RESEARCH QUESTION: What is the prevalence of DNA contamination in embryo biopsy samples taken for preimplantation genetic testing for aneuploidy (PGT-A), and how often might contamination lead to incorrect interpretation of results? DESIGN: A retrospective study of 57,292 clinical PGT-A samples from 32 IVF clinics was carried out to determine the frequency of DNA contamination affecting trophectoderm biopsy specimens. Testing employed a highly validated and widely used PGT-A method, which includes genotyping of >4000 single-nucleotide polymorphisms. RESULTS: Validation experiments confirmed the ability of the method to detect DNA contamination reliably. Overall, contamination was detected in 0.45% (256/57,292) of trophectoderm biopsy specimens, but rates at individual clinics varied significantly (P < 0.0001, chi-squared test). In total, 155 embryos with a contaminated biopsy specimen had a result from a second (uncontaminated) sample. Comparison of the two biopsies revealed that errors in classification of chromosomal status could have occurred in 44.5% (69/155) of these embryos if contamination had not been detected. CONCLUSIONS: DNA contamination affected one in 222 samples. However, frequencies below one in 1000 were achieved in some clinics, while the incidence of contamination was more than 10 times higher at other clinics. Contamination was associated with a substantial risk of diagnostic error, potentially leading to the discard of viable embryos or the transfer of aneuploid embryos. Validated PGT-A methods, capable of detecting DNA contamination, should be used, and the incidence of contamination should be considered a key performance indicator for clinics offering PGT-A.