Large numbers of patients with deletions of the long arm of chromosome 13 have been described. However, only a few instances have been reported of monosomy 13/r(13) mosaicism. A 31-year-old Japanese woman underwent an ultrasound tomographic screening, which detected a fetus with a nuchal translucency (NT) of >5.8mm, indicating an increased risk of fetal chromosomal abnormality. An amniocentesis (AC) was performed, and the karyotype was 46,XX,r(13)(p11q33)[18] / 45XX[12]. Ultrasound showed echogenic skin edema. Phenotype of the fetus after delivery revealed some anomalies, including hyponasal bridge, hypertelorism, ambiguous genitalia with huge clitoris, low-set ear, neck edema and webbing.Deletion of the long arm of chromosome 13 is associated with a wide spectrum of abnormalities, including retinoblastoma, mental and growth retardation, brain malformations, heart defects, distal limb deformities, and digestive, urogenital, and other abnormalities. The present case, however, had anomalies which were too faint to be detected by ultrasound tomography. Prenatal diagnosis of deletion 13q syndrome is rare. A number of reports have documented an association between increased NT and chromosomal defects. Ultrasound did not identify any major anomaly in this case, however amniocentesis was able to detect this rare abnormality.