Amniocentesis

Written by Heather Ross | Published on July 18, 2012
Medically Reviewed by George Krucik, MD

Overview

Amniocentesis is a test used to determine whether your unborn baby has any genetic abnormalities (genetic amniocentesis) or whether its lungs are developed enough for birth (maturity amniocentesis).

Your doctor will use a long, thin needle to collect a small amount of amniotic fluid. This fluid surrounds and protects the baby while it is in your womb. It is then tested for certain genetic conditions, including Down syndrome, spina bifida, and cystic fibrosis.

The test results can help you determine whether to continue with your pregnancy. The test can also tell you whether or not your baby is mature enough to be born. It is also helpful for determining whether you need to deliver early to prevent complications from your pregnancy.

Why Is Amniocentesis Performed?

You might consider genetic amniocentesis if the results of your prenatal screening test are abnormal. Amniocentesis can confirm or rule out a diagnosis made during the initial screening test. If you have already had a child with a birth defect or neural tube defect (a serious abnormality of the brain or spinal cord), you can use amniocentesis to check whether your unborn child also has such a condition.

If you are 35 or older, your baby is at a higher risk for chromosomal abnormalities, such as Down syndrome. Amniocentesis can identify these abnormalities. If you or your partner is a known carrier of a genetic disorder, such as cystic fibrosis, amniocentesis can detect whether your unborn child will be affected.

You might consider maturity amniocentesis if you need to deliver your baby early due to complications from your pregnancy. This test can determine whether your baby’s lungs are developed enough to allow the child to survive outside the womb.

Amniocentesis may also be performed if your doctor suspects that your unborn child may have an infection or anemia. Your doctor may also order amniocentesis if he or she thinks that you have a uterine infection. The procedure can also decrease the amount of amniotic fluid in your womb, if necessary.

How Is Amniocentesis Performed?

This test is performed on an outpatient basis, so you won’t need to stay in the hospital. Your doctor will first perform an ultrasound to determine the exact location of your baby in the uterus. An ultrasound is a non-invasive procedure that uses high-frequency sound waves to create an image of your unborn baby. Your bladder must be full during the ultrasound, so drink plenty of fluids beforehand.

After the ultrasound, your doctor may apply numbing medication onto a section of your belly. Then, they’ll insert a needle through your belly and into your womb, withdrawing a small amount of amniotic fluid. This portion of the procedure usually takes about two minutes.

The results of genetic tests on your amniotic fluid are usually available within a few days. The results of tests to determine the maturity of your baby’s lungs are usually available within a few hours.

What Risks Are Associated With Amniocentesis?

Amniocentesis is usually performed during your second trimester. There is a slight risk of miscarriage (between one in 300 and one in 500) if the test is performed after 16 weeks of pregnancy. The risk is slightly higher if the test is done before 15 weeks of pregnancy.

Some women experience cramping or a small amount of vaginal bleeding after the procedure. Rarely, amniocentesis may cause amniotic fluid to leak out of your body or it may trigger a uterine infection.

In rare cases, this test may cause some of your baby’s blood cells to enter your bloodstream. If you have a type of protein called the Rh factor on the surface of your red blood cells, you are considered Rh positive. If you do not, then you are Rh negative. It is possible for you to have Rh negative blood and your baby to have Rh positive blood. In this case, when your blood cells mix, your body may react as if it were allergic to your baby. If this happens, your doctor will give you a drug called RhoGAM. This medication will prevent your body from making antibodies that will attack your baby’s blood cells.

If you have an infection, such as hepatitis C or HIV, amniocentesis could cause your infection to be transmitted to your unborn baby.

Understanding Your Test Results

If the results of your amniocentesis are normal, your baby most likely does not have genetic or chromosomal abnormalities. In the case of maturity amniocentesis, normal test results will assure you that your baby is ready to be born.

Abnormal results may mean that your baby has a genetic problem or serious birth defect. Discuss all test results with your doctor and your partner so that you can make an informed decision about whether or not to continue your pregnancy.

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