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 accepted—if rarely used—technique. 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

A doctor and nurse perform amniocentesis on a patient. The procedure is used to learn about the health of unborn babies.
A doctor and nurse perform amniocentesis on a patient. The procedure is used to learn about the health of unborn babies.
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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