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Amniocentesis facts

  • Amniocentesis may be performed for chromosome analysis or the detection of genetic defects and conditions in the fetus.
  • Pregnant women over the age of 35 years are candidates for amniocentesis to detect chromosomal abnormalities in the fetus.
  • Amniocentesis can also be used to determine the maturity of the fetal lungs or the cause of a fever in the mother.
  • For genetic testing, chromosome analysis, and the evaluation of an abnormal alpha feto-protein test, amniocentesis is usually performed between the 16th and 21st weeks of pregnancy.
  • The risk of fetal loss from the amniocentesis procedure is less than 1%.

What is amniocentesis?

Amniocentesis is a procedure wherein a sample of fluid is removed from the amniotic sac for analysis. The amniotic sac is found within the uterine cavity. The amniotic sac is the fluid-filled structure inside a pregnant woman's uterus within which the baby lives. Fetal cells, proteins, and fetal urine freely move within this sac.


During amniocentesis, fluid is removed by placing a long needle through the abdominal wall into amniotic sac. Sometimes, the woman's skin is injected first with a local anesthetic, but this is not usually necessary. The amniocentesis needle is typically guided into the sac with the help of ultrasound imaging performed either prior to or during the procedure. Once the needle is in the sac, a syringe is used to withdraw the clear amber-colored amniotic fluid, that resembles urine. The volume of fluid withdrawn depends upon the age of the fetus and the reason for the procedure.


The fluid can then be sent for evaluation of fetal lung maturity, genetic evaluation, evidence of spina bifida (a birth defect in spinal cord development) or other neural-tube defects, the presence of infection, or fetal chromosomal analysis. Chromosomes are structures which contain all of the genetic information within our cells. Amniotic fluid contains numerous free-floating fetal cells that can be grown in the laboratory. When these cells multiply and reach a certain number, their chromosomes are extracted and analyzed. It takes approximately two weeks to perform chromosomal analysis. The fluid also contains proteins, minerals and other compounds that can be tested. These additional studies may require 1 to 7 days to perform. Data obtained from amniotic fluid can help women make informed decisions regarding their pregnancies.


For most patients, amniocentesis is a fairly rapid and comfortable procedure. Some women experience uterine cramping after the procedure. They may also feel faint during or following the test.