Genetic amniocentesis was made possible in the 1970s after it was shown that fetal cells could be cultured and grown.  Fetal epithelial (skin) and other cells are sloughed from the fetus and are present in the amniotic fluid.  By getting a sample of the fluid, there is no need to sample the fetus directly. Fetal cells are cultured and grown because there are relatively few cells initially.  After culturing the cells, a process of karyotypic banding is performed.  The DNA is stained with a special stain (Giemsa stain) during a critical part of cell division.  Using a high powered microscope, the DNA can then be visualized and analyzed.  A full karyotype analysis looks for not only major chromsome abnormalities- such as abnormal number of chromosomes- but also much smaller abnormalities.
  • Usually performed 14-20 weeks
  • Risk of fetal death estimated as 1:200.  This risk is not due to hurting the fetus at the time of the procedure, but rather because the hole made by the needle may not seal properly, allowing amniotic fluid to escape.