What Is an Amniocentesis?

What to expect when undergoing this test

Amniocentesis. John Fedele Creative #: 594826963

An amniocentesis is a medical procedure used for the prenatal diagnosis of chromosomal abnormalities and other medical conditions in the fetus. It involves using a long needle inserted through the mother's abdomen to collect a sample of amniotic fluid. It is often recommended for women who are over the age of 35 or who have risk factors for having a baby with a genetic condition. The procedure is usually done between 15 and 20 weeks gestation, and carries some potential risks, such as miscarriage. Potential benefits include the opportunity for parents to plan for a special needs child, to initiate in utero treatment (if applicable), and make a more educated decision about continuing a pregnancy.

At the current time, the American College of Obstetricians and Gynecologists (ACOG) recommends genetic screening for all pregnant women, either via non-invasive prenatal tests or through more invasive tests such as amniocentesis or chorionic villus sampling. No matter what your doctor recommends, however, the decision whether to have an amniocentesis or not belongs to you and your partner.

Purpose of Test

If you are pregnant, your doctor may recommend an amniocentesis for a number of reasons, such as your age or family medical history.

Indications

Some of the specific reasons why an amniocentesis may be recommended include:

  • Maternal age of 35 or older: The risk of chromosomal abnormalities increases with age, especially after the age of 35. (Keep in mind that most babies born to mothers over the age of 35, or even over the age of 40, do not have chromosomal abnormalities.)
  • Abnormal results on prenatal blood screening tests such as a Quad screen or prenatal cell-free DNA screening
  • Abnormal results on a first trimester or second trimester ultrasound
  • Previous child with a chromosomal abnormality or neural tube defect
  • Family history of certain genetic conditions (you or your partner)
  • Suspected infection
  • To evaluate Rh disease
  • To deliver medications into the uterus
  • Lung maturity testing
  • For polyhydramnios (too much fluid in the uterus): If too much fluid is present, an amniocentesis may be done to remove the excess fluid.

Conditions That May Be Diagnosed

Conditions that may be detected with an amniocentesis include:

  • Chromosomal abnormalities such as Down syndrome and more (see below)
  • Genetic conditions such as sickle cell disease, cystic fibrosis, and more
  • Congenital infections that can cause birth defects such as cytomegalovirus, herpes, rubella (German measles), toxoplasmosis, parvovirus B19, and Zika virus
  • Neural tube defects (spina bifida or anencephaly)

Potential Benefits

There are a number of reasons why women choose to have an amniocentesis. While the following are benefits of the test from a medical standpoint, many people consider and weigh these differently based on personal beliefs.

Learning about chromosomal abnormalities in a fetus can:

  • Help parents better prepare physically and/or emotionally for a special needs child
  • Lead to a diagnosis so that treatment may be started before birth (for example, stem cell treatments in fetuses with sickle cell disease)
  • Give parents information that may factor into considering termination of a pregnancy (therapeutic termination) when birth defects will be incompatible with life or seriously reduce quality of life for the fetus

Limitations

While amniocentesis is a very accurate test for detecting chromosomal problems, it cannot diagnose all potential genetic problems or birth defects in a fetus.

Similar Tests

A similar test for evaluating chromosomal abnormalities is chorionic villus sampling (CVS). CVS is also an invasive test in which a sample of the placenta (chorionic villi) is obtained via the cervix or abdominal wall between weeks 10 and 13 of pregnancy.

CVS has the advantage of being able to be done earlier in the pregnancy (especially if termination is a consideration), but is unable to diagnose some conditions that can be diagnosed with amniocentesis (such as neural tube defects) and has a slightly increased chance of causing a miscarriage.

Other Tests

Noninvasive prenatal testing options are blood tests that can give clues to the presence of chromosomal abnormalities or neural tube defects in a fetus, but cannot conclusively diagnose these conditions. A Quad screen is a screening test that tests the combination of alpha-fetoprotein (AFP), estriol, human chorionic gonadotropin (HCG), and inhibin A. Prenatal cell-free DNA testing is a newer test that can also give clues to the presence of chromosome abnormalities.

Genetic testing may also include other types of tests, such as preimplantation screening and carrier screening (for example, to see if either parent-to-be carries the gene for cystic fibrosis).

Risks and Contraindications

There are potential risks associated with an amniocentesis that need to be weighed against the possible benefits. While the emotional impact of an amniocentesis (either the procedure and/or potential results) are not usually listed as risks, it's important to consider how the procedure or results obtained may affect you psychologically. The decision to proceed with the test is entirely yours.

Potential Risks

Potential risks and complications of an amniocentesis include:

  • Miscarriage: The risk of miscarriage due to amniocentesis varies based on the study, but ranges from 1 in 1000 to 1 in 200. A 2018 review of studies from 2000 to 2017 found that the overall risk of miscarriage directly related to amniocentesis was slightly more than 1 in 300 women. This risk is greater for women who have an amniocentesis before 15 weeks gestation.
  • Amniotic fluid leak: Roughly 2 percent of women experience leakage of amniotic fluid after an amniocentesis, but this resolves on its own in the majority of cases.
  • Needle injury: There is a small risk that a baby's arm, leg, or other regions could be penetrated by the spinal needle, but this is very uncommon.
  • Infection: There is a small risk of infection related to the needle placement (from bacteria on the skin).
  • Transmission of infections from the mother to the baby: Infections such as hepatitis B, hepatitis C, HIV, and toxoplasmosis could be transmitted from the mother's blood to the fetus (see contraindications below).
  • Rh sensitization: Also due to exposure of the fetus to the mother's blood, the possibility of Rh sensitization from an Rh-negative mother to an Rh-negative fetus could occur. This is rare since mothers who are Rh negative are given Rhogam after the procedure.
  • Clubfoot: There is a risk of clubfoot (talipes equinovarus) when an amniocentesis is done before 15 weeks gestation.
  • Preterm labor

Contraindications

There are several relative contraindications for an amniocentesis. With these conditions, an amniocentesis may be done in certain circumstances, but the risks need to be very carefully weighed against possible benefits. These include:

  • Mothers who have an infection on the skin of the abdomen
  • Mothers who have chronic infections with hepatitis B, hepatitis C, or HIV
  • If there is a low amount of amniotic fluid (oligohydramnios)
  • An anterior placenta

Before the Test

When you talk to your doctor about having an amniocentesis, she will want to know about any risk factors you have for chromosomal abnormalities, any medical conditions you have, and any medications you are taking. She will ask you about any allergies, including an allergy to latex. She will also want to review your prenatal blood tests and determine your blood type and Rh.

Timing

An amniocentesis procedure usually takes only about five minutes, but it's important to set aside at least a few hours before the procedure. This includes the time to check your vital signs and the baby's heart rate, and to do an ultrasound to determine the baby's position and more before the procedure. You will also be observed for a short time after the procedure to make sure you and the baby are doing well before going home. Most people spend around an hour on the table in the procedure room.

Location

An amniocentesis may be done in an obstetrician's office or at a medical center/hospital.

What to Wear

You will be undressed from the waist down during the procedure, but it's a good idea to wear clothing that is loose fitting around your abdomen, as you may feel some abdominal cramping after the procedure.

Food and Drink

There are no special dietary restrictions before an amniocentesis, but it's important to drink plenty of fluids. In some cases, a physician will encourage you to drink extra fluids and refrain from urinating so that you have a full bladder during the test (a full bladder is more important if you are closer to 15 weeks and less so if you are closer to 20 weeks).

Cost and Health Insurance

Many insurance companies cover at least part of the cost of an amniocentesis. For those who do not have insurance, the average cost is between $1,500 to $2,500, but can range from less than $1,000 to over $7,000. If you will need to pay out of pocket for the procedure, some clinics offer reduced fees if you set up a payment plan ahead of time.

What to Bring

You should bring your insurance card to your appointment, as well as any forms you were advised to bring. If a physician other than your regular doctor will be performing the test, she will want to review your prenatal records before the test. It's always a good idea to bring a book or something else to occupy you should the doctor be backed up or your procedure otherwise delayed. Even though you will probably be able to drive yourself, it's helpful to bring your partner or a friend who can support you during the procedure.

Other Considerations

You may wish to speak with a genetic counselor before (or after) your amniocentesis. There are many conditions that cannot be detected by amniocentesis, and sometimes a genetic counselor (by looking at your family history) can help you explore concerns for which testing does not yet exist.

These counselors are also invaluable should your results be abnormal. Most of the conditions that can be diagnosed by amniocentesis can cause a spectrum of symptoms, and no two people with these conditions are the same. For example, while children with Down syndrome can be affected differently depending on the type they have, even children with the same type can have unique experiences and outcomes.

During the Test

During your test, there will be a few people in the room with you, including a nurse, an obstetrician, and sometimes a radiologist or radiology technician.

Pre-Test

Before your procedure begins, you will be asked to sign a consent form indicating you understand the purpose of the procedure and potential risks. Your nurse will check your temperature, pulse, and blood pressure, as well as your baby's heart rate.

Since the ultrasound may reveal the baby's sex, and the chromosomal analysis will for certain, it's important to let your nurse know if you do not want to know if you're having a boy or a girl. She can then attach a note to your records so that your doctor won't inadvertently tell you when sharing your results.

Throughout the Test

When you are ready for the test, you will be asked to change into a gown and lie on your back on the procedure table. Ultrasound gel will be applied to your abdomen and an ultrasound used to determine your baby's position, the location of the placenta, and to look for pockets of amniotic fluid. This may take some time, especially if you have an anterior placenta.

Once the obstetrician has done a basic ultrasound evaluation of your baby, your abdomen will be cleaned with an antiseptic. With ultrasound guidance, a long hollow needle will then be inserted through your abdominal wall and into your uterus. You may be asked to move around somewhat to redistribute amniotic fluid. Most of the time, a local anesthetic is not used and you will feel a sting when the needle is inserted.

When the needle is in the proper location, your doctor will withdraw roughly 20 to 30 milliliters (about 4 to 6 teaspoons) of fluid. This amount of fluid loss will not hurt your baby in any way, and your body will quickly replenish it. You may feel some discomfort and cramping as the fluid is removed.

Once the fluid is retrieved, the needle is removed and a bandage is placed over the insertion site.

Post-Test

After your test, your nurse will continue to monitor your baby's heart rate for several minutes and will ask you about any discomfort or leakage of amniotic fluid. If you are Rh negative, your nurse will administer a shot of Rhogam (Rh immune globulin) to prevent your baby from developing Rh sensitization.

The fluid that was removed will be sent to the lab for a pathologist to evaluate.

After the Test

When your test is done, you will be allowed to return home. Your doctor will let you know roughly how long it will be until results are available. Women are usually advised to avoid strenuous activity for several days and to abstain from sexual activity for a day or two.

Managing Side Effects

Some cramping is normal for the first couple of hours after the procedure, but roughly 1 percent to 2 percent of women experience more cramping and spotting that can raise concern about miscarriage. It's important to note that, even when these symptoms occur, most people do not go on to miscarry.

Call your doctor if you experience any significant cramping, abdominal pain, or contractions, as well as any vaginal bleeding. It's also important to seek medical attention if you develop a fever or chills, suspect you are leaking amniotic fluid, or notice a change in your baby's activity (either increased or decreased movement) if you are far enough along to feel these movements.

Interpreting Results

The time it takes to get your results can vary from a few days to two weeks or longer. The specific results reported will depend on the reason the amniocentesis was performed and what tests were performed on the sample.

Genetic Testing

A karyotype is a picture of your baby's chromosomes and is the test that takes the longest to complete. It is 98 percent to 99 percent accurate for chromosomal abnormalities.

A fluorescent in situ hybridization (FISH) analysis is another test that may be done on amniotic fluid. This test is often available much more rapidly (only a few days), but it can't discriminate between the different types of Down syndrome.

Many people wonder how often chromosomal abnormalities are found on an amniocentesis. In a 2017 review of over 40,000 amniocentesis procedures (done for a maternal age of 35 or over or abnormal prenatal screening tests), it was found that 3.36 percent of these studies found a chromosomal condition. Again, it's important to note that an amniocentesis cannot diagnose all birth defects or potential genetic conditions.

Structural chromosomal abnormalities may include:

  • Trisomies: Normally, each person has 46 chromosomes (23 pairs), but with a trisomy, there is an extra chromosome (for a total of 47 chromosomes). Possible findings may include Down syndrome (trisomy 21, in which there are three copies of chromosome 21), Edwards syndrome (trisomy 18), and Patau syndrome (trisomy 13). There can also be trisomies of the sex chromosomes such as Klinefelter's syndrome (XXY) and triple X syndrome (XXX).
  • Monosomies: The most common monosomy is Turner syndrome (XO) in which there are only 45 chromosomes, with one of the sex chromosomes missing.
  • Translocations: With translocations, the genetic material from one chromosome is attached to another chromosome. These may be balanced (in which parts of two chromosomes are simply exchanged or unbalanced).
  • Deletions: When part of a chromosome is missing
  • Additions: When extra material is added to a chromosome
  • Inversions: The genetic material on a chromosome is "turned around"
  • Duplications: Where part of a chromosome is duplicated (there are two copies of one part of a chromosome)
  • Mosaicism: Mosaicism occurs when one of the conditions above affects only some of the cells. For example, a baby may be born live with mosaic trisomy 9 is which there are three copies of chromosome 9 in only a small percent of cells in the body (full trisomy 9 is incompatible with life and usually causes an early miscarriage).

Sometimes more than one of these genetic changes may be responsible for a condition.

Molecular abnormalities, or changes in one or more genes, may also be found on an amniocentesis. Some conditions that may be diagnosed include:

  • Sickle cell disease
  • Cri-du-Chat syndrome
  • Fragile X syndrome
  • Tay-Sachs disease
  • Muscular dystrophy
  • Cystic fibrosis
  • Thalassemia

Infection Tests

If your amniocentesis was done to check for the presence of a congenital infection in the baby, antibody tests will be returned as either positive or negative.

Follow-Up

Follow-up after an amniocentesis will depend on the results that are found. If enough fluid isn't obtained, an amniocentesis may need to be repeated. (It takes roughly one ounce of amniotic fluid to perform diagnostic testing.)

A Word From Verywell

The decision to have an amniocentesis is often accompanied by significant anxiety. Not only are there concerns about the potential risks of the procedure, but many people are nervous about conditions that may be found. Finding an experienced physician and learning about the procedure and potential findings may help to reduce some of this anxiety.

Amniocentesis is also surrounded by strong opinions both for and against the procedure. It's important to remember that choosing to have the procedure is a personal decision—one that rests on the personal preferences of you and your partner alone. Mothers-to-be and their partners should work with their doctors to make decisions that best fit their own philosophical beliefs and medical circumstances.

As advances take place in molecular testing, especially with the advent of tests to evaluate circulating DNA and DNA fragments, there will hopefully be less invasive methods of obtaining the same results of an amniocentesis in the near future.

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