Patau syndrome is associated with extra chromosome 13 material, either free as in the 47,+13 or in a Robertsonian translocation or another rearrangement. We report on two fetuses with trisomy 13 who were diagnosed prenatally via cord blood and amniocentesis, respectively. They showed de novo Robertsonian translocation between chromosome 13 and 13, and had normal parents. One was detected cytogeneticaly at 24 weeks of gestation with a karyotype of 46, XX, rob(13;13) and lived only 1 month after birth. Holoprosencephaly, proboscis, microphthalmia and heart septal defects were present. The other fetus was examined at 14 weeks gestation because of cystic hygroma, hydrothorax and hyperechogenic kidneys and had the karyotpe 46, XY, rob( 13; 13). After abortion the fetus was found to have a cleft lip and palate, postaxial polydactyly of the feet, micrognathia, omphalocele, low-set ears with abnormal helix and to be small for the gestational age. Due to the difference in chromosomal makeup seen in non disjunction, there may be differences in expression of several of the features often seen with trisomy 13, either classical type ( as in the 47,+13) or de novo Robertsonian translocation type (as in the 46).