Amniocentesis

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Amniocentesis, or an amniotic fluid test (AFT), is a diagnostic, surgical procedure whereby a long needle is used to extract a sample of amniotic fluid from the amnion around a developing fetus. Fetal cells collected in the sample are used to diagnose the condition of the fetus, including genetic abnormalities such as Down syndrome and Tay-Sachs disease.

Amniocentesis offers the patient an opportunity to make informed decisions about prenatal treatment options and intervention. For example, one can evaluate whether an Rh-positive baby, being born to an Rh-negative mother, needs to be delivered early or requires a fetal blood transfusion, or whether a child will be born with hemophilia and need particular treatments to deal with that condition. While amniocentesis reveals some of the best of human creativity, there is also the potential for misuse of this procedure by potential parents to preselect or abort otherwise healthy, normal children that do not fit their own preferences or preconceived notions of the "correct" sex, eye color, height, intelligence, and so forth.

Contents

Overview and purpose

Amniocentesis involves passing a long, fine needle through the abdominal wall and the wall of the pregnant uterus and collecting a sample of the amniotic fluid that surrounds the fetus (Blakemore and Jennett 2001; Longe 2006). This invasive surgical procedure is designed to collect stray cells shed from the fetus into the fluid.

These cells can then be tested for evidence of fetal abnormalities, such as chromosomal abnormalities, genetic diseases caused by defective genes, and metabolic problems, as well as whether an expected premature baby can survive outside the womb, and to evaluate the condition of Rh-positive fetuses being carried by Rh-negative mothers (Zaret 1997).

Graph showing increased chance of Down syndrome compared to maternal age.

Amniocentesis has been used since the mid-1970s to test for Down syndrome, the most common, nonhereditary, genetic birth defect, impacting about one of every 1,000 babies (Longe 2006). By 1997, there have been about 800 diagnostic tests available, including for such hereditary genetic diseases as Tay-Sachs disease, sickle cell anemia, hemophilia, cystic fibrosis, and muscular dystrophy (Knapp and Wilson 2005).

A side effect of cytogenetic analysis is determination of the sex of the fetus with 100 percent accuracy, although the sex of the fetus can be discovered using the much more common and less intrusive means of examining ultrasound images.

Amniocentesis is considered to be an elective procedure, but it is usually recommended for women who will be older than 35 on their due date. This is because the risk of having a child with a nonhereditary genetic defect, such as Down syndrome, is directly related to the age of the woman, and 35 years old is the age at which the risk of carrying a fetus with such a defect is about equal to the risk of miscarriage from the procedure, or about one in 200 (Longe 2006). Notably, however, about 75 percent of all Down syndrome infants born in the United States each year occur to women younger than 35 (Longe 2006). In cases of family histories with chromosomal abnormalities or problematic pregnancies, some doctors will suggest younger women to take the test even though they do not fit the typical age profile.

Procedure

The basic idea of the procedure is to somehow get a hold of the fetus' cells to examine the chromosomes and/or DNA from these cells in order to rule out genetic abnormalities that could lead to birth defects. The most accessible fetal cells are in the amniotic fluid, where one would find traces of the fetus' skin and other cells that have sloughed off the fetus during its growth.

Amniocentesis means "puncture of the amnion," and involves withdrawal of about one ounce of amniotic fluid (Longe 2006). Amniotic fluid often is obtained using a long syringe. It can be done on an outpatient basis and sometimes using local anesthesia. Ultrasound is usually used to guide the placement of the needle (Knapp and Wilson 2005).

The syringe is usually inserted in to the mother's abdominal wall or at the end of the vagina, and through the uterus wall. The physician would aim for an area of the amniotic sac that is away from the fetus so to avoid stabbing it. A small amount of amniotic fluid then gets sucked out and the syringe is withdrawn. The puncture wound should close up by itself, just like any shots one would get, and the amniotic sac should then automatically replenish the liquid over a day or so.

Amniocentesis can be done as soon as there is enough amniotic fluid surrounding the fetus that a sample can be removed safely. Early amniocentesis can be performed as early as 13 weeks gestation. Standard amniocentesis is usually performed between 15 and 20 weeks gestation. Amniocentesis is not performed earlier because there is not as much amniotic fluid when the fetus is younger. There is a greater risk to the fetus if fluid is taken out. The test is typically not performed later than the second trimester.

Once the amniotic fluid is extracted, fetal cells are separated from the fluid using a centrifuge, and the lab will check the chromosomes in the cells for abnormalities. Results take about one to two weeks. Various types of genetic testing could be done but it would be up to the individual lab doing the work to decide on the list of diseases they are able to detect and look for. They could either be cytogenetics screening or simply looking for abnormal genes. The doctor might order additional screening to look for particular genetic signature if there is family history of that particular disease. In any case, it is impossible to perform a comprehensive screening of all known human genetic diseases at this time, regardless of which lab the patient or doctor choose.

In addition to the fetal cells, some labs would use the remainder amniotic fluid to find indications of non-genetic diseases the fetus might have. Levels of chemical substances can be measured to detect other health problems such as hydrocephalus.

Often, genetic counseling is done before amniocentesis, or other types of genetic testing are offered. Possible fetal defects are sometimes signaled by another test, the alpha-fetoprotein (AFP) test, which involves a simple blood screening for this protein; an abnormal AFP test may be followed by amniocentesis to confirm whether there indeed is a fetal defect.

Risks

Amniocentesis is an invasive surgical procedure and thus there as some, albeit small, risks involved.

During the procedure, and immediately afterward, the patient may experience nausea, dizziness, cramping, and a rapid heartbeat (Longe 2006). Some complications with the procedure include the introduction of pathogens into the amniotic sac from the needle and the puncture wound not healing properly after the syringe has been withdrawn, which leads to leakage or infections. Other complications may include vaginal bleeding, premature labor, maternal or fetal hemorrhaging, fetal injury due to contact with the needle, and miscarriage (Longe 2006), as well as respiratory distress, postural deformities, and alloimmunisation (rhesus disease).

There also are emotional concerns. These can include the anxiety of waiting for results and the trauma of family-planning decisions based on the results (Longe 2006). There also is the reality that the results are not 100 percent accurate at detecting the targeted conditions (Longe 2006).

Amniocentesis is not as risky to the child as chorionic villus sampling, although villus sampling can be done earlier (Rhoads et al. 1989).

The risk of amniocentesis-related miscarriage is generally thought to be one out of every 200, but a recent study suggests the risk may be closer to one out of every 1,600 (Hitti 2006).

References


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