician as the amniotic fluid is withdrawn. This amniotic fluid contains cells that are shed from the developing fetus and the sample is sent to the laboratory to test for certain genetic diseases and birth defects.

Understanding results

A routine amniocentesis looks for chromosome disorders, most notably Down syndrome and trisomy 18, but there are other chromosomal abnormalities that may be detected at the same time. 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.


  1. Amniocentesis. March of Dimes website. Accessed August 2, 2017.