Normal prenatal ultrasound findings in a case with de novo mosaic small supernumerary marker chromosome 18 - how to counsel?

Introduction. We report on a prenatally diagnosed de novo small supernumerary marker chromosome (sSMC) derived form chromosome 18. Molecular cytogenetic studies led to information about the clinical relevance of the sSMC-induced chromosomal imbalance. As prenatal ultrasound was normal, detailed information with respect to prenatal counseling of the parents was necessary. In general, detection of an sSMC requires as much information on the exact genetic content with its possible impact on the phenotype as achievable. Material and methods. Amniocentesis was performed in a 37-year-old Gravida IV Para II with a history of an induced abortion due to a prenatally diagnosed trisomy 21. Fluorescence in situ hybridization quick test gave hint on a possible mosaic trisomy 18, whereas the conventional banding cytogenetic analysis revealed an sSMC. The amount of euchromatin was estimated to be less than 5 MB. Conclusions. sSMC are rare, being present in less than 0.08 of all pregnancies. Going together with an abnormal ultrasound, counseling of the parents is relatively easy to perform. In cases of normal prenatal ultrasound, profound knowledge about the surplus genetic content is necessary for the estimation of the fetal outcome prognosis. In the present case, detailed molecular cytogenetics techniques led the parents to continue the pregnancy.