A Rare Finding of Mosaic 45,XX,der(13;21)(q10;q10)[15]/46,XX,r(13)(p11.2q33) Following Abnormal Prenatal Chromosomal Microarray Testing

Mosaicism for both a ring chromosome and Robertsonian translocation is a rarely reported cytogenetic phenomenon. We describe a case referred for testing following abnormal noninvasive prenatal screening for trisomy 13. Prenatal chromosomal microarray (CMA) testing identified a mosaic 3.6-Mb terminal loss in 13q34, which was confirmed by postnatal CMA. At birth, the phenotype included a small head circumference (3rd percentile) and a patent ductus arteriosus. Conventional chromosome analysis identified mosaicism of two abnormal cell lines, one containing der(13;21)(q10;q10) and the other containing r(13)(p11.2q33). This case demonstrates the utility of conventional chromosomal analysis following an abnormal CMA result.