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2. What can amniocentesis detect?




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This article is from the Pregnancy Screening FAQ, by Lynn Gazis-Sax (gazissax@netcom.com) with numerous contributions by others.

2. What can amniocentesis detect?

One of the main uses of amniocentesis is to detect chromosomal
abnormalities. With amniocentesis, it is possible to reconstruct the
chariotype of an individual, i.e. map his or her chromosomes. This allows
the detection of trisomies (extra chromosomes), monosomies (missing
chromosomes), and other structural defects in the chromosomes.

Most people have 46 chromosomes, in 23 pairs, but some have an extra copy
of one chromosome, called a trisomy because there are three of one
chromosome pair. This extra chromosome can lead to a variety of
abnormalities. The most common trisomy is called Down syndrome, a trisomy
of chromosome pair 21, and it leads to mental retardation and various
physical problems. Other common trisomies are trisomies 13 and 18 (which
generally cause a baby to die shortly after birth) and sex chromosome
trisomies. Examining the chromosomes also allows the sex to be
determined, which may be of particular interest to women who are carriers
of sex-linked disorders, such as hemophilia or Duchenne muscular
dystrophy.

In addition to examining the fetal cells for chromosome abnormalities, the
amniotic fluid can be tested for levels of AFAFP, acetylcholinesterase
(AChe), and hemoglobin F. It is possible to detect neural tube defects,
including anencephaly, spina bifida, and meningomyelocele (though the use
of amniocentesis to detect neural tube defects has been mostly superceded
by a combination of the AFP test and high resolution detailed ultrasound).

It is also possible to detect about 70 metabolic disorders. The tests for
metabolic disorders, however, are only done if family history warrants,
and will not be done for women being referred for amnio due to age or
results on the AFP test or Down's screen. If there is no history of
genetic disease in the family, a genetic analysis will not be performed,
only a chromosomal analysis. Chromosomal analysis is performed at most
large hospitals and some private labs. Genetic analysis is only performed
in a few labs in the country and is significantly more expensive. Tay
Sachs, which is common among Eastern European Ashkenazi Jews and French
Canadians, can be detected by amniocentesis. One of my sources (Blatt)
states that amnio can be used for certain experimental DNA studies,
detecting cystic fibrosis, sickle cell anemia, thalassemia, and other
blood disorders with varying degrees of accuracy. Another source (Scher
and Dix) says that sickle cell anemia and thalassemia cannot be detected
by amniocentesis, but can be detected by an experimental procedure called
fetoscopy.

Clarification by Dr. T. Reynolds:

It is possible to collect blood samples from the fetus by amnio by guiding the
needle into an umbilical vessel. We don't do it in my hospital because our
ethnic mix doesn't warant the development of the expertise.


Since several different prenatal tests are used to detect the same disabilities,
information on the disabilities which amnio and other tests
detect is included in the Prenatal Testing overview FAQ.

 

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