Prevalence of maternal cell contamination in amniotic fluid samples.

PURPOSE: The purpose of this study is to evaluate the incidence of maternal cell contamination (MCC) in the first few milliliters of amniotic fluid withdrawn during amniocentesis.

METHODS: A prospective observational study was performed. The initial 2-3 ml of amniotic fluid withdrawn during amniocentesis was divided into direct analysis (uncultured) and cultured samples. A matching maternal buccal swab was obtained for MCC testing. MCC was determined by short-tandem repeat analysis. The primary outcome was measurement of clinically significant contamination (MCC >5%). Secondary outcomes included the determination of risk factors associated with MCC >5%. Outcomes were assessed by fisher's exact, independent t-test, binary logistic regression, and ANOVA.

RESULTS: Direct analysis measured clinically significant contamination (MCC > 5%) in 26% of specimens, while any amount of MCC was present in 68% of specimens. Cultured specimens had MCC > 5% in 2%, and any amount of MCC in 24%. Only blood-tinged fluid was associated with an increased risk for MCC > 5%. Larger volumes of the discard sample were not associated with increased incidence of MCC greater than 5%.

CONCLUSION: A significant amount of MCC is present with direct analysis of the initial few milliliters of amniotic fluid withdrawn and is not influenced by the volume of the discard sample. Our results suggest that the first few milliliters of amniotic fluid be removed and discarded when direct analysis is utilized for prenatal genetic testing.