Pallister Killian syndrome (PKS) is the most frequent form of partial autosomal 
tetrasomy 12p in humans. Sufferers have a mosaic of isochromosome 12p [i(12p)]. 
We report the first pre-natal diagnosis on fetal blood cells after cordocentesis 
during the second trimester. The extra chromosome was first diagnosed by in situ 
hybridization. Fluorescence in situ hybridization (FISH) was used to count the 
interphase and/or metaphase cells containing the isochromosome. A review of the 
literature identified 27 other reports of PKS diagnosed pre-natally. We showed 
that the most consistent pre-natal ultrasound findings include hypertelorism, 
broad neck, shorts limbs, abnormal hands or feet, diaphragmatic hernia and 
hydramnios. Recognition of this congenital malformation pattern pre-natally may 
allow utilization of FISH.