This case report describes a 37-year-old pregnant woman who initially presented with discordant results from maternal cell-free DNA (cfDNA) testing, suggesting full or partial monosomies for chromosomes 13, 18, 21, and X. Despite normal fetal karyotype and chromosomal microarray results, further analysis revealed a "saw-tooth" pattern of genome-wide imbalance, indicating potential maternal malignancy. Postpartum, the patient was diagnosed with stage IV colon cancer, which was confirmed by imaging and biopsy. The report highlights the importance of follow-up and deeper analysis when cfDNA results are discordant with diagnostic testing, as well as the challenges in identifying maternal malignancies. The case also underscores the need for a systematic multidisciplinary approach to manage pregnant women with discordant cfDNA results and to establish consistent recommendations for diagnosis and treatment.This case report describes a 37-year-old pregnant woman who initially presented with discordant results from maternal cell-free DNA (cfDNA) testing, suggesting full or partial monosomies for chromosomes 13, 18, 21, and X. Despite normal fetal karyotype and chromosomal microarray results, further analysis revealed a "saw-tooth" pattern of genome-wide imbalance, indicating potential maternal malignancy. Postpartum, the patient was diagnosed with stage IV colon cancer, which was confirmed by imaging and biopsy. The report highlights the importance of follow-up and deeper analysis when cfDNA results are discordant with diagnostic testing, as well as the challenges in identifying maternal malignancies. The case also underscores the need for a systematic multidisciplinary approach to manage pregnant women with discordant cfDNA results and to establish consistent recommendations for diagnosis and treatment.