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Amniocentesis

Amniocentesis

Definition

Amniocentesis is a procedure used to diagnose fetal defects in the early second trimester of pregnancy. A sample of the amniotic fluid, which surrounds a fetus in the womb, is collected through a pregnant woman's abdomen using a needle and syringe. Tests performed on fetal cells found in the sample can reveal the presence of many types of genetic disorders, thus allowing doctors and prospective parents to make important decisions about early treatment and intervention.

Purpose

Since the mid-1970s, amniocentesis has been used routinely to test for Down syndrome , by far the most common, nonhereditary, genetic birth defect, afflicting about one in every 1,000 babies. By 1997, approximately 800 different diagnostic tests were available, most of them for hereditary genetic disorders such as Tay-Sachs disease, sickle cell anemia, hemophilia, muscular dystrophy , and cystic fibrosis .

Amniocentesis, often called amnio, is recommended for women who will be older than 35 on their due-date. It is also recommended for women who have already borne children with birth defects, or when either of the parents has a family history of a birth defect for which a diagnostic test is available. Another reason for the procedure is to confirm indications of Down syndrome and certain other defects which may have shown up previously during routine maternal blood screening.

The risk of bearing a child with a nonhereditary genetic defect such as Down syndrome is directly related to a woman's agethe older the woman, the greater the risk. Thirty-five is the recommended age to begin amnio testing because that is the age at which the risk of carrying a fetus with such a defect roughly equals the risk of miscarriage caused by the procedureabout one in 200. At age 25, the risk of giving birth to a child with this type of defect is about one in 1,400; by age 45 it increases to about one in 20. Nearly half of all pregnant women over 35 in the United States undergo amniocentesis and many younger women also decide to have the procedure. Notably, some 75% of all Down syndrome infants born in the United States each year are to women younger than 35.

One of the most common reasons for performing amniocentesis is an abnormal alpha-fetoprotein (AFP) test. Alpha-fetoprotein is a protein produced by the fetus and present in the mother's blood. A simple blood screening, usually conducted around the 15th week of pregnancy, can determine the AFP levels in the mother's blood. Levels that are too high or too low may signal possible fetal defects. Because this test has a high false-positive rate, another test such as amnio is recommended whenever the AFP levels fall outside the normal range.

Amniocentesis is generally performed during the 16th week of pregnancy, with results usually available within three weeks. It is possible to perform an amnio as early as the 11th week, but this is not usually recommended because there appears to be an increased risk of miscarriage when done at this time. The advantage of early amnio and speedy results lies in the extra time for decision making if a problem is detected. Potential treatment of the fetus can begin earlier. Important, also, is the fact that elective abortions are safer and less controversial the earlier they are performed.

Precautions

As an invasive surgical procedure, amnio poses a real, although small, risk to the health of a fetus. Parents must weigh the potential value of the knowledge gained, or indeed the reassurance that all is well, against the small risk of damaging what is in all probability a normal fetus. The serious emotional and ethical dilemmas that adverse test results can bring must also be considered. The decision to undergo amnio is always a matter of personal choice.

Description

The word amniocentesis literally means "puncture of the amnion," the thin-walled sac of fluid in which a developing fetus is suspended during pregnancy. During the sampling procedure, the obstetrician inserts a very fine needle through the woman's abdomen into the uterus and amniotic sac and withdraws approximately one ounce of amniotic fluid for testing. The relatively painless procedure is performed on an outpatient basis, sometimes using local anesthesia.

The physician uses ultrasound images to guide needle placement and collect the sample, thereby minimizing the risk of fetal injury and the need for repeated needle insertions. Once the sample is collected, the woman can return home after a brief observation period. She may be instructed to rest for the first 24 hours and to avoid heavy lifting for two days.

The sample of amniotic fluid is sent to a laboratory where fetal cells contained in the fluid are isolated and grown in order to provide enough genetic material for testing. This takes about seven to 14 days. The material is then extracted and treated so that visual examination for defects can be made. For some disorders, like Tay-Sachs, the simple presence of a telltale chemical compound in the amniotic fluid is enough to confirm a diagnosis. Depending on the specific tests ordered, and the skill of the lab conducting them, all the results are available between one and four weeks after the sample is taken.

Cost of the procedure depends on the doctor, the lab, and the tests ordered. Most insurers provide coverage for women over 35, as a follow-up to positive maternal blood screening results, and when genetic disorders run in the family.

An alternative to amnio, now in general use, is chorionic villus sampling, or CVS, which can be performed as early as the eighth week of pregnancy. While this allows for the possibility of a first trimester abortion, if warranted, CVS is apparently also riskier and is more expensive. The most promising area of new research in prenatal testing involves expanding the scope and accuracy of maternal blood screening as this poses no risk to the fetus.

Preparation

It is important for a woman to fully understand the procedure and to feel confident in the obstetrician performing it. Evidence suggests that a physician's experience with the procedure reduces the chance of mishap. Almost all obstetricians are experienced in performing amniocentesis. The patient should feel free to ask questions and seek emotional support before, during and after the amnio is performed.

Aftercare

Necessary aftercare falls into two categories, physical and emotional.

PHYSICAL AFTERCARE During and immediately following the sampling procedure, a woman may experience dizziness, nausea , a rapid heartbeat, and cramping. Once past these immediate hurdles, the physician will send the woman home with instructions to rest and to report any complications requiring immediate treatment, including:

  • Vaginal bleeding. The appearance of blood could signal a problem.
  • Premature labor. Unusual abdominal pain and/or cramping may indicate the onset of premature labor. Mild cramping for the first day or two following the procedure is normal.
  • Signs of infection. Leaking of amniotic fluid or unusual vaginal discharge, and fever could signal the onset of infection.

EMOTIONAL AFTERCARE Once the procedure has been safely completed, the anxiety of waiting for the test results can prove to be the worst part of the process. A woman should seek and receive emotional support from family and friends, as well as from her obstetrician and family doctor. Professional counseling may also prove necessary, particularly if a fetal defect is discovered.

Risks

Most of the risks and short-term side effects associated with amniocentesis relate to the sampling procedure and have been discussed above. A successful amnio sampling results in no long-term side effects. Risks include:

  • Maternal/fetal hemorrhaging. While spotting in pregnancy is fairly common, bleeding following amnio should always be investigated.
  • Infection. Infection, although rare, can occur after amniocentesis. An unchecked infection can lead to severe complications.
  • Fetal injury. A very slight risk of injury to the fetus resulting from contact with the amnio needle does exist.
  • Miscarriage. The rate of miscarriage occurring during standard, second trimester amnio appears to be approximately 0.5%. This compares to a miscarriage rate of 1% for CVS. Many fetuses with severe genetic defects miscarry naturally during the first trimester.
  • The trauma of difficult family-planning decisions. The threat posed to parental and family mental health from the trauma accompanying an abnormal test result can not be underestimated.

Normal results

Negative results from an amnio analysis indicate that everything about the fetus appears normal and the pregnancy can continue without undue concern. A negative result for Down syndrome means that it is 99% certain that the disease does not exist.

KEY TERMS

Alpha fetoprotein (AFP) A substance produced by a fetus' liver that can be found in the amniotic fluid and in the mother's blood. Abnormally high levels of this substance suggests there may be defects in the fetal neural tube, a structure that will include the brain and spinal cord when completely developed. AFP may also be found at elevated levels in the blood of adults with liver, testicular, and ovarian cancer.

Anencephaly A genetic defect resulting in the partial to complete absence of the brain and malformation of the brainstem.

Chorionic villus sampling A procedure used for prenatal diagnosis at 1012 weeks gestation. Under ultrasound guidance a needle is inserted either through the mother's vagina or abdominal wall and a sample of the chorionic membrane. These cells are then tested for chromosome abnormalities or other genetic diseases.

Chromosome A microscopic thread-like structure found within each cell of the human body and consisting of a complex of proteins and DNA. Humans have 46 chromosomes arranged into 23 pairs. Chromosomes contain the genetic information necessary to direct the development and functioning of all cells and systems in the body. They pass on hereditary traits from parents to child (like eye color) and determine whether the child will be male or female.

Down syndrome A chromosomal disorder caused by an extra copy or a rearrangement of chromosome 21. Children with Down syndrome have varying degrees of mental retardation and may have heart defects.

Genetic Refers to genes, the basic units of biological heredity, which are contained on the chromosomes.

Hereditary Something which is inherited, that is passed down from parents to offspring. In biology and medicine, the word pertains to inherited genetic characteristics.

Maternal blood screening Screening that is normally done early in pregnancy to test for a variety of conditions. Abnormal amounts of certain proteins in a pregnant woman's blood raise the probability of fetal defects. Amniocentesis is recommended if such a probability occurs.

Tay-Sachs disease An inherited disease caused by a missing enzyme that is prevalent among the Ashkenazi Jewish population of the United States. Infants with the disease are unable to process a certain type of fat which accumulates in nerve and brain cells, causing mental and physical retardation, and, finally, death.

Ultrasonography A medical test in which sound waves are directed against internal structures in the body. As sound waves bounce off the internal structure, they create an image on a video screen. Ultrasonography is often used to diagnose fetal abnormalities, gallstones, heart defects, and tumors. Also called ultrasound imaging.

An overall "normal" result does not, however, guarantee that the pregnancy will come to term, or that the fetus does not suffer from some other defect. Laboratory tests are not 100% accurate at detecting targeted conditions, nor can every possible fetal condition be tested for.

Abnormal results

Positive results on an amnio analysis indicate the presence of the fetal defect being tested for, with an accuracy approaching 100%. Prospective parents are then faced with emotionally and ethically difficult choices regarding treatment options, the prospect of dealing with a severely affected newborn, and the option of elective abortion. At this point, the parents need expert medical advice and counseling.

Parental concerns

There is a risk of miscarrage with this procedure.

When to call a doctor

If there is excess bleeding, a doctor should be contacted.

Resources

BOOKS

Hassold, Terry and Schwartz, Stuart. "Chromosome Disorders." In Harrison's Principles of Internal Medicine, ed. Eugene Braunwald, et al. Philadelphia: McGraw-Hill, 2001.

Miesfeldt, Susan and Jameson, J. Larry. "Screening, Counseling, and Prevention of Genetic Disorders." In Harrison's Principles of Internal Medicine, ed. Eugene Braunwald, et al. Philadelphia: McGraw-Hill, 2001.

Wallach, Jacques. Interpretation of Diagnostic Tests, 7th ed. hiladelphia, PA: Lippincott Williams & Wilkens, 2000.

ORGANIZATIONS

American College of Obstetricians and Gynecologists. 409 12th St., S.W., P.O. Box 96920, Washington, DC 20090-6920. <http://www.acog.org>.

ORGANIZATIONS

National Institutes of Health. <http://www.nlm.nih.gov/medlineplus/encyclopedia.html>.

Mark A. Best

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"Amniocentesis." Gale Encyclopedia of Children's Health: Infancy through Adolescence. . Encyclopedia.com. 19 Oct. 2017 <http://www.encyclopedia.com>.

"Amniocentesis." Gale Encyclopedia of Children's Health: Infancy through Adolescence. . Encyclopedia.com. (October 19, 2017). http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/amniocentesis

"Amniocentesis." Gale Encyclopedia of Children's Health: Infancy through Adolescence. . Retrieved October 19, 2017 from Encyclopedia.com: http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/amniocentesis

Amniocentesis

Amniocentesis

Definition

Amniocentesis is a procedure used to diagnose fetal defects in the early second trimester of pregnancy. A sample of the amniotic fluid, which surrounds a fetus in the womb, is collected through a pregnant woman's abdomen using a needle and syringe. Tests performed on fetal cells found in the sample can reveal the presence of many types of genetic disorders, thus allowing doctors and prospective parents to make important decisions about early treatment and intervention.

Purpose

Since the mid-1970s, amniocentesis has been used routinely to test for Down syndrome, by far the most common, nonhereditary, genetic birth defect, afflicting about one in every 1,000 babies. By 1997, approximately 800 different diagnostic tests were available, most of them for hereditary genetic disorders such as Tay-Sachs disease, sickle cell anemia, hemophilia, muscular dystrophy and cystic fibrosis.

Amniocentesis, often called amnio, is recommended for women who will be older than 35 on their due-date. It is also recommended for women who have already borne children with birth defects, or when either of the parents has a family history of a birth defect for which a diagnostic test is available. Another reason for the procedure is to confirm indications of Down syndrome and certain other defects which may have shown up previously during routine maternal blood screening.

The risk of bearing a child with a nonhereditary genetic defect such as Down syndrome is directly related to a woman's agethe older the woman, the greater the risk. Thirty-five is the recommended age to begin amnio testing because that is the age at which the risk of carrying a fetus with such a defect roughly equals the risk of miscarriage caused by the procedure-about one in 200. At age 25, the risk of giving birth to a child with this type of defect is about one in 1,400; by age 45 it increases to about one in 20. Nearly half of all pregnant women over 35 in the United States undergo amniocentesis and many younger women also decide to have the procedure. Notably, some 75% of all Down syndrome infants born in the United States each year are to women younger than 35.

One of the most common reasons for performing amniocentesis is an abnormal alpha-fetoprotein (AFP) test. Alpha-fetoprotein is a protein produced by the fetus and present in the mother's blood. A simple blood screening, usually conducted around the 15th week of pregnancy, can determine the AFP levels in the mother's blood. Levels that are too high or too low may signal possible fetal defects. Because this test has a high false-positive rate, another test such as amnio is recommended whenever the AFP levels fall outside the normal range.

Amniocentesis is generally performed during the 16th week of pregnancy, with results usually available within three weeks. It is possible to perform an amnio as early as the 11th week but this is not usually recommended because there appears to be an increased risk of miscarriage when done at this time. The advantage of early amnio and speedy results lies in the extra time for decision making if a problem is detected. Potential treatment of the fetus can begin earlier. Important, also, is the fact that elective abortions are safer and less controversial the earlier they are performed.

Precautions

As an invasive surgical procedure, amnio poses a real, although small, risk to the health of a fetus. Parents must weigh the potential value of the knowledge gained, or indeed the reassurance that all is well, against the small risk of damaging what is in all probability a normal fetus. The serious emotional and ethical dilemmas that adverse test results can bring must also be considered. The decision to undergo amnio is always a matter of personal choice.

KEY TERMS

Alpha-fetoprotein (AFP) A protein normally produced by the liver of a fetus and detectable in maternal blood samples. AFP screening measures the amount of alpha-fetoprotein in the blood. Levels outside the norm may indicate fetal defects.

Anencephaly A hereditary defect resulting in the partial to complete absence of a brain and spinal cord. It is fatal.

Chorionic villus sampling (CVS) A procedure similar to amniocentesis, except that cells are taken from the chorionic membrane for testing. These cells, called chorionic villus cells, eventually become the placenta. The samples are collected either through the abdomen, as in amnio, or through the vagina. CVS can be done earlier in the pregnancy than amnio, but carries a somewhat higher risk.

Chromosome Chromosomes are the strands of genetic material in a cell that occur in nearly identical pairs. Normal human cells contain 23 chromosome pairs-one in each pair inherited from the mother, and one from the father. Every human cell contains the exact same set of chromosomes.

Down syndrome The most prevalent of a class of genetic defects known as trisomies, in which cells contain three copies of certain chromosomes rather than the usual two. Down syndrome, or trisomy 21, usually results from three copies of chromosome 21.

Genetic The term refers to genes, the basic units of biological heredity, which are contained on the chromosomes, and contain chemical instructions which direct the development and functioning of an individual.

Hereditary Something which is inherited-passed down from parents to offspring. In biology and medicine, the word pertains to inherited genetic characteristics.

Maternal blood screening Maternal blood screening is normally done early in pregnancy to test for a variety of conditions. Abnormal amounts of certain proteins in a pregnant woman's blood raise the probability of fetal defects. Amniocentesis is recommended if such a probability occurs.

Tay-Sachs disease An inherited disease prevalent among the Ashkenazi Jewish population of the United States. Infants with the disease are unable to process a certain type of fat which accumulates in nerve and brain cells, causing mental and physical retardation, and death by age four.

Ultrasound A technique which uses high-frequency sound waves to create a visual image (a sonogram) of soft tissues. The technique is routinely used in prenatal care and diagnosis.

Description

The word amniocentesis literally means "puncture of the amnion," the thin-walled sac of fluid in which a developing fetus is suspended during pregnancy. During the sampling procedure, the obstetrician inserts a very fine needle through the woman's abdomen into the uterus and amniotic sac and withdraws approximately one ounce of amniotic fluid for testing. The relatively painless procedure is performed on an outpatient basis, sometimes using local anesthesia.

The physician uses ultrasound images to guide needle placement and collect the sample, thereby minimizing the risk of fetal injury and the need for repeated needle insertions. Once the sample is collected, the woman can return home after a brief observation period. She may be instructed to rest for the first 24 hours and to avoid heavy lifting for two days.

The sample of amniotic fluid is sent to a laboratory where fetal cells contained in the fluid are isolated and grown in order to provide enough genetic material for testing. This takes about seven to 14 days. The material is then extracted and treated so that visual examination for defects can be made. For some disorders, like Tay-Sachs, the simple presence of a telltale chemical compound in the amniotic fluid is enough to confirm a diagnosis. Depending on the specific tests ordered, and the skill of the lab conducting them, all the results are available between one and four weeks after the sample is taken.

Cost of the procedure depends on the doctor, the lab, and the tests ordered. Most insurers provide coverage for women over 35, as a follow-up to positive maternal blood screening results, and when genetic disorders run in the family.

An alternative to amnio, now in general use, is chorionic villus sampling, or CVS, which can be performed as early as the eighth week of pregnancy. While this allows for the possibility of a first trimester abortion, if warranted, CVS is apparently also riskier and is more expensive. The most promising area of new research in prenatal testing involves expanding the scope and accuracy of maternal blood screening as this poses no risk to the fetus.

Preparation

It is important for a woman to fully understand the procedure and to feel confident in the obstetrician performing it. Evidence suggests that a physician's experience with the procedure reduces the chance of mishap. Almost all obstetricians are experienced in performing amniocentesis. The patient should feel free to ask questions and seek emotional support before, during and after the amnio is performed.

Aftercare

Necessary aftercare falls into two categories, physical and emotional.

Physical aftercare

During and immediately following the sampling procedure, a woman may experience dizziness, nausea, a rapid heartbeat, and cramping. Once past these immediate hurdles, the physician will send the woman home with instructions to rest and to report any complications requiring immediate treatment, including:

  • vaginal bleeding. The appearance of blood could signal a problem.
  • premature labor. Unusual abdominal pain and/or cramping may indicate the onset of premature labor. Mild cramping for the first day or two following the procedure is normal.
  • signs of infection. Leaking of amniotic fluid or unusual vaginal discharge, and fever could signal the onset of infection.

Emotional aftercare

Once the procedure has been safely completed, the anxiety of waiting for the test results can prove to be the worst part of the process. A woman should seek and receive emotional support from family and friends, as well as from her obstetrician and family doctor. Professional counseling may also prove necessary, particularly if a fetal defect is discovered.

Risks

Most of the risks and short-term side effects associated with amniocentesis relate to the sampling procedure and have been discussed above. A successful amnio sampling results in no long-term side effects. Risks include:

  • maternal/fetal hemorrhaging. While spotting in pregnancy is fairly common, bleeding following amnio should always be investigated.
  • infection. Infection, although rare, can occur after amniocentesis. An unchecked infection can lead to severe complications.
  • fetal injury. A very slight risk of injury to the fetus resulting from contact with the amnio needle does exist.
  • miscarriage. The rate of miscarriage occurring during standard, second trimester amnio appears to be approximately 0.5%. This compares to a miscarriage rate of 1% for CVS. Many fetuses with severe genetic defects miscarry naturally during the first trimester.
  • the trauma of difficult family-planning decisions. The threat posed to parental and family mental health from the trauma accompanying an abnormal test result can not be underestimated.

Normal results

Negative results from an amnio analysis indicate that everything about the fetus appears normal and the pregnancy can continue without undue concern. A negative result for Down syndrome means that it is 99% certain that the disease does not exist.

An overall "normal" result does not, however, guarantee that the pregnancy will come to term, or that the fetus does not suffer from some other defect. Laboratory tests are not 100% accurate at detecting targeted conditions, nor can every possible fetal condition be tested for.

Abnormal results

Positive results on an amnio analysis indicate the presence of the fetal defect being tested for, with an accuracy approaching 100%. Prospective parents are then faced with emotionally and ethically difficult choices regarding treatment options, the prospect of dealing with a severely affected newborn, and the option of elective abortion. At this point, the parents need expert medical advice and counseling.

Resources

PERIODICALS

Dreisbach, Shaun. "Amnio Alternative." Working Mother (March 1997): 11.

ORGANIZATIONS

American College of Obstetricians and Gynecologists. 409 12th St., S.W., P.O. Box 96920, Washington, DC 20090-6920. http://www.acog.org.

OTHER

Holbrook Jr., Harold R. Stanford University School of MedicineWeb Home Page. February 2001. http://www.stanford.edu/holbrook.

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"Amniocentesis." Gale Encyclopedia of Medicine, 3rd ed.. . Encyclopedia.com. 19 Oct. 2017 <http://www.encyclopedia.com>.

"Amniocentesis." Gale Encyclopedia of Medicine, 3rd ed.. . Encyclopedia.com. (October 19, 2017). http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/amniocentesis-1

"Amniocentesis." Gale Encyclopedia of Medicine, 3rd ed.. . Retrieved October 19, 2017 from Encyclopedia.com: http://www.encyclopedia.com/medicine/encyclopedias-almanacs-transcripts-and-maps/amniocentesis-1

Amniocentesis

Amniocentesis

Definition

Amniocentesis is a procedure used to diagnose fetal defects in the early second trimester of pregnancy. A sample of the amniotic fluid, which surrounds a fetus in the womb, is collected through a pregnant woman's abdomen using a needle and syringe. Tests performed on fetal cells found in the amniotic fluid can reveal the presence of many types of genetic disorders. Early diagnosis allows doctors and prospective parents to make important decisions about treatment and intervention prior to birth.


Purpose

Since the mid-1970s, amniocentesis has been used routinely to test for Down syndrome, by far the most common, nonhereditary, genetic birth defect, afflicting about one in every 1,000 babies. By 1997, approximately 800 different diagnostic tests were available, most of them for hereditary genetic disorders such as Tay-Sachs disease, sickle cell disease, hemophilia, muscular dystrophy, and cystic fibrosis.

Amniocentesis, often called amnio, is recommended for women who will be older than 35 on their due-date. It is also recommended for women who have already borne children with birth defects, or when either of the parents has a family history of a birth defect for which a diagnostic test is available. Another reason for the procedure is to confirm indications of Down syndrome and certain other defects that may have shown up previously during routine maternal blood screening.

The risk of bearing a child with a nonhereditary genetic defect such as Down syndrome is directly related to a woman's agethe older the woman, the greater the risk. Thirty-five is the recommended age to begin amnio testing because that is the age at which the risk of carrying a fetus with such a defect roughly equals the risk of miscarriage caused by the procedureabout one in 200. At age 25, the risk of giving birth to a child with this type of defect is about one in 1,400; by age 45 it increases to about one in 20. Nearly half of all pregnant women over 35 in the United States undergo amniocentesis and many younger women also decide to have the procedure. Notably, some 75% of all Down syndrome infants born in the United States each year are to women younger than 35.

One of the most common reasons for performing amniocentesis is an abnormal alpha-fetoprotein (AFP) test. Alpha-fetoprotein is a protein produced by the fetus and present in the mother's blood. A simple blood screening, usually conducted around the fifteenth week of pregnancy, can determine the AFP levels in the mother's blood. Levels that are too high or too low may signal possible fetal defects. Because this test has a high false-positive rate, another test such as amniocentesis is recommended whenever the AFP levels fall outside the normal range.

Amniocentesis is generally performed during the sixteenth week of pregnancy, with results usually available within three weeks. It is possible to perform an amnio as early as the eleventh week but this is not usually recommended because there appears to be an increased risk of miscarriage when done at this time. The advantage of early amnio and speedy results lies in the extra time for decision making if a problem is detected. Potential treatment of the fetus can begin earlier. Important, also, is the fact that elective abortions are safer and less controversial the earlier they are performed.


Precautions

As an invasive surgical procedure, amniocentesis poses a real, although small, risk to the health of a fetus. Parents must weigh the potential value of the knowledge gained, or indeed the reassurance that all is well, against the small risk of miscarriage. The serious emotional and ethical dilemmas that adverse test results can bring must also be considered. The decision to undergo amnio is always a matter of personal choice.


Description

The word amniocentesis literally means "puncture of the amnion," the thin-walled sac of fluid in which a developing fetus is suspended during pregnancy. During the procedure, the obstetrician inserts a very fine needle through the woman's abdomen into the uterus and amniotic sac and withdraws approximately 1 oz (28.3 g) of amniotic fluid for testing. The relatively painless procedure is performed on an outpatient basis, sometimes using local anesthesia.

The physician uses ultrasound images to guide needle placement and collect the sample, thereby minimizing the risk of fetal injury and the need for repeated needle insertions. Once the sample is collected, the woman can return home after a brief observation period. She may be instructed to rest for the first 24 hours and to avoid heavy lifting for two days.

The sample of amniotic fluid is sent to a laboratory where fetal cells contained in the fluid are isolated and grown in order to provide enough genetic material for testing. This takes about seven to 14 days. The material is then extracted and treated so that visual examination for defects can be made. For some disorders, like Tay-Sachs, the simple presence of a telltale chemical compound in the amniotic fluid is enough to confirm a diagnosis. Depending on the specific tests ordered, and the skill of the lab conducting them, all the results are available one to four weeks after the sample is taken.

Cost of the procedure depends on the doctor, the lab, and the tests ordered. Most insurers provide coverage for women over 35, as a follow-up to positive maternal blood screening results, and when genetic disorders run in the family.

An alternative to amnio, now in general use, is chorionic villus sampling (CVS), which can be performed as early as the eighth week of pregnancy. While this allows for the possibility of a first trimester abortion, if warranted, CVS is apparently also riskier and is more expensive. The most promising area of new research in prenatal testing involves expanding the scope and accuracy of maternal blood screening as this poses no risk to the fetus.


Preparation

It is important for a woman to fully understand the procedure and to feel confident in the obstetrician performing it. Evidence suggests that a physician's experience with the procedure reduces the chance of mishap. Almost all obstetricians are experienced in performing amniocentesis. The patient should feel free to ask questions and seek emotional support before, during, and after amniocentesis is performed.


Aftercare

Necessary aftercare falls into two categories, physical and emotional.


Physical aftercare

During and immediately following the sampling procedure, a woman may experience dizziness, nausea, a rapid heartbeat, and cramping. Once past these immediate hurdles, the physician will send the woman home with instructions to rest and to report any complications requiring immediate treatment, including:

  • Vaginal bleeding. The appearance of blood could signal a problem.
  • Premature labor. Unusual abdominal pain and/or cramping may indicate the onset of premature labor. Mild cramping for the first day or two following the procedure is normal.
  • Signs of infection. Leaking of amniotic fluid or unusual vaginal discharge, and fever could signal the onset of infection.

Emotional aftercare

Once the procedure has been safely completed, the anxiety of waiting for the test results can prove to be the worst part of the process. A woman should seek and receive emotional support from family and friends, as well as from her obstetrician and family doctor. Professional counseling may also prove necessary, particularly if a fetal defect is detected.


Risks

Most of the risks and short-term side effects associated with amniocentesis relate to the sampling procedure. A successful amnio sampling results in no long-term side effects. Risks include:

  • Maternal/fetal hemorrhaging. While spotting in pregnancy is fairly common, bleeding following amnio should always be investigated.
  • Infection. Infection, although rare, can occur after amniocentesis. An unchecked infection can lead to severe complications.
  • Fetal injury. A very slight risk of injury to the fetus resulting from contact with the amnio needle does exist.
  • Miscarriage. The rate of miscarriage occurring during standard, second trimester amnio is approximately 0.5%. This compares to a miscarriage rate of 1% for CVS. Many fetuses with severe genetic defects miscarry naturally during the first trimester.
  • The trauma of difficult family-planning decisions. The threat posed to parental and family mental health from the trauma accompanying an abnormal test result can not be underestimated.

Normal results

Negative results from an amnio analysis indicate that everything about the fetus appears normal and the pregnancy can continue without undue concern. A negative result for Down syndrome means that it is 99% certain that the disease does not exist.

An overall "normal" result does not, however, guarantee that the pregnancy will come to term, or that the fetus does not suffer from some other defect. Laboratory tests are not 100% accurate at detecting targeted conditions, nor can every possible fetal condition be tested for.

Abnormal results

Positive results on an amnio analysis indicate the presence of a fetal defect, with an accuracy approaching 100%. With such a diagnosis, prospective parents face emotionally and ethically difficult choices regarding prenatal treatment options, the prospect of treating the defect at birth, and the option of elective abortion. At this point, the parents need expert medical advice and counseling.


Resources

books

Hassold, Terry and Stuart Schwartz. "Chromosome Disorders." In Harrison's Principles of Internal Medicine, edited by Eugene Braunwald, et al. Philadelphia: McGraw-Hill, 2001.

Miesfeldt, Susan and J. Larry Jameson. "Screening, Counseling, and Prevention of Genetic Disorders." In Harrison's Principles of Internal Medicine, edited by Eugene Braunwald, et al. Philadelphia: McGraw-Hill, 2001.

Wallach, Jacques. Interpretation of Diagnostic Tests. 7th ed. Philadelphia, PA: Lippincott Williams & Wilkens, 2000.

organizations

American College of Obstetricians and Gynecologists. 409 12th St., S.W., P.O. Box 96920, Washington, DC 20090-6920. <http://www.acog.org>.

other

National Institutes of Health. [cited April 4, 2003]. <http://www.nlm.nih.gov/medlineplus/encyclopedia.html>.


Kurt Richard Sternlof Mark A. Best

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Amniocentesis

Amniocentesis

Amniocentesis is the process of removing a sample of amniotic fluid from the mother's uterus (a pear-shaped organ located in the pelvis where unborn young develop) in which the fetus (growing baby) floats. The fluid and fetal cells in the fluid are then analyzed to check for and diagnose possible genetic disorders.

The Bevis Study

Until amniocentesis became available, prenatal (prebirth) diagnostic techniques were severely limited and risky. By the late 1920s or early 1930s, using a needle to obtain samples of amniotic fluid was an acceptedif rarely usedtechnique. It was only after a British doctor published the results of his study in the February, 1952 issue of Lancet that the use of amniocentesis became widespread. Douglas Bevis, the doctor who conducted the study at St. Mary's Hospital in Manchester, England, chemically analyzed the iron and urobilinogen content of amniotic fluid to determine the possibility of hemolytic (blood) disease in unborn children. The doctor used amniocentesis to determine fetal risk if an Rh-negative woman was impregnated (made pregnant) by an Rh-positive man. Bevis's study of amniocentesis is considered a landmark event in promoting the procedure. His technique was later refined by another researcher who measured amounts of bilirubin (a reddish-yellow organic compound made from homoglobin) in the amniotic fluid of Rh-sensitized women. These test results were published in 1961.

Using Amniocentesis as a Diagnostic Tool

Amniocentesis eventually enabled doctors to predict fetal sex. This ability was based on the 1949 observations of doctors Murray Barr and Ewart Bartram, who noted that all female cells, but no male cells, contain a chromatin mass (made of nucleic acid and protein) on the edge of the nucleus (a complex body within a cell that contains the cell's hereditary material and controls its growth). If the fetal cells found in the amniotic fluid contain this mass (known as a "Barr Body"), then the fetus is female. Knowing the sex of the fetus is important in assessing the risk of a child being born with a sex-linked (affecting one sex only) disease such as hemophilia.

Other Prenatal Diagnostic Tools

Amniocentesis is one of the most common prenatal diagnostic tools. While the development of this procedure marked an important advance, amniocentesis is just one tool doctors use to determine fetal status. Other prenatal diagnostic techniques include ultrasound scanning (the use of sound waves to produce a picture of the developing fetus), and fetal blood sampling (in which a fetalscope is inserted surgically through the uterine wall to collect a clear blood sample). Nuclear Magnetic Resonance Imaging (NMR) reveals biochemical information about fetal tissues and organ structure, while DNA testing, introduced in 1976, is used to identify specific gene disorders.

During the mid-1960s it became possible to grow human cells in the laboratory and perform chromosomal testing. Chromosomes (the hereditary material found in the cell's nucleus) carry genes, which contain the chemical instructions for inherited characteristics. Chromosomal testing made it possible to determine whether a fetus was affected by Down's syndrome, which causes severe mental retardation as well as physical and developmental deficiencies. The first such diagnosis was made in 1968 by Dr. Carlo Valenti in New York. Testing the fetus for genetic disease is now widely practiced, particularly for pregnant women over the age of 35 (who are at greater risk of conceiving a child with Down's syndrome) and parents with a family history of genetic problems. There are now over 500 hereditary (family) diseases that can be diagnosed through amniocentesis and other diagnostic techniques.

How Amniocentesis Is Performed

During amniocentesis, a doctor inserts a fine needle into the amniotic sack inside the uterus. A sample of the amniotic fluid is drawn out and cultured (grown) in the laboratory. In the early days of the procedure, doctors guided the needle into the uterus by touch and tried to be careful not to prick the placenta (sack), the fetus (baby), or the umbilical cord. Since the 1980s ultrasound devices have decreased the risk of damage during the procedure by providing a visual image of the fetus inside the uterus, which allows the doctor to guide the needle while watching the device's monitor.

The amniotic sample is taken from the fifteenth to the eighteenth weeks of the pregnancy (40 weeks is considered the normal length of a human pregnancy). Before the fifteenth week the amount of amniotic fluid present is insufficient to allow sampling. Culture and analysis of the specimen takes 10 to 21 days, which means that diagnosis of any fetal problems is not available until the twentieth or twenty-first week (fifth month) of the pregnancy. Chorionic villus sampling (CVS), an alternative method of fetal diagnosis, can be done much earlier in the pregnancy, but CVS carries a higher risk of causing spontaneous abortion (miscarriage). Amniocentesis, which causes miscarriage at a rate of 0.5 percent to 1 percent, is now being tried earlier in the pregnancy than 15 weeks.

The rise of amniocentesis as a tool for accurate prenatal diagnosis has made it possible to treat some medical problems before birth while the baby is in the uterus. In cases where treatment is not available, parents have faced the difficult option of giving birth to a child with life-threatening conditions or terminating the pregnancy (by abortion). Amniocentesis can also reveal how developed a fetus is. This knowledge is especially important when early delivery may be necessary. For example, when amniocentesis shows that the fetal lungs are not mature enough to work properly after birth, a hormone can be injected into the fetus to help the lungs develop.

[See also Rh factor ]

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amniocentesis

amniocentesis (ăm´nēō´sĕntē´sĬs), diagnostic procedure in which a sample of the amniotic fluid surrounding a fetus is removed from the uterus by means of a fine needle inserted through the abdomen of the pregnant woman (see pregnancy). The procedure can be done in a hospital or in a doctor's office. Ultrasound is used to determine the location of the fetus during the procedure. Fetal cells in the fluid can be grown in the laboratory and studied to detect the presence of certain genetic disorders (e.g., Down syndrome, Tay-Sachs disease) or physical abnormalities (e.g., anencephaly, or incomplete development of the brain). The sample also can be examined to determine the gender of the fetus and has been used to preselect the sex of the baby, a practice that, although controversial, is much used in some parts of the world. Amniocentesis cannot be used to detect such defects as congenital heart disease or cleft palate.

Amniocentesis is generally recommended when there is a family history of genetic disorders or when the woman is over age 35 and therefore at a higher risk of having a baby with a chromosomal abnormality. The procedure is usually carried out around the 14th or 15th week of pregnancy, when there is sufficient amniotic fluid and abortion is still an option. It can also be used in the third trimester (after 30 weeks) when Rh incompatibility (see erythroblastosis fetalis) is suspected, or to determine the status of the fetus in early or late delivery or when there are signs of fetal distress.

See also birth defects; chorionic villus sampling; embryo screening.

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amniocentesis

amniocentesis Removal of fluid from the amniotic cavity. This is accomplished by passing a long, fine needle through the abdominal wall and through the underlying wall of the pregnant uterus, and aspirating a sample into a syringe. The purpose is to obtain stray cells shed from the fetus into the fluid, which can provide evidence of genetic abnormalities, especially Down's syndrome. Because the risk of conceiving such an infant increases with age, amniocentesis is commonly advised at about 16 weeks of pregnancy in any mother over about 35. The procedure is combined with ultrasound scanning, which allows the fetus, the placenta, and the needle to be visualized, and damage thus avoided. There is a very small risk of inducing miscarriage (less than 1% in good hands). Preliminary tests on the mother's blood may indicate how strongly amniocentesis is to be advised.

Stuart Judge


See antenatal development; congenital abnormalities; pregnancy; ultrasound.

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amniocentesis

amniocentesis (am-ni-oh-sen-tee-sis) n. withdrawal of a sample of amniotic fluid surrounding a fetus in the uterus, by means of a syringe inserted through the abdominal wall under direct ultrasound guidance, to enable chromosome analysis of fetal cells in the fluid and hence prenatal diagnosis of chromosomal abnormalities (such as Down's syndrome) and metabolic and other congenital disorders (such as spina bifida). The procedure is usually performed after the 15th completed week of gestation and carries a 1–2% risk of miscarriage.

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amniocentesis

amniocentesis The taking of a sample of amniotic fluid from a pregnant woman to determine the condition of an unborn baby. A hollow needle is inserted through the woman's abdomen and wall of the uterus and the fluid drawn off. Chemical and microscopical examination of cells shed from the embryo's skin into the fluid are used to detect spina bifida, Down's syndrome, or other serious biochemical or chromosomal abnormalities.

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amniocentesis

am·ni·o·cen·te·sis / ˌamnē-ōsenˈtēsis/ • n. (pl. -ses / -sēz/ ) Med. the sampling of amniotic fluid using a hollow needle inserted into the uterus, to screen for developmental abnormalities in a fetus.

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amniocentesis

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