"Possibly" de novo translocations: prenatal risk counseling.

De novo rearrangements incidentally found at prenatal diagnosis may carry an increased risk of abnormal outcome. Situations in which the paternal chromosomes cannot be analyzed (e.g., artificial insemination, nonpaternity, deceased or otherwise unavailable father) are only "possibly" de novo. In our series of 23,495 prenatal diagnoses we unexpectedly found 67 balanced translocations. Of these, 26 were Robertsonian and 41 were reciprocal. A total of 45 (67%) of these translocations were inherited, and 19 (28%) were apparently de novo. Inherited translocations were maternal in 25 cases (55%) and paternal in 20 cases (45%). Follow-up was available in 87% overall, and revealed abnormal outcomes in 2/15 de novo cases, and 2/38 inherited cases. We suggest that 10% may be a reasonable estimate of risk for de novo translocations found at prenatal diagnosis. Given the observed distributions of inherited cases, a lower risk of abnormal outcome should be counseled in "possibly" de novo cases.