When amniocentesis reveals a mosaic karyotype and the baby presents with multiple malformations, an analysis of the baby's peripheral blood typically reveals a mosaic karyotype. We present a boy who was prenatally diagnosed by amniocentesis as having trisomy 9 mosaicisim but who had normal G-banding results on postnatal blood karyotyping; the patient also exhibited multiple malformations, including a diaphragmatic hernia, arthrogryposis, undescended testes, and characteristic facies. Because of the discrepancy between the phenotype and karyotype, we repeated the chromosomal studies on multiple occasions. Interphase FISH performed on abdominal wall muscle tissue revealed a mosaic trisomy 9 karyotype: 47,XY, + 9(159)/46,XY (19). Based on these findings, we finally diagnosed the patient as having trisomy 9 mosaicism and counseled the parents that the risk of recurrence was low. We conclude that it is important to be aware of the possibility that the patient can have a normal postnatal blood karyotype and an abnormal phenotype with multiple malformations when trisomy 9 mosaicism is detected prenatally. When the baby's phenotype is abnormal, karyotyping on multiple tissues is useful for confirming clinical impression as well as determining the prognosis and providing accurate genetic counseling.
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