Amniocentesis (amnio = amnion; kentesis = puncture)
Amniocentesis involves withdrawing some of the amniotic fluid that bathes the developing fetus and analyzing the fetal cells and dissolved substances.
It is used to test for the presence of certain genetic disorders such as down syndrome (DS), Spina bifida, hemophilia, Tay-Sachs disease, sickle-cell anemia, and certain muscular dystrophies or to determine fetal maturity and well being near the time of the delivery.
To detect suspected genetic abnormalities, the test is usually done at 14 to 16 week of gestation.
To assess fetal maturity, it is usually done after the 35th week of gestation. About 300 chromosomal disorders and over 50 biochemical defects can be detected through amniocentesis.
It can also reveal gender. This information is important for diagnosis of sex-linked disorders, in which an abnormal gene is carried by the mother but affects only her male offspring. If the fetus is female, it will not be afflicted.
When both parents are known or suspected to be genetic carriers of any of these disorders, or when the mother is past age 35, amniocentesis is advised.
The procedure is also advised when there is concern of a premature delivery, a medical condition that necessitates an early delivery, and for Rh- women who have Rh antibodies in their blood and a fetus that is possibly Rh+.
Using ultrasound and palpation, the position of the fetus and placenta are first determined.
After the skin is prepared with an antiseptic, a local anesthetic is given, a hypodermic needle is inserted through the mother's abdominal wall and uterus into the amniotic cavity, and about 10ml of fluid are aspirated.
The fluid and suspended cells are subjected to microscopic examination and biochemical testing.
Elevated levels of alphafetoprotein (AFP) and acetylcholine esterase may indicated failure of the nervous system to develop properly, For example, anencephaly (absence of the cerebrum) or spina bifida.
Chromosome studies, which require growing the cells for two to four weeks in a culture medium, may reveal rearranged, missing, or extra chromosomes.
There is about a 0.5% chance of spontaneous abortion after the test.
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