Prenatal diagnosis and outcome of mosaicism for a de novo unbalanced translocation identified in amniocytes.

Mosaicism for an unbalanced reciprocal translocation was identified in cultured amniocytes of a 16-week-old fetus; mos46,XX,der(4)t(4;5)(q34;q12)/46,XX. Parental karyotypes were normal, indicating a de novo origin of the unbalanced translocation in the fetus. The additional chromosomal material on the der(4) was derived from chromosome 5 as demonstrated by both GTG banding and fluorescence in situ hybridization with a chromosome 5 paint. Two subsequent amniocenteses, at 18 and 20 weeks, confirmed the presence of the abnormal cell line. A percutaneous umbilical blood sample (PUBS) contained only normal cells, 46,XX, and a high resolution ultrasound revealed no fetal abnormalities or growth retardation. The pregnancy was continued and a normal female was born at term. No evidence of the unbalanced translocation cell line was found in cord blood or placental samples at birth. The finding of mosaicism for an unbalanced translocation at amniocentesis is rare, and is associated with a high risk of fetal abnormality. This case illustrates the importance of follow-up studies by PUBS and high-resolution ultrasound for further assessing the risk of phenotypic abnormality.