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 baby. Amniocentesis can be done after the 14th week of pregnancy, when there is enough amniotic fluid to take a sample. It is done by gently putting a needle through your belly into your uterus. About 30 mL (2 Tbsp) of the amniotic fluid is taken out and looked at.

Amniocentesis is often done between weeks 15 and 18 (during your second trimester) or later in your pregnancy.

Amniocentesis is offered to women whose pregnancies may be high-risk. These include women:

• With a close family member who has a genetic problem.
• Who will be older than 35 years of age on their due date.
• Who have an abnormal screening test for Down syndrome 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.
• With Rh sensitization.

Amniocentesis in early pregnancy

Amniocentesis is often done between weeks 15 and 18 to see whether your baby has certain types of birth defects. Amniocentesis can also tell the sex of your baby.

Amniotic fluid has cells that have been shed by your developing baby. 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 between the 15th and 18th weeks of pregnancy, when the pregnancy can be ended if your baby is severely disabled. However, 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 is done when the risk of a birth defect or disease is higher than the risk of the test. 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

Amniocentesis may be done late in pregnancy (during the third trimester) to see whether your baby's lungs are mature enough for early delivery. Your developing baby makes substances that can be found and measured in amniotic fluid. The amounts of these substances show the maturity of your baby's lungs 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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Author:	Jan Nissl, RN, BS Sydney Youngerman-Cole, RN, BSN, RNC	Last Updated: October 12, 2006
Medical Review:	Renée M. Crichlow, MD - Family Medicine Siobhan M. Dolan, MD, MPH - Reproductive Genetics Liisa Honey, MD, FRCSC - Obstetrics and Gynecology
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