Test Overview

Amniocentesis is a test to look at the fluid that surrounds your baby (fetus) in the uterus. Amniotic fluid has cells and other substances that can give clues about the health of your fetus.

For this test, a needle is put gently through your belly into your uterus. About 30 mL (2 Tbsp) of fluid is taken out and looked at.

This test may be done between weeks 15 and 20 to look at the baby's genes. It can also be used later in pregnancy.

Amniocentesis for genetic testing

Amniocentesis is often done around week 16 to see if a fetus has certain genetic conditions. It can also tell the sex of a fetus and help find certain neural tube defects.

Amniotic fluid has cells that have been shed by your growing fetus. The cells are checked for the number and size of chromosomes. This can show if there are any problems that put the baby at risk for certain conditions, such as Down syndrome. But this test can't find many common birth defects, such as cleft lip, cleft palate, or heart problems.

Testing is most often done as soon as possible so women and their families have time to think about their options.

Amniocentesis in late pregnancy

This test may also be done later in pregnancy if your doctor thinks you may have an infection of the amniotic fluid (chorioamnionitis).

Why It Is Done

Amniocentesis may be done during your second trimester to find certain genetic conditions as well as neural tube defects.

You may choose to have this test because:

• You are concerned about your baby because of your age. As you get older, you have a greater chance of having a baby with a genetic condition.
• You want to know for sure if your baby has a certain health problem. This may help you decide early whether you want to continue your pregnancy or make plans to care for a sick child.
• You or the baby's father carries an abnormal gene that is known to cause a disease. These diseases include Tay-Sachs disease, sickle cell disease, and cystic fibrosis.
• You or the baby's father has a family history of a genetic condition or neural tube defect.
• You've had a baby with a genetic condition or neural tube defect.
• Screening tests suggest that your chance of having a baby with a genetic condition or neural tube defect is higher than average.

Amniocentesis can tell the sex of your baby. This is important when you or the father may pass on a disease that occurs mainly in one sex (sex-linked). Examples include hemophilia and Duchenne muscular dystrophy. These both occur mainly in males.

Amniocentesis may also be done to:

• Remove excess amniotic fluid (polyhydramnios).
• See if the amniotic fluid is infected (chorioamnionitis).

Learn more

How To Prepare

You may want to talk to a genetic counsellor before or after the test. This person is trained to give you detailed information about the test. The counsellor can help you make decisions about testing. The counsellor can also help you understand the results of the tests.

You will be asked to sign a consent form that says you understand the risks of the test and agree to have it done.

How It Is Done

Amniocentesis is done by your obstetrician in the doctor's office or in the hospital. You probably won't need to stay overnight in the hospital unless problems occur during the test.

You will lie on your back on an examination table. Your belly will be cleaned with an antiseptic.

Your doctor will use a fetal ultrasound to check the position of your fetus and the placenta. Ultrasound uses sound waves to make a picture of the uterus, your fetus, and the placenta on a TV screen. Your fetus's heart rate can also be watched during the test using ultrasound.

With the ultrasound picture as a guide, your doctor will gently put a thin needle through your belly and into your uterus. This will not hurt your fetus or the placenta. If your fetus moves too close to the needle, the doctor will take the needle out and try again in another spot.

The doctor will take about 30 mL (2 Tbsp) of amniotic fluid. Then the needle is removed. The site is covered with a bandage.

Your fetus's heart rate and your blood pressure, pulse, and breathing will be checked before, during, and after the test.

How long the test takes

The test will take about 15 minutes. The thin needle is in your belly for only 1 to 2 minutes.

How It Feels

You may feel a sharp sting or burn in your belly where the needle is put in. This lasts for only a few seconds. When the needle is put into your uterus, you may feel a sharp cramp for a few seconds.

When the amniotic fluid is taken out, you may get a feeling of pulling or pressure in your belly. To keep yourself comfortable, breathe slowly and relax your belly muscles during the test.

Risks

Some risks of amniocentesis may include:

• Leakage of amniotic fluid.
• Vaginal bleeding.
• Infection in the uterus.
• Miscarriage. About 3 out of 1000 women have a miscarriage after this test.^{footnote 1}

Amniocentesis has a very small risk of causing bleeding that could mix your blood and your fetus's blood. So if you have Rh-negative blood, you will be given the Rh immunoglobulin shot (such as WinRho). This can prevent Rh sensitization, which could cause harm if your baby has Rh-positive blood.

Results

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

• Cells from your fetus are looked at carefully. They are checked 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 sex 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.

Normal results from amniocentesis do not guarantee that your baby will be healthy.