amniocentesis

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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 age—the 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〉.

amniocentesis

 [am″ne-o-sen-te´sis]
transabdominal perforation of the amniotic sac in order to obtain a sample of amniotic fluid, which contains cells shed from the skin of the fetus as well as biochemical substances. Analyses of changes in chemical and cellular composition of the fluid are helpful in assessing the maturation and viability of the fetus. Amniocentesis also provides for prenatal diagnosis of certain genetically transmitted errors of metabolism, congenital abnormalities, and chromosomal disorders. It is feasible to detect by amniocentesis the presence of over 40 different types of inherited disorders in embryos and fetuses as young as 16 weeks' gestation.
 A diagram of the technique of amniocentesis. From Mueller and Young, 2001.


A common indication for amniocentesis is the suspected presence of erythroblastosis fetalis, which results from incompatibility of Rh factors in the fetal and maternal blood. Samples of amniotic fluid are analyzed for concentrations of protein and bilirubin. The higher the concentration, the stronger the evidence that erythrocytes are being destroyed, and measures must be taken to forestall the effects of the disorder on the fetus.

Another biochemical study involves measuring the level of alpha-fetoprotein (AFP) in the amniotic fluid. Abnormally high levels indicate an open defect of the spine, e.g., spina bifida or anencephaly.

Because of the ability to determine fetal maturation by amniotic fluid studies, it is possible to predict whether an infant will suffer from hyaline membrane disease at birth. A favorable ratio of lecithin to sphingomyelin indicates sufficient lung maturity.
Patient Care During and After Amniocentesis. While the procedure for removal of amniotic fluid has minimal risk for the fetus and mother, there are slight risks of bleeding, leakage of fluid, or infection. There also is a remote chance of miscarriage.

When being performed for prenatal diagnosis, amniocentesis is usually offered to women 35 years or older, women with a child with a chromosomal abnormality, parents with a child with neural tube defect, and women with elevated serum alpha-fetoprotein, or when one parent has a chromosomal abnormality or is a carrier for a metabolic disease. Parents who agree to amniocentesis must be prepared to make a decision whether or not to abort the fetus if the laboratory tests indicate the presence of a birth defect. Frequently, however, parents who have a high risk for having an abnormal child find that the developing fetus is normal and the prognosis for delivery of a healthy baby is excellent.

During the procedure, the physician inserts a long pudendal needle into the mother's abdomen and into the amniotic cavity, avoiding the fetus and the placenta (which is located via ultrasonography). Local anesthesia is used to minimize discomfort. The patient is cautioned not to move during the procedure lest the needle become displaced. A syringe is attached and fluid is withdrawn. It is sometimes necessary to repeat the procedure because of an insufficient sample or unsuccessful laboratory testing. The repeat procedure is done several days after the first.

Following amniocentesis the patient is observed for changes in blood pressure, excessive leakage of fluid, and signs of infection. Hemorrhage from the placenta must be considered a possibility if the blood pressure begins to drop. Increased fetal activity or other signs of fetal distress such as changes in the fetal heart rate must be reported to the physician at once as they may warrant immediate measures such as delivery of the infant if it is considered to be viable. The Centers for Disease Control and Prevention publish a useful patient education booklet on amniocentesis titled “Amniocentesis for Prenatal Chromosomal Diagnosis,” which can be obtained by request from the CDC: Chronic Disease Division, Bureau of Epidemiology, Atlanta, GA 30333.

am·ni·o·cen·te·sis

(am'nē-ō-sen-tē'sis),
Transabdominal aspiration of fluid from the amniotic sac.
[amnio- + G. kentēsis, puncture]

amniocentesis

/am·nio·cen·te·sis/ (am″ne-o-sen-te´sis) surgical transabdominal or transcervical penetration of the uterus for aspiration of amniotic fluid.
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Amniocentesis.

amniocentesis

(ăm′nē-ō-sĕn-tē′sĭs)
n. pl. amniocente·ses (-sēz)
A procedure in which a small sample of amniotic fluid is drawn out of the uterus through a needle inserted in the abdomen. The fluid is then analyzed to detect genetic abnormalities in the fetus or to determine the sex of the fetus.

amniocentesis

[am′nē·ōsentē′sis]
Etymology: Gk, amnos, lamb's caul, kentesis, pricking
an obstetric procedure in which a small amount of amniotic fluid is removed for laboratory analysis. It is usually performed between the sixteenth and twentieth weeks of gestation to aid in the diagnosis of fetal abnormalities, especially genetic disorders.
method With the use of ultrasound imaging techniques the position of the fetus and the location of the placenta are determined. The skin on the mother's abdomen is aseptically prepared, and a local anesthetic is usually injected. A needle attached to a syringe is introduced into a part of the uterus where there is the least chance of perforating the placenta or scratching the fetus. Between 20 and 25 mL of amniotic fluid is aspirated. Amniocentesis is performed to diagnose various inherited disorders, including chromosomal aberrratiions, neural tube defects, and Tay-Sachs disease. It is also performed to discover the sex of the fetus if certain sex-linked disorders are suspected. Later in pregnancy, amniocentesis may be performed to assess fetal lung maturity by testing the lecithin/sphingomyelin ratio and the presence of phosphatidyl-glycerol in the laboratory before elective delivery. The fluid may be tested for the concentration of creatinine, another indicator of fetal maturity. When postmaturity is suspected, amniocentesis is performed to examine the amniotic fluid for meconium.
nursing considerations A woman must sign an informed consent form before amniocentesis. Specifically stated in the consent form are the reasons for performing the procedure and the facts that fluid is to be removed after needle puncture of the uterus, that ultrasound imaging techniques are usual adjuncts, that the procedure may fail to give the results intended, and that spontaneous abortion, nausea, abdominal pain, or fetal injury may occur. The woman is reassured that complications and failure are rare; she is given emotional support before, during, and after the procedure. In testing for inherited disorders, 10 days to 2 weeks is usually necessary for tissue culture before a diagnosis may be made; this waiting period may be extremely stressful for the mother. The woman is warned to report any signs of infection or of the onset of labor. Rho(D) immunoglobulin should be given to pregnant women who are Rh negative.
outcome criteria Spontaneous abortion occurs in approximately 1% of women undergoing amniocentesis. Perforation of the placenta or a blood vessel in the umbilical cord or placenta may cause hemorrhage or isoimmunization and hemolytic disease of the fetus, possibly leading to fetal death. Maternal and fetal infection with attendant morbidity or mortality may occur but it is rare. Premature rupture of membranes, preterm labor, or trauma to the fetus or umbilical cord may occur. Compare chorionic villus sampling.
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Amniocentesis: placement of needle

amniocentesis

A procedure in which fluid is obtained by an ultrasonographically guided needle from the amniotic cavity—usually between weeks 15–17 of pregnancy—to detect foetal abnormalities by karyotyping or chemical analysis; accuracy is reported to be 99.4%; complication rate < 0.5% above the background pregnancy loss of 2–3%; foetal loss minimal.

Defects identified by amniocentesis
Cultured amniotic cells can be used for cytogenetic studies, DNA analysis (karyotyping) and enzyme assays; amniocentesis may not be necessary in women with increased AFP and a normal foetus by ultrasonography.

Indications
• Maternal age > 35;
• 3+ spontaneous abortions;
• Previous child with chromosome defect, metabolic disease, neural tube defect; 
• Parent with or family history of chromosome defect(s);
• Possible carrier of X-linked disease.

amniocentesis

Obstetrics A procedure in which fluid is obtained by an ultrasonographically-guided needle from the amniotic cavity–usually between wks 15-17 of pregnancy to detect fetal abnormalities by karyotyping or chemical analysis; accuracy is reported to be 99.4%; complication rate < 0.5% above the background pregnancy loss of 2-3%; fetal loss minimal  Defects identified by amniocentesis Cultured amniotic cells can be used for cytogenetic studies, DNA analysis–karyotyping, and enzyme assays. Cf Chorionic villus biopsy.
Amniocentesis—indications for
Maternal age > 35 3+ spontaneous abortions Previous child with chromosome defect, metabolic disease, neural tube defect Parent, or family Hx of chromosome defect(s) Possible carrier of X-linked disease.

am·ni·o·cen·te·sis

(am'nē-ō-sen-tē'sis)
Transabdominal aspiration of fluid from the amnionic sac for diagnostic purposes.
[amnio- + G. kentēsis, puncture]

amniocentesis

(am?ne-o-sen-te'sis) [ amnion + -centesis]
Enlarge picture
AMNIOCENTESIS
Transabdominal puncture and aspiration of the amniotic sac by ultrasound to remove amniotic fluid. The sample is studied chemically and cytologically to detect genetic and biochemical disorders and maternal-fetal blood incompatibility and, later in the pregnancy, to determine fetal maturity. The procedure also allows for transfusion of the fetus with platelets or blood and instillation of drugs for treating the fetus.

This procedure is usually performed no earlier than at 14 weeks' gestation. It is important that the analysis be done by experts in chemistry, cytogenetics, and cell culture. Cell cultures may require 30 days, and, if the test has to be repeated, the time required may be insufficient to allow corrective action. See: illustration

CAUTION!

The procedure can cause abortion or trauma to the fetus.

Patient care

The patient's knowledge about the procedure is evaluated, misconceptions corrected, and information provided as necessary. The patient is informed about sensations that she may experience and signs a consent form. The amniocentesis equipment is assembled; amber-colored test tubes are used (or clear test tubes covered with aluminum foil) to shield the fluid from light, which could break down bilirubin. Baseline vital signs and fetal heart rate are obtained, and the fundus is palpated for fetal position and fetal and uterine activity for 30 min before, during, and 30 min after the procedure. The patient is assessed for light-headedness, nausea, and diaphoresis as well as for anxiety, pain, and labor onset. During the procedure, emotional support is provided. After the procedure, the patient is positioned on her left side and is instructed to report unusual fetal hyperactivity or hypoactivity, clear or bloody vaginal drainage, uterine contractions, abdominal pain, or fever and chills, any of which is indicative of complications. Rh-negative women with an Rh-positive fetus should be given RhoGam. See: chorionic villus sampling

therapeutic amniocentesis

Amniodrainage.

amniocentesis

A method of obtaining early information about the health and genetic constitution of the growing fetus, by taking a sample of the fluid from the womb (AMNIOTIC FLUID) for analysis usually between the 15th and 18th week of pregnancy. Cellular debris in the fluid provides DNA for chromosome analysis and sex determination and the fluid contains specific substances characteristic of various diseases. The risk of fetal loss from this procedure is estimated to be 0.5% to 1% above the natural level of spontaneous abortion.
Amniocentesisclick for a larger image
Fig. 29 Amniocentesis . Sampling amniotic fluid by means of a surgical syringe.

amniocentesis

a technique for the diagnosis of congenital abnormalities before birth. In this procedure a sample of 10–15 cm3 of amniotic fluid surrounding the foetus is extracted through the abdominal wall using a surgical syringe, usually at about the 16th week of pregnancy. The sloughed-off embryonic cells contained in the fluid can be cultured to enable KARYOTYPE analysis, which shows the number and condition of the chromosomes. The fluid can also be analysed for the presence of chemicals which indicate biochemical and other defects (e.g. spina bifida).

amniocentesis

analysis of a sample of amniotic fluid to determine the genetic status of the fetus

amniocentesis (amˈ·nē·ō·sen·tēˑ·sis),

n procedure in obstetrics in which a needle is inserted into a pregnant woman's uterus in order to draw out a sample of amniotic fluid for laboratory analysis. Often used in older mothers to detect possible birth defects before birth.

amniocentesis

transabdominal perforation of the amniotic sac for the purpose of obtaining a sample of amniotic fluid, which contains cells shed from the skin of the fetus as well as biochemical substances.

Patient discussion about amniocentesis

Q. Can I get a little more information on amniocentesis…. hi all…..I am 38 years pregnant. I am expecting my 3rd child and I will have my amniocentesis test next week ………can I get a little more information on amniocentesis…. what else I can find and why do they take this and can it harm….. Please.

A. Hi………. Amniocentesis is done in 15 – 18 weeks for genetic screening. It’s mainly done for the pregnant women of more than 35 years of age. This is mainly carried out, if the prenatal test had shown any likelihood of problem with the baby or If found with any genetic problem with previous child and any previous miscarriages also. It’s used to check the baby`s heart rate and lung maturity. They check for any chromosomal abnormalities. Generally risk is very less such as high loss of blood, infection, premature labor.

Q. what does enlarged heart mean? is it linked to down syndrome? My wife is 16 weeks pregnant. we did a prenatal US and we were told that the baby hes an enlarged heart and that we are advised to do amniocentesis. A friend of mine had a baby with down syndrome, and he was toled that if they have done this procedure, the doctors could have find it on time and abort the pregnancy. Is there a clear connection between the enlarged hart and the fact that the doctor advised us to do this procedure to down syndrome?

A. Down syndrome is associated with several kinds of heart problems. those problems can cause an enlarged heart but usually it will appears as a post-natal complication.
The protocol that most obstetrics use is that if there is anything abnormal in the US there is an automatic referral to an amniocentesis procedure.
the US has a lot of false alarms, so they must check with the more accurate procedure.

More discussions about amniocentesis
References in periodicals archive ?
All the amniocenteses were conducted at 15-23 weeks' gestation, using a 22-gauge needle and guided by the same sonography team.
No frequency threshold for maintaining skills is set by the American College of Obstetricians and Gynecologists or by the American Board of Obstetrics and Gynecology Nor does the Accreditation Council for Graduate Medical Education set a figure on the number of amniocenteses that residents need to perform to graduate.
The reference method for prenatal diagnosis of CMV, requiring combined viral isolation and positive CMV PCR from amniotic fluid after gestational week 21 or 7 weeks after maternal symptoms (3,11), was applied for all amniocenteses.
In two recent case series with 1,500 third-trimester amniocenteses, nearly 1% of cases required an emergency cesarean section due to fetal or obstetric complications.