Genetic
information about a fetus can
be obtained by analysis of amniotic
fluid or the shed cells in the amniotic
cavity. In the amniocentesis procedure,
fluid
is withdrawn by means of a needle inserted through the amnion
into the placental sac. Biochemical
analysis of the
fluid itself may indicate a genetic condition (e.g., high levels of
phenylalanine
may indicate PKU).
Karyotypic analysis of shed
fetal cells can be used
to determine
gender (Barr body
determination), or more specific karyotypic
conditions (e.g., Down Syndrome
or Trisomy-21).
Direct analysis of DNA by various RFLP and PCR
tests
allows for
much more rapid diagnosis, an important consideration in deciosions
about second
trimester
pregnancies.
The amniocentesis procedure entail some risk of
damage to the mother and/or fetus. In general, in the absence of
indications of specific genetic risk from family history, amniocentesis
is not recommended as a general prenatal screen.
