Small supernumerary marker chromosomes (sSMC) are still a major problem in clinical cytogenetics as they cannot be identified or characterized unambiguously by conventional cytogenetics alone. On the other hand, and perhaps more importantly in prenatal settings, there is a challenging situation for counseling how to predict the risk for an abnormal phenotype, especially in cases with a de novo sSMC. Here we report on the prenatal diagnosis of a mosaic tetrasomy 18p due to presence of an sSMC in a fetus without abnormal sonographic signs. For a 26-year-old, gravida 2 (para 1) amniocentesis was done due to consanguineous marriage and concern for Down syndrome, based on borderline risk assessment. Parental karyotypes were normal, indicating a de novo chromosome aberration of the fetus. FISH analysis as well as molecular karyotyping identified the sSMC as an i(18)(pter->q10:q10->pter), compatible with tetrasomy for the mentioned region. Cordocentesis was done due to normal sonography and the results from amniocentesis were confirmed. The parents opted for pregnancy termination and post mortem examination now noted, low anterior hairline, large philtrum, low-set posteriorly rotated malformed ears with prominent antihelix, lower limbs joint contracture and digital anomalies, including long and narrow toes with clinodactyly of the 1st and 5th toes and postaxial polydactyly of one hand. De novo i(18p) can be considered as a special case in the sense that the major relevant phenotypes mentioned for it, i.e. feeding difficulties, abnormalities in muscle tone and developmental/mental retardation, cognitive and behavioral characteristics, recurrent otitis media and seizures, are mostly postnatal. This emphasizes the necessity to determine the nature of a de novo euchromatic marker chromosome, especially in cases with normal ultrasound result and the suitability of a cordocentesis in order to better predicting the pregnancy outcome and parental counseling.