Chromosomal analysis of fetal cells is a commonly used, safe, and highly accurate procedure. The rate of false-negative results is unknown. Recent experience at four centers suggests that there may be a particular likelihood for mosaic trisomy 8 to be missed with routine antenatal diagnostic procedures. This report reviews these cases, the characteristic findings of mosaic trisomy 8, and the tissue-specific differential yield of chromosomal analysis that may contribute to the increased risk of missed antenatal diagnosis in patients with this disorder.