If 50 out of 200 interphase nuclei have 3 signals in a prenatal aneusomy FISH analysis for chromosome 18, what does this indicate?

The presence of 3 signals in 50 out of 200 interphase nuclei during a prenatal aneuploidy FISH analysis for chromosome 18 suggests a potential increase in chromosome 18 copy number. In a normal diploid cell, you would expect to see 2 signals for chromosome 18. Observing 3 signals indicates that there may be trisomy 18, which is a condition characterized by having three copies of chromosome 18 instead of the usual two.

However, it's important to consider that the presence of 3 signals could also be indicative of a structural anomaly involving chromosome 18. Such anomalies can sometimes lead to a situation where an abnormal chromosome configuration presents as an increase in the number of detected signals in FISH analysis. Therefore, the situation described is consistent with both trisomy 18 and potentially a structural anomaly involving chromosome 18, making the correct conclusion that the results could point to either condition. This highlights the ambiguity that can arise in cytogenetic analysis, where further testing might be needed to clarify the exact nature of the chromosomal abnormality.