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Introduction: Trisomy 18 is a constitutional chromosomal disorder defined by the presence of an extra chromosome 18. The diagnosis is suspected clinically and confirmed by cytogenetic analysis. Genetic counseling for patients’ families is important. The objective of this study is to report our experience in the medical genetics department at the Mohammed VI University Hospital in Oujda of the diagnosis and genetic counseling of trisomy 18 through dysmorphological expertise and cytogenetic analysis. Materials and methods: We report a retrospective descriptive study over a period of four years (2018–2022) of nine patients with polymalformative syndrome suggestive of trisomy 18 who underwent cytogenetic analysis. Results: The median age of patients at diagnosis was 2 days, with a male predominance. The mean maternal age at birth of the patients in our series was 40 years. Consanguinity was found in only one patient. All patients had a typical phenotype of trisomy 18. The postnatal constitutional karyotype showed a homogeneous trisomy 18 in all patients. In our series, only one patient was still alive at the age of 7 months, while the other 8 patients died with a median postnatal survival of 5 days. Conclusion: Through this study, we highlight the contribution of the medical geneticist in the clinical and cytogenetic diagnostic approach of rare chromosomal disorders, in order to provide adequate genetic counseling to families.