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Objective: To report a case of monozygotic dichorionic (DC) twins after a single cryopreserved blastocyst embryo transfer followed by genetic determination of zygosity postpartum. Design: Case report. Setting: University hospital. Patients: A 26-year-old woman with polycystic ovary syndrome and her 36-year-old male partner with severe oligozoospermia, resulting in a 1.5-year history of primary infertility. Interventions: Controlled ovarian stimulation and intracytoplasmic sperm injection treatment with single cryopreserved embryo transfer at blastocyst stage. Main Outcome Measures: Ultrasound images of the fetuses and short tandem repeat genotyping postpartum. Results: A DC twin pregnancy following a single cryopreserved blastocyst embryo transfer was confirmed at the first trimester screening. Confirmatory testing performed postpartum included short tandem repeat analysis determining monozygosity and pathology examination reporting DC placental configuration. Conclusions: Dichorionic monozygotic twins are thought to arise from the splitting of an embryo before the blastocyst stage. This case suggests that placental configuration of monozygotic twins may not strictly depend on timing of embryo division. Genetic analysis is the only tool to confirm the zygosity.