Pregnancy and Amniocentesis

During pregnancy, the fetus is surrounded by amniotic fluid, a substance much like water. Amniotic fluid contains live fetal cells and other things, such as alpha-fetoprotein (AFP). They provide important information about your baby's health before birth.

Your doctor may suggest a test called amniocentesis (or amnio) if your baby is at a higher risk of some birth defects. You may also need it if you have signs of an infection or if you may deliver early. Unlike blood testing, which shows only whether you are at risk, amniocentesis is used to make a diagnosis.

What Is Amniocentesis?

Amniocentesis is an invasive prenatal test in which your doctor takes a sample of the amniotic fluid. The sample (less than an ounce) is removed through a fine needle inserted into the uterus through your abdomen, under ultrasound guidance. A lab tests the sample, checking your baby’s chromosomes. Those tests can include the karyotype test, the FISH test, and microarray analysis.

Why Is an Amniocentesis Performed?

A complete anatomical ultrasound will be done prior to amniocentesis. but amniocentesis is performed to look for certain types of birth defects, such as Down syndrome, a chromosomal abnormality.

Because amniocentesis presents a small risk for both the mother and their baby, the prenatal test is generally offered to women who have a significant risk for genetic diseases, including those who:

  • Have an abnormal ultrasound or abnormal lab screens
  • Have a family history of certain birth defects
  • Have previously had a child or pregnancy with a birth defect
  • Had an abnormal genetic test result in the current pregnancy 

Amniocentesis does not detect all birth defects, but it can be used to detect the following conditions if the parents have a significant genetic risk:

Amniocentesis can  detect certain neural tube defects (diseases where the brain and spinal column don't develop properly), such as spina bifida and anencephaly. 

Because ultrasound is performed at the time of amniocentesis, it may detect birth defects that are not detected by amniocentesis (such as cleft palate, cleft lip, club foot, or heart defects). There are some birth defects, however, that will not be detected by either amniocentesis or ultrasound.

If you are having an amniocentesis, you may ask to find out the baby's sex; amniocentesis is the most accurate way.

Although it's rarely performed in the third trimester, an amniocentesis can determine if the baby's lungs are mature enough for delivery, or to evaluate the amniotic fluid for infection.

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When Is Amniocentesis Performed?

If your doctor has recommended an amniocentesis, the procedure is usually scheduled once between the 15th and 18th weeks of pregnancy.

How Accurate Is Amniocentesis?

The accuracy of amniocentesis is about 99.4%.

Amniocentesis may occasionally be unsuccessful due to technical problems, such as being unable to collect an adequate amount of amniotic fluid or failure of the collected cells to grow when cultured.

Does Amniocentesis Have Risks?

Yes. There is a small risk that an amniocentesis could cause a miscarriage (less than 1%, or approximately 1 in 1,000 to 1 in 43,000). Injury to the baby or mother, infection, and preterm labor are other potential complications that can happen but are extremely rare.

The risks are higher in women carrying twins.

Can I Choose Not to Have an Amniocentesis?

Yes. You will receive genetic counseling before the procedure. After the risks and benefits of amniocentesis have been thoroughly explained to you, you can choose whether or not you want to have the procedure.

What Happens During an Amniocentesis?

A small area of the abdomen is cleansed with an antiseptic to prepare for the amniocentesis. You may receive a local anesthetic (pain-relieving medication) to reduce any discomfort. The doctor first locates the position of the fetus and placenta with an ultrasound. Under ultrasound guidance, the doctor inserts a thin, hollow needle through your abdomen and uterus, and into the amniotic sac, away from the baby. A small amount of fluid (less than an ounce) is removed through the needle and sent for laboratory analysis.

If you’re carrying twins and each baby has their own amniotic sac, your doctor will take two samples. Amniocentesis is a little trickier with twins than it is with single babies. Make sure your doctor or technician has experience doing amniocentesis with multiples.

You may feel minor menstrual-like cramping or discomfort during the amniocentesis or for a few hours after the procedure.

Can I Resume Normal Activities After an Amniocentesis?

After an amniocentesis, it is best to go home and relax for the remainder of the day. You should not exercise or perform any strenuous activity, lift anything over 20 pounds (including children), or have sex.

You may take two Tylenol (acetaminophen) every 4 hours to relieve discomfort. The day after the procedure, you may resume all of your normal activities unless otherwise directed by your doctor.

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When Should I Call My Doctor After an Amniocentesis?

Call your doctor if you develop a fever or have bleeding, vaginal discharge, or abdominal pain that is more severe than cramps.

When Will I Receive the Results of the Amniocentesis?

The results of the amniocentesis are generally available within 2 or 3 weeks. If you have not received the results within 3 weeks, call your health care provider.

If the test shows a problem, you'll meet with a counselor to talk about your options. Doctors can sometimes treat certain birth defects, such as spina bifida, while your baby is still in the womb. Knowing about an issue can also help by increasing monitoring during pregnancy and helping you and your doctor to prepare.

WebMD Medical Reference Reviewed by Jennifer Robinson, MD on August 29, 2019

Sources

SOURCES:

The March of Dimes.

Reproductive Genetics Institute.

American Academy of Pediatrics healthychildren.org: "Detecting Genetic Abnormalities."

UCSF Medical Center: FAQ: "Amniocentesis."

The Ohio State University Wexner Medical Center: "Amniocentesis."

Children's Hospital Boston: "Karyotype, extended banding, fluorescent in situ hybridization (FISH)."

Johns Hopkins Manual of Gynecology and Obstetrics, 4th Edition. Lippincott Williams & Williams, 2010.

Mark I. Evans MD, professor of obstetrics and gynecology, Mt. Sinai School of Medicine, New York, N.Y.

James D. Goldberg MD, medical director, San Francisco Perinatal Associates, San Francisco.

© 2019 WebMD, LLC. All rights reserved.

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