Abstract
Karyotyping and fluorescence in situ hybridization (FISH) detect fetal chromosome
abnormalities. The choice between karyotyping and FISH is still debatable. In a developing
country, parents face an emotional and economic constraint of a prenatal test. Therefore,
the results of karyotyping and FISH were analyzed to determine the percentage of clinically
abnormal fetuses which would be missed by using standalone FISH. Amniotic fluid samples
from 9033 high-risk pregnancies were subjected to karyotyping and FISH for chromosomes
13, 18, 21, X, and Y. Karyotype and FISH were normal in 8680 (96.1 %) of these samples
and 353 (3.9 %) had abnormal karyotypes: 285 (3.2 %) were aneuploidies, also detected
by FISH and 68 (0.7 %) were structural chromosomal aberrations not detected by FISH.
Out of these 68 structural aberrations, 40 (0.4 %) were balanced rearrangements with
no apparent clinical significance and 28 (0.3 %) were unbalanced rearrangements with
potential clinical significance. By standalone FISH, 0.3 % clinically-significant
samples would have been missed. A 0.2 % risk of procedure-related abortion may be
acceptable but a 0.3 % risk of having an abnormal child may not be acceptable to the
parents. FISH may be offered as a first test, followed by karyotyping. Although, karyotyping
increases the cost, it is preferable to carry this out once an invasive procedure
has been opted for, with its accompanying risk of miscarriage.
Keywords
Karyotype - FISH - Fetal chromosomes - Aneuploidy - Prenatal
