What is a fish test on amniotic fluid?

Rapid look for Down.: Fish stands of fluorescent in-situ hybidization of the chromosomes in fetal cells obtained after invasive prenatal diagnosis (such as amniocentesis). When time is of the essence, fish can give a preliminary result of fetal aneuploidy (e.g. Down syndrome) within 24 hours from the procedure. It should be followed by a full karyotype and chromosomal microarray analysis.