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Amniocentesis is a test to look at the fluid (amniotic fluid) that surrounds your baby (fetus).

Normal amniotic fluid is clear to light yellow in color and does not contain any harmful bacteria. The cells can be tested for problems.

  • Cells from your fetus are looked at carefully for the proper number and arrangement of the cell parts (chromosomes) that show genetic disease. Normally there are 46 chromosomes in each cell, arranged in 23 pairs. Chromosomes also tell the gender of your fetus.
  • The amounts of some substances in the amniotic fluid may be measured. These results can find some birth defects, genetic diseases, and the maturity of your fetus.

What Affects the Test

Reasons you may not be able to have the test or why the results may not be helpful include:

  • If there is blood from your fetus in the amniotic fluid. This can falsely increase the level of the substances alpha-fetoprotein (AFP) and acetylcholinesterase (AChE) levels, which test for neural tube defects.
  • If the amniotic fluid is exposed to light. This can falsely lower bilirubin levels.
  • If there is blood or meconium in the fluid. This may cause an incorrect result for the test that checks to see whether your baby's (fetus's) lungs are mature.

What To Think About

  • Normal results from amniocentesis do not guarantee that your fetus will be healthy.
  • Amniocentesis can be done to help you prepare if your fetus has a possible birth defect or to help you make a decision about ending the pregnancy if a serious problem is found.
    Pregnancy: Should I Have Amniocentesis?
  • In very rare cases, amniocentesis may be done before 15 weeks of pregnancy. This is rare, because there may be greater risks to your fetus. Talk to your doctor about the risks and benefits of early amniocentesis.
  • Amniocentesis can't be done easily if the amount of amniotic fluid is very small or if the placenta is in front of your fetus.
  • Chorionic villus sampling (CVS) is another test that can find many fetal problems. CVS can be done earlier in pregnancy than amniocentesis, and results are ready sooner. For more information, see the topic Chorionic Villus Sampling (CVS).
  • Amniocentesis has a very small chance of causing bleeding that could lead to mixing your blood and your fetus's. So if you have Rh-negative blood, you will be given a vaccine (RhoGAM) to prevent Rh sensitization, which could harm your fetus if he or she has Rh-positive blood.
  • Amniotic fluid has cells that have been shed by your developing fetus. The cells are checked for the number and size of chromosomes (karyotype) to see if there are any problems. For more information, see the topic Karyotype Test.
  • If you have abnormal results from amniocentesis, you should ask your doctor or a genetic counselor for help in making decisions about the problems your fetus may have and about continuing the pregnancy. It will also be helpful to understand your possible risks with future pregnancies.

WebMD Medical Reference from Healthwise

Last Updated: April 04, 2012
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.

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