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procedure, fluid is withdrawn by means of a needle inserted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 Phenylketonuria.
Karyotypic analysis of
shed fetal cells can be used to determine gender (Barr
body determination), or more specific chromosomal
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
decisions about second trimester pregnancies.
The amniocentesis procedure entails 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.
