Medical Tests
Information on medical tests, including how to prepare, what to expect, and what the results mean.

Amniocentesis

Test Overview

Amniocentesis is a test to look at the fluid (amniotic fluid) that surrounds your baby (fetus). Amniotic fluid has cells and other substances that can give clues about the health of your fetus. Amniocentesis is done by gently putting a needle through your belly into your uterus. About 2 Tbsp(30 mL) of the amniotic fluid is taken out and looked at.

Amniocentesis is generally done between weeks 15 and 20 (usually around week 16) to look at genetic information. It can also be used later in pregnancy to see how the fetus is doing.

Amniocentesis is offered to women who would like a test to find problems with the fetus. These include women:

• With a close family member who has a genetic problem.
• Who have an abnormal screening test for birth defects in the first trimester.
• Who have an abnormal triple or quadruple screen test (alpha-fetoprotein, estriol, human chorionic gonadotropin, hormone inhibin A).
• Who have a fetal ultrasound that shows problems, such as a heart problem or too much amniotic fluid.

Amniocentesis

Amniocentesis is often done around week 16 to see whether a fetus has certain types of birth defects. Amniocentesis can also tell the sex of your fetus.

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 that put the baby at risk for certain conditions. Testing is most commonly done as early as possible so that women and their families have time to consider their options. But amniocentesis cannot find many common birth defects, such as cleft lip, cleft palate, heart problems, and some types of mental retardation.

Amniocentesis is not done as a general screening test for birth defects because it has an increased risk to your fetus. One study showed the chance of miscarriage from amniocentesis was small (about 1 in 400) when it is done by highly trained providers.¹ In some studies, the risk is a little higher, about 2 to 4 in 400.² 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.

Should I have an amniocentesis?

Amniocentesis in late pregnancy

If you are at risk of having your baby early amniocentesis may be done during the third trimester to see whether your baby's lungs are developed. Your developing fetus makes substances that can be found and measured in amniotic fluid. The amounts of these substances show how mature the lungs are and if your baby will be able to breathe without help if delivered early.

Amniocentesis may also be done later in pregnancy if an infection of the amniotic fluid (chorioamnionitis) is suspected.

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