Genetically unbalanced embryos usually miscarry. Most of the miscarriages occur in the first trimester as is true for all miscarriages, regardless of the cause [1]. The incidence of having a baby with this syndrome is in the range of 0.04-0.162% (unpublished data, our laboratory results). Although our two cases had rob(13;13) in their karyotypes, the expression of several of the features often seen with trisomy 13 were different in these two cases. Due to the difference in chromosomal makeup seen in non disjunction vs. translocation, there may be differences in expression of several of the features often seen with tri somy 13 [2]. Bugge et al. [13] used 20 polymerase chain reaction (PCR)-based DNA polymorphisms to determine whether trisomy 13 due to de novo rea(13q;13q) in six cases is caused by translocation (13q;13q) or isochromosome (13q;13q), and to determine the parental origin of the rearrangements and the mechanisms of formation. They found that the six probands were three live born children with clinical features characteristic of Patau’s syndrome and three fetuses diagnosed prenatally by amniocentesis for chorionic villus sampling (CVS). Five cases were isochro mosomes with two identical q arms, one of maternal and four of paternal origin. Only one case was a Robertsonian translocation of maternal origin. On the cytogenetic level, it was not possible to distinguish an isochromosome i(13q) from a Robertsonian translocation of two homologous chromosomes 13. DNA studies of trisomy 21 due to de novo rea(21q;21q) have concluded that the majority is due to isochromosomes i(21q) and not translocations between two chromosomes 21 [14-17]. Similar studies of trisomy 13 with de novo rea(13q;13q) have shown similar results, but the number of cases studied has been very limited [18-21]. Our cases of trisomy 13 due to de novo rob(13q;13q) were translocations on the cytogenetic level. We could not determine the parental origin and mechanisms of formation, and compare these findings with previously published cases. Trisomy 13 remains a lethal disorder. The geneticist and/or genetic counselor can provide specifics on inheritance, recurrence risk, and the implications for further genetic testing of family members.