During amniocentesis the gynaecologist takes 20 to 30 ml of amniotic fluid. This contains mainly the cells of the foetus, which in the lab are partly processed immediately and partly cultured.
As soon as sufficient cells are present:
The cytogenetics lab carries out a chromosome test on the sample and produces a molecular karyotype.
But FISH too (another technique for examining chromosomes) is a very common test after amniocentesis.
Biochemical tests are also possible on cultured amniotic fluid cells.
On the DNA that is immediately isolated from the amniotic fluid a small number of DNA tests can be carried out.
If possible the cytogenetics lab keeps cultured amniotic fluid cells as a back-up in case the amniocentesis contains too few cells.