[A case of pseudoisodicentric chromosome 18q detected at prenatal diagnosis].

Although trisomy 18 (Edwards' syndrome) or the terminal deletion syndromes of 18p and 18q have been occasionally detected, pseudoisodicentric chromosome 18 is a very rare constitutional chromosomal abnormality. We describe a case of pseudoisodicentric chromosome 18q without mosaicism, which was confirmed from fetal cells in the amniotic fluid used for prenatal diagnosis of multiple congenital anomalies. A 23-yr-old pregnant woman was suspected of having a fetal anomaly at 18(+3) weeks gestation. In sonography, the fetus showed multiple anomalies: bilateral overt ventriculomegaly in the brain, ventricular septal defect and valve anomaly in the heart, bilateral club foot, polydactyly, meningocele, and a single umbilical artery. The pregnancy was terminated and a conventional G-banded chromosome study was performed using amniotic fluid. Twenty metaphase cells among the cultured amniocytes showed a 46,XX,psu idic(18)(q22). Consequently, the fetus had partial trisomy (18pter-->q22) and partial monosomy (18q22-->qter). Both parents were confirmed to have a normal karyotype.