Test : Amniotic fluid, chromosome & genetic abnormality analysis
Indications : Prenatal testing through amniocentesis is done for the following reasons:
• Couples with a family history of a genetic abnormality
• Couples who have an older child with a genetic defect
• Mother above the age of 35 yrs
The test can also be done in any pregnant woman who wishes to do it, even if her risks are not above normal.
Amniotic fluid is a liquid that surrounds and bathes the fetus. Besides maternal cells, amniotic fluid also contains cells shed by the fetus. Amniocentesis is carried out under USG guidance. No preparation is needed prior to the test.
During the procedure, the doctor inserts a needle through the abdominal wall and draws a sample of amniotic fluid. This is then cultured in the laboratory by incubating along with growth media and a mitogen, PHA. The chromosomes in the cultured lymphocytes are analyzed under a microscope. The results of the test will be available within two weeks.
A part of the amniotic fluid is tested for alpha feto protein levels, to detect neural defects in the fetus.
After the procedure, women who are Rh-negative are given Rh0 (D) immune globulin to prevent them from producing Rh factor antibodies which can harm the fetus. Some women experience soreness while others may experience vaginal spotting or amniotic fluid leakage afterwards.
Physiology : Amniotic fluid analysis, also known as
amniocentesis, is one of the most common prenatal diagnostic procedures carried out on an unborn fetus to detect hereditary or spontaneous genetic abnormalities.
Usually amniocentesis is carried out to determine the risk of children being born with
genetic defects (such as neural tube defects,
Down syndrome), especially to mothers above age 35.
Couples who consider amniocentesis may have to think about
abortion. If they are unwilling to abort they may have to face the consequences of raising a child with an abnormality. Many couples require counseling to help them understand the situation in order to make the right decision.
Amniocentesis is invasive and complicated; hence the necessity to do this test is evaluated after doing blood tests and an ultrasound. It is usually carried out during 15th week of
pregnancy or later and can also be done on women who are pregnant with twins or more.
Normal Range : Prenatal diagnosis is possible for more than 1000 inherited diseases, including inborn errors of metabolism.
Interpretation : Microscopic observation of the karyotype of fetal chromosomes may reveal an underlying chromosomal abnormality. However, an absence of a chromosomal abnormality does not indicate ‘normal’. It merely indicates that no detectable chromosomal abnormality can be seen. More specific results may be obtained by doing a procedure called Fluorescent insitu hybridization (FISH), which uses DNA probes.
Sample : 22 sets of normal autosomal chromosomes and normal number and appearance of sex chromosmes.
Test Method : cell culturing, harvesting and chromosome analysis
Related Tests : Amniotic fluid
References :
1. http://www.merckmanuals.com/home/womens_health_issues/genetic_disorders_detection/prenatal_diagnostic_testing.html
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