Purpose: To determine the prevalence of chromosomal abnormalities in fetuses with prenatally diagnosed pleural effusions and to identify factors associated with an increased risk of aneuploidy. Methods: A retrospective analysis of the Genzyme Genetics database was performed for samples submitted from October 1994 to April 2003 with an indication of fetal pleural effusion. Results: There were 246 samples in which pleural effusion was identified as an indication for prenatal chromosome analysis.

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... inety-four were from fetuses with isolated pleural effusions and 152 had other abnormalities in addition to pleural effusion. The prevalence of chromosome abnormalities was 35.4% (95% confidence interval, 29.2-41.4%). Among the eight first trimester samples, the aneuploidy rate was 63%. Pleural effusion cases associated with additional sonographic findings had a significantly higher aneuploidy rate than the isolated pleural effusion cases (50% vs. 12%, P Ͻ 0.001). Conclusions: Chromosome analysis is warranted after the prenatal detection of a fetal pleural effusion. The risk of aneuploidy is greater with first trimester detection and is significantly increased in the presence of other associated anomalies.