Amniocentesis: What to expect

By Jeffrey Boyle, MD Apr 24, 2017

Amniocentesis is a procedure involving the removal of a small amount of amniotic fluid from the uterus for testing during pregnancy. Amniotic fluid surrounds the baby during development in the womb. It contains cells and other substances that can be tested to determine the baby’s genetic makeup, presence of infection and lung maturity.

The amniocentesis procedure is fairly simple. An ultrasound is performed on the pregnant mother as she lies on an exam room table to assess how much fluid is present around the baby, the location of the placenta and the baby’s position. This information is helpful for a safe amniocentesis. It is also used to determine if the baby is growing normally and if there are any signs of birth defects.
After the ultrasound, the doctor performing the amniocentesis — a perinatologist (maternal-fetal medicine specialist) — will talk to you about the reasons for performing the procedure and what it involves, its risks and the availability of other tests.

You will be asked to read and sign a consent form stating you have discussed the procedure with your doctor, you understand what is being done and know the risks involved.

Why is amniocentesis done?

An amniocentesis can be performed at any time from 15 weeks gestation until term. It usually takes place earlier in pregnancy — between 15 and 24 weeks — when being done for genetic testing. Lung maturity testing is typically done after 32 weeks. Amniocentesis to rule out infection or remove excess fluid can take place any time in pregnancy.

Reasons to perform amniocentesis include:
Genetic testing. This most common reason to do an amniocentesis determines the baby’s number of chromosomes.

To look for genetic abnormalities, two main tests can be performed. A rapid fluorescence in-situ hybridization (FISH) test looks for the most common chromosomal disorders like Down syndrome, trisomy 18 and trisomy 13. Sex chromosome (X and Y) disorders can also be detected with the rapid test, with results usually available three days after the amniocentesis.

A microarray test examines all the baby’s chromosomes in great detail, providing the most complete information available for many different genetic conditions. Results from the test are usually available 7 to 10 days after the amniocentesis.

A genetic amniocentesis can be done for any patient, with or without risk factors for genetic abnormalities. Some women request a genetic amniocentesis when they have no risk factors because they want to know the results before delivery. Others request it only if the risk of a genetic condition is increased due to maternal age over 35, abnormal blood screening test, abnormal ultrasound findings, family history of genetic conditions or a chromosomal condition in a previous pregnancy.

Genetic abnormalities are fairly common, something many women don’t realize going into pregnancy. A particular misconception is Down syndrome is the only major genetic anomaly to be concerned about. The common trisomies — like Down syndrome and including trisomy 21, 13 and 18 — occur in 1 in 500 babies. Chromosome abnormalities other than the common trisomies occur in 1 in 250 babies. Missing or extra pieces of chromosomes, called microdeletion and duplication syndromes, are more common than Down syndrome, occurring in one in 80 newborns. Many of these conditions are actually more severe than Down syndrome. Genetic testing with microarray can also detect some of the 1-3 percent of infants with developmental delay, and 1 child in 80 with autism.

When an amniocentesis is performed for genetic testing, you will also be asked to visit with a genetic counselor who will take your family and genetic history, explaining all the genetic testing options available. The counselor will also provide you with your results and explain what they mean.

Each patient or family should decide if genetic testing with amniocentesis is in their best interests and will provide helpful information before delivery. The process involves speaking with a genetic counselor, the provider taking care of you during pregnancy and the physician performing the amniocentesis.
Lung maturity. Chemicals in the amniotic fluid are tested to determine if the baby’s lungs are mature enough to survive outside the womb. Usually done at 32-39 weeks gestation, the information can help ascertain if delivery is safe.

Infection. In patients with early rupture of membranes, amniocentesis helps establish if there is infection in the amniotic fluid that could have caused the membranes to rupture. Fluid is tested for bacteria and other signs of infection. Amniocentesis can also rule out amniotic fluid infection in patients with fever, abdominal pain, rapid heart rate and elevated white blood count.

Removal of excess fluid. In cases of polyhydramnios, or excess amniotic fluid, with a patient experiencing signs of preterm labor, shortness of breath, difficulty lying down and difficulty sleeping, amniocentesis is performed to remove the excess fluid. It can be done multiple times to treat the condition.

Summary of reasons to do an amniocentesis

If you want the most complete genetic information about your baby before delivery, or you have abnormal screening tests such as first trimester screen, quad screen or noninvasive prenatal test, you may want an amniocentesis. Other reasons include:

  • Birth defect on ultrasound exam
  • Determining lung maturity of the baby
  • Family history of genetic condition
  • Maternal age 35 or older
  • Parent is a carrier of genetic disorder
  • Previous child with chromosome abnormality
  • Slow or abnormal growth on ultrasound
  • Signs or symptoms of infection in the amniotic fluid
  • Treating excess fluid

What are the risks of amniocentesis?

The procedure carries with it a very low risk. Very rarely, patients experience a small amount of fluid leaking from the cervix or vagina after the procedure. In most cases, the amount is small and stops within a few days.

Whether or not you have an amniocentesis, there is a small risk of miscarriage with every pregnancy. An amniocentesis during the second trimester has a very low risk of miscarriage — less than 1 in 500. Recent studies have even shown there is no increased risk of miscarriage in amniocentesis performed by experienced doctors.

During the amniocentesis, the baby might move an arm or leg near the amniocentesis needle. Because the procedure is done using ultrasound, the needle can be easily moved and injury to the baby is rare.

There is also a small risk of introducing an infection into the amniotic fluid from the procedure, thought to be less than 1 in 1,000. Risk is minimized through proper cleansing with antiseptic solution before the procedure.

Women with Rh negative blood type having amniocentesis are given an injection of Rh immune globulin (Rhogam) after the procedure to prevent the baby’s blood cells from mixing with the mother’s.

How is amniocentesis performed?

After the ultrasound is done, a small area of your abdomen is cleansed with a special antiseptic soap solution to help prevent infection. A local anesthetic (skin-numbing medicine) is usually not needed because the needle is very thin and anesthetic can’t numb the uterus. The needle is in the abdomen for a very short time, usually 20-30 seconds. Most women report only mild discomfort.

A fine needle is inserted into the uterus through the mother’s abdomen under ultrasound guidance to remove the sample of amniotic fluid. The needle is placed into a pocket of fluid away from the baby, not into or near the baby. An ultrasound scan is performed throughout the procedure so the doctor can see where the needle is.

The amount of amniotic fluid removed is usually two to three tablespoons, replaced naturally within a day. After removal, the fluid is placed into sterile test tubes for testing. After cleaning, an ultrasound is performed to confirm a normal fetal heartbeat.

What can I expect after the procedure?

After the amniocentesis, patients can return to work or other normal daily activity. For a few hours to a day following the procedure, there might be some mild cramping or soreness of the uterus, similar to a flu shot in the arm. It is generally recommended to avoid strenuous exercise, heavy lifting and sexual activity for a day or two. Call your doctor if you experience increasing uterine pain, fever, redness or inflammation where the needle was inserted, bleeding or heavy leaking after the procedure.

How accurate are the results?

For genetic testing, amniocentesis is considered a diagnostic — not a screening — test. Results are very accurate, providing certain information as to whether or not your baby has a genetic condition.

Because the testing is done on actual cells shed from the baby and the risk of contamination is low, false positive and false negative results are very uncommon. Providing the most complete genetic information available about your baby, genetic amniocentesis is accurate more than 99 percent of the time.

Screening tests, however, provide a risk estimate, meaning they tell you whether it is likely or unlikely that the baby has a condition. They do not provide definite yes or no answers. Abnormal screening tests usually need to be followed up with a diagnostic test like amniocentesis.

Amniocentesis cannot detect all types of birth defects or genetic abnormalities. For example, amniocentesis results do not provide information about the physical development of the baby. Also, there remain many undiscovered genetic conditions.

Testing for lung maturity and infection from chemicals in the fluid is also accurate. There are no substances in the fluid that can cause a false test result.

Fear of amniocentesis

The thought of a needle going into your abdomen and near your baby can seem scary. As a physician who performs amniocentesis, I frequently hear patients say they are afraid of needles. This is a normal response. Women seem to be more afraid of this procedure than having an IV placed into their arm or having an epidural for labor. Most women I have seen experience an amniocentesis will say it wasn’t as bad as they thought it was going to be and was less painful than having an IV for labor.

Many people think the needle is large. In fact, it is long, but very thin and therefore doesn’t cause much pain. You might feel slight cramping as it goes through the uterus and a pulling sensation as the fluid is withdrawn, but these feelings aren’t necessarily painful — they just feel “weird”. When an amniocentesis is performed by a physician who has experience doing them and does so often, it is usually quick with minimal discomfort.

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