What Is the FISH Test for Pregnant Woman?

FISH, or Fluorescence in situ hybridization, gives researchers a way to map the genetic material in the body's cells by allowing them to look at specific genes or pieces of genetic material, according to the National Human Genome Research Institute 1. Knowing the location of particular genes is important for understanding a number of chromosomal abnormalities and genetic mutations 3. As a result, FISH can be a useful prenatal screening tool in helping to detect chromosome conditions, or genetic diseases, in unborn babies 4.

Analyzing Cells

In performing the FISH test, medical researchers analyze cells from amniotic fluid samples. They begin by labeling what they refer to as FISH probes, or single-stranded DNA, with a specific color of fluorescent dye. Since DNA is made up of two strands of molecules that bind to each other, the probes then bind to complimentary strands of DNA, identifying the location of a gene on a chromosome. The test also can help to determine if a fetus has the correct number of chromosomes or is missing genetic material.

Detecting Chromosome Abnormalities

FISH detects abnormalities in the number of chromosomes X, Y, 13, 18 and 21 3. Doctors order the test to help identify Down syndrome, Turner syndrome, Klinefelter syndrome, and triploidy, a chromosomal abnormality in which the zygote -- the cell formed when the egg is fertilized -- receives three instead of two copies of a chromosome. Since it cannot detect abnormalities involving chromosomes other than X, Y, 13, 18 and 21, FISH is not useful for identifying structural chromosome abnormalities or mosaicism, a condition where some of the body's cells have a different number of chromosomes, points out Quest Diagnostics 2.

Considering the Contributing Factors

FISH and prenatal chromosome analysis are tests available when a woman becomes pregnant after age 35, if there is a known family history of chromosomal abnormalities, or if abnormal ultrasound results or other prenatal screens suggest Down syndrome 34. Although babies inherit some chromosomal disorders, many occur randomly. Chromosomal abnormalities can cause a wide variety of mental and physical birth defects, according to the March of Dimes 3. Problems come about when cells pass down an incorrect number of chromosomes each time a cell divides and replicates DNA.

Getting Unclear Results

FISH results generally are consistent with routine cytogenetic analysis results but, in some cases, the results can be false-positive or inconclusive. Consequently, the American College of Medical Genetics recommends that doctors should not rely solely on the findings of FISH analysis. Although health care providers can order the test individually, prenatal chromosome analysis is required to confirm the results 4.