Amniocentesis

What Is Amniocentesis?

Amniocentesis is a procedure performed during pregnancy in which amniotic fluid is withdrawn from a woman’s uterus to test for certain problems in the fetus, such as genetic defects, fetal infections, fetal lung immaturity, or Rh sensitization. The word “amniocentesis” literally means “puncture of the amnion,” the fluid-filled sac that encloses the fetus during pregnancy.

During amniocentesis, a needle is inserted through a mother’s belly into the sac of amniotic fluid, the watery fluid that surrounds the fetus, and a small amount of the fluid is removed.

The fluid, which is produced by the fetal lungs and kidneys and the umbilical cord, contains skin cells the fetus has shed and biochemical substances that the fetus has produced. These cells can be sent to a laboratory, where they are isolated and grown so that their genetic material (chromosomes) can be tested, or so other laboratory tests can be done.

What Can Amniocentesis Reveal?

There are two types of tests that can be performed on amniotic fluid to show different types of problems with the fetus:

• Genetic amniocentesis
• Chemical tests

The cells in the amniotic fluid contain genetic material that can be used to observe chromosomal abnormalities or defects in specific genes. Biochemical substances, like proteins and lipids, can give clues about the health or maturity of the fetus.

Genetic Amniocentesis

The need for genetic testing of amniotic fluid depends on:

• The age of the parents
• Family history of genetic abnormalities
• Problems found during blood tests on the mother
• Ultrasound results

For genetic testing, fetal cells collected by amniocentesis, isolated, and allowed to grow in the laboratory and divide. The chromosomes, which carry genetic material, are stained with dye and examined under a microscope. Normally, every cell has two copies of each of the 23 human chromosomes (46 total).

A commonly detected abnormality is Down syndrome, where a baby has trisomy 21 – an extra copy of chromosome 21. Other trisomy chromosome abnormalities include Edward’s syndrome, where there is an extra copy of the 18th chromosome, and Patau’s syndrome, where there is an extra copy of the 13th chromosome.

Sex chromosome abnormalities can also be detected. Some, likeTurner syndrome – where a girl has a single X chromosome – have significant consequences, such as a high chance of dying before birth, abnormalities in some organs, and infertility. Other sex chromosome abnormalities, such as the presence of an extra X chromosome in either sex or of an extra Y chromosome in males, have relatively slight consequences.

The chromosome test can also reveal the sex of the fetus, since males have an X and a Y chromosome, while females have two X chromosomes.

Chemical Tests

Many other tests can also be performed on amniotic fluid. A test for the presence of alpha-fetoprotein (or AFP) reveals whether there is an anomaly of the abdominal wall or the spine of the fetus, with aneural tube defect, such as spina bifida or anencephaly.

Other diseases, such as Tay-Sachs disease and sickle-cell disease, are detected by the presence of a specific chemical compound in the amniotic fluid. Many other gene abnormalities of the fetus can also be tested in amniotic fluid.

When a mother is Rh negative, meaning her red blood cells lack a specific protein on their surface, amniocentesis can reveal if the fetus is Rh positive. The resulting fetal anemia from destruction of red blood cells in Rh immunization disease can also be detected from levels of bilirubin in amniotic fluid.

In the second and third trimesters, analyzing the amniotic fluid can help show how mature the fetal lungs are by measuring the fetal lung lipid (surfactant) in the fluid. This can help prevent a premature delivery. In a pregnancy complicated by diabetes, monitoring the levels of glucose and insulin in amniotic fluid can be helpful. In cases of preterm labor or preterm ruptured membranes, checking for the presence of bacteria in the amniotic fluid is an important diagnostic test.

For more detailed information about preterm labor, go to Premature Labor.

Amniocentesis: How Is Amniocentesis Done?

In amniocentesis, ultrasound (sound wave) monitoring is used to find the amniotic fluid and the placenta, to keep track of the fetus, and to insert the amniocentesis needle away from the baby.

For more detailed information about ultrasound, go to Ultrasound.

Drawing Amniotic Fluid

The doctor or technician conducting the test will carefully clean the skin on the woman’s abdomen where the needle will be inserted. The needle insertion site on the skin may be locally anesthetized.

The needle, which is very thin, will be placed through the abdomen into the uterus. Local anesthetic may not be used since the needle administering an anesthetic is frequently as painful as the amniocentesis needle. However, sometimes a topical anesthetic is used. After the needle is withdrawn, the clinician will observe the fetus on the ultrasound monitor to make sure that its heart is beating normally and that everything appears normal.

The Rest Of The Day

A woman is advised not to lift anything heavy and to report any bleeding, fluid leakage, fever, or severe cramps after the test.

Special Case: Rh Negative Mother

Rh negative women will be given medicine after the amniocentesis, which is an antibody to D protein called Rhogans. During amniocentesis there is a chance that the mother’s and baby’s blood will mix; the drug can prevent the potential complication of having the mother develop an immune response against her Rh D protein, which may affect future babies.

Amniocentesis: What Are The Risks And Complications?

Millions of women have safely undergone amniocentesis. However, the procedure does carry some risks:

• Miscarriage: The rate of miscarriage is estimated to be between one in 200 and one in 500.
• Infection: The procedure carries a low risk of uterine infection (less than one in 1,000), which can cause labor. If such an infection occurs, it will happen within a few days of the procedure.
• Maternal or fetal hemorrhaging.
• Fetal injury: There is a very slight risk that the needle used during amniocentesis can hit a crucial fetal area and cause permanent damage.

Preparing For Genetic Amniocentesis

Doctors typically encourage a formal counseling session – not just a short discussion in the doctor’s office – before parents decide about genetic amniocentesis. In the counseling session, parents can weigh the benefits against the modest risk from the procedure. They can also discuss specific gene or chemical tests that family and maternal histories may warrant. Parents may consult a genetic counselor, who is trained to identify potential risk factors and suggest specific tests.

Genetic amniocentesis is normally performed between the 15th and 17th week of pregnancy because the amount of fluid bathing the fetus doubles in volume between 14 and 16 weeks of gestation.

A woman does not have to prepare for amniocentesis in any special way, such as by keeping her bladder full or fasting beforehand. However, she should avoid strenuous activity for the rest of the day following the procedure.

Most test results will be available within about 3 weeks. Much of that time is devoted to growing the small amount of cells found in the amniotic fluid. Since the rate of cell growth varies, sometimes results take longer. The doctor will be called immediately if the tests reveal an abnormality.

Alternatives To Genetic Amniocentesis

Amniocentesis is highly accurate, but expectant parents must wait until at least the 15th week of pregnancy, and it does involve some risks. There are some other tests available that can either be done earlier or are less invasive.

Chorionic Villus Sampling (CVS)

Chorionic Villus Sampling (CVS) is an alternative to amniocentesis that can be performed between 10 and 12 weeks of pregnancy. As with amniocentesis, ultrasound is used to determine the position of the baby. Once that has been established, the doctor or technician will then remove a tiny sample of the placenta, either through the cervix (the neck of the womb) or through a small needle inserted into the abdomen.

Like amniocentesis, CVS can detect chromosomal abnormalities, but it does not detect neural tube defects (although these can sometimes be detected with an ultrasound). It carries a slightly higher risk of miscarriage than amniocentesis (one in 100 to one in 200), but it can be performed earlier in the pregnancy, allowing for the possibility of ending the pregnancy in the first trimester.

Multiple Marker Screening

The multiple maternal marker screening, also known as the triple screen or the quadruple screen, is a blood test given between 15 and 20 weeks of pregnancy. It tests for three or four substances in the blood whose presence can indicate a risk of certain birth defects:

• Alpha-fetoprotein (AFP): A high AFP signifies the risk of neural tube defects, such as spina bifida, or abdominal wall defects. A low AFP is associated with Down syndrome.
• Human chorionic gonadotropin (hCG): High hCG is associated with Down syndrome.
• Unconjugated estriol (uE3): Low estriol may indicate Down syndrome. Low levels of all three hormones indicate a risk for trisomy 18, also known as Edward’s syndrome.
• Inhibin A (inhibin) if it’s a quadruple screen: The detection rate for Down syndrome increases with inhibin screening.

The multiple marker test may also indicate risk of pregnancy complications such as pre-eclampsia, premature birth, or miscarriage.

Remember that the multiple marker test is a screening test used to help a woman understand the risk of her baby suffering from certain conditions. It should be used to help determine if further tests, such as amniocentesis, are necessary.

Ultrasound Studies

There are many markers on a fetus, such as neck skin thickness or anomaly, that can raise the suspicion of a chromosomal abnormality. These markers, which can be detected through ultrasound, may help determine the need for amniocentesis.

Frequently Asked Questions: Amniocentesis

Here are some frequently asked questions related to amniocentesis:

Q: Does amniocentesis hurt?

A: Most women will feel slight pain during amniocentesis. For the first day or so afterward, some women may feel cramps like those during their menstrual period, but that is the most discomfort a woman undergoing the procedure is likely to experience. Bleeding, fever, or leaking of amniotic fluid after amniocentesis may occur and should signal a call to your doctor.

Q: Why does it take so long to get results back from the lab with genetic amniocentesis?

A: There are only a few stray cells in the amniotic fluid. The cells must grow and divide before enough are available for genetic tests, and cell growth rates vary so result times can differ from patient to patient.

Q: Do normal amniocentesis results guarantee a normal baby?

A: No test can guarantee a healthy baby. The results of amniocentesis are very likely to be accurate. (The accuracy of the chromosome test is estimated to be between 99.4% and 100%.) So a normal result can accurately exclude some problems and should be reassuring. Still, amniocentesis does not test for everything. Most of the approximately 4% of babies who are born with some abnormality do not have a chromosome defect or other disorder that amniocentesis detects.

Q: Can amniocentesis determine the sex of the baby?

A: Yes, amniocentesis is used to observe chromosomal abnormalities and will also show if the baby has an X and a Y chromosome (in which case it’s a boy) or two X chromosomes (in which case it’s a girl).

Q: Should I have CVS or wait and have amniocentesis?

A: It depends. If you know you’re at risk for having a baby with a chromosomal abnormality and you want to know if there’s a problem during the first trimester, you may want to consider CVS. But if you want to have a multiple marker screening (which is a blood test) before deciding to undergo a more invasive procedure, amniocentesis may be a better choice. A session with a genetic counselor can help you make an informed decision.

Q: Aren’t there risks involved in the procedure?

A: Amniocentesis increases the risk of miscarriage by less than 1% over the baseline risk for miscarriage. There is also a slight risk of uterine infection (less than one in 1,000) and bleeding. Generally, the more experience a clinician has in performing amniocentesis, the lower the risk of complications will be.

Putting It All Together: Amniocentesis

Here is a summary of the important facts and information related to amniocentesis:

• Amniocentesis is a relatively painless procedure that is performed in a doctor’s office or hospital.
• A small amount of the amniotic fluid bathing a fetus is removed during amniocentesis to identify potential birth defects and many other problems.
• The procedure involves inserting a needle through the abdomen skin into the uterus to remove a sample of amniotic fluid. Ultrasound is used to determine the position of the fetus so that the needle can avoid the placenta and fetus.
• Amniocentesis is used to diagnose chromosome abnormalities, neural tube defects such as spina bifida, and many specific genetic and other abnormalities that may concern a family, usually because of a family history.
• Amniocentesis will diagnose the sex of the baby.
• Genetic amniocentesis increases the risk of miscarriage by less than 1% over the baseline risk for miscarriage.
• Doctors and genetic counselors typically recommend genetic amniocentesis when the statistical risk of an abnormal pregnancy exceeds the risk of miscarriage from the procedure.
• Many other studies of the fetus can be done with amniotic fluid obtained by amniocentesis in the second half of pregnancy. These include fetal infection, fetal lung maturity, and fetal anemia.

Glossary: Amniocentesis

Here are definitions of medical terms related to amniocentesis:

Alpha-fetoprotein: A protein, also called AFP, which the fetus produces in its liver and brain. The level of alpha-fetoprotein tends to increase in the amniotic fluid when a fetus has an open neural tube defect or abdominal wall defect. The protein can be found in adults as well; it’s particularly abundant in the blood of adults with certain forms of liver cancer.

Amniocentesis: A prenatal test in which amniotic fluid is taken out of the uterus.

Amniotic fluid: The fluid that surrounds the fetus.

Anencephaly: A congenital defect in the development of the brain. Bones in the skull and parts of the brain may be missing or poorly developed. An infant with anencephaly will either be stillborn or will die shortly after birth.

Cervix: The lower part of the uterus, often referred to as the neck of the uterus.

Chorionic villus sampling (CVS): A prenatal test to detect chromosomal abnormalities in which cells from the placenta are analyzed. CVS can be done at 10 to 12 weeks of pregnancy.

Chromosomes: The units along which genes are arranged. Humans have two sets of 23 chromosomes, with one set coming from each parent.

Down syndrome: A common genetic disorder in which there is an extra copy of chromosome number 21 (trisomy 21), resulting in mental retardation and physical malformations.

Edward’s syndrome: A genetic disorder in which there is an extra copy of chromosome number 18, resulting in severe mental retardation.

Genetic counseling: A counseling process for those with either a family history of a genetic disorder or those who are at risk for having a child with a genetic disease.

Karyotype: The array of chromosomes in a cell.

Multiple marker screening (MMS): A prenatal blood test that tests for substances in the blood that are linked to genetic abnormalities. The MMS can be a triple screen, which tests for three substances, or a quadruple screen, which tests for four substances.

Neural tube defect: A common birth defect in which the neural tube, which gives rise to the central nervous system, develops abnormally, causing death or serious disability. Spina bifida and anencephaly are examples of neural tube defects.

Patau’s syndrome: A genetic disorder in which there is an extra copy of chromosome number 13, resulting in severe mental retardation.

Pre-eclampsia: Also called toxemia; an illness that occurs during the second half of pregnancy characterized by high blood pressure, swelling of the hands, feet and face, and large amounts of protein in the urine.

Premature labor: Labor occurring before 37 full weeks of pregnancy.

Rh factor: A protein D found on the surface of red blood cells. Those lacking the protein are Rh negative, while those who have it (the majority of the population) are Rh positive.

Sickle cell anemia: An inherited blood disease in which the red blood cells contain an abnormal from of hemoglobin, the protein that transports oxygen from the lungs to the rest of the body.

Spina bifida: A failure during embryonic development of the vertebral column to fuse completely.

Tay-Sachs disease: A hereditary disorder in which a deficiency of the enzyme hexosaminidase causes progressive mental retardation, paralysis, dementia and blindness, which usually results in death by age 5. Tay-Sachs is most common in families of Eastern European descent.

Turner syndrome: A genetic disorder affecting only females where there is only one X chromosome or one of the two X chromosomes is damaged.

Additional Sources Of Information: Amniocentesis

Here are some reliable sources that can provide more information about amniocentesis:

The March of Dimes Birth Defects Foundation 
Phone: 1-888-MODIMES (663-4637)
http://www.modimes.org

American Academy of Obstetricians and Gynecologists 
http://www.acog.org

National Down Syndrome Society 
Phone: 1-800-221-4602
http://www.ndss.org

National Tay-Sachs & Allied Disease Association 
Phone: 1-800-906-8723
http://www.ntsad.org

Sickle Cell Disease Association of America 
Phone: 1-800-421-8453
http://www.sicklecelldisease.org

Cord Blood Banking Information

Provides information on public and private umbilical cord blood collection and storage.

Online: http://www.cordbankings.com/

Spina Bifida Association of America 
Phone: 1-800- 621-3141
http://www.sbaa.org

Cystic Fibrosis Foundation 
Phone: 1-800-FIGHT CF (344-4823)
http://www.cff.org