Analysis of fetal tissue to detect targeted numeric or structural chromosome aberrations.
Interphase FISH performed on uncultured cells is used to provide a rapid (1 day) result for targeted chromosome enumeration. Typically, chromosomes 13, 18, 21, X, and Y are assessed. Metaphase FISH performed on cultured cells is used to assess chromosome aberrations too small to be detected by conventional chromosome analysis.
Generally used only in cases with specific risk (specific ultrasound anomalies, family history).
Chromosome analysis
Analysis of fetal tissue to detect numeric and structural chromosome aberrations. Most chromosome aberrations are numeric (e.g., trisomies 13, 18, 21 [Down syndrome], 45,X [Turner syndrome], 47,XXY [Klinefelter syndrome]).
Principle indications:
Increased risk determined from maternal screening
Ultrasound anomaly
Family history of chromosome anomaly (previous affect pregnancy, balanced rearrangement carrier parent)
Fetal sexing for history of X-linked disorders
Limitations
FISH
Targeted test that assesses only specific region on chromosome; does not ensure that entire chromosome is normal and does not assess every chromosome.
Mosaicism may also confound results.
Chromosome analysis
Analysis cannot detect aberrations smaller than 5 " “10 megabases; requires cell culture to obtain actively dividing metaphase cells.
Mosaicism, the presence of two cell lines, may be difficult to interpret, because chromosome anomalies can arise in vitro during specimen culture.