Couples, where one partner carries a balanced chromosome rearrangement or other structural chromosome abnormality, are at the risk of genetically unbalanced conceptions, which may result in failure to implant, early or late miscarriage, or liveborn children with physical and mental disability. Chromosome rearrangements include Robertsonian and reciprocal translocations, peri- and para-centric inversions, inter- and intrachromosomal insertions, deletions, duplications, and complex chromosome rearrangements (CCRs). Balanced reciprocal translocations, which are typically an exchange of two terminal segments from different chromosomes, occur in approximately one in 500 live births, and are usually associated with a normal phenotype. Robertsonian translocations occur with a prevalence of one in 1000 in the general population. Carriers of balanced pericentric inversions are at risk of pregnancies with imbalance for the noninverted segments. Imbalance for the noninverted segments may be ascertained by using subtelomeric probes for the relevant segments.