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Persistent right umbilical vein (PRUV) is a common anomaly of the venous system. Although candidates for future PRUV were expected to occur more frequently in earlier specimens, evaluation of serial horizontal sections from 58 embryos and fetuses of gestational age 5-7 weeks found that only two of these embryos and fetuses were candidates for anomalies. In a specimen, a degenerating right umbilical vein (UV) joined the thick left UV in a narrow peritoneal space between the liver and abdominal cavity, and in the other specimen, a degenerating left UV joined a thick right UV in the abdominal wall near the liver. In these two specimens, the UV drained into the normal, umbilical portion of the left liver. These results strongly suggested that, other than the usual PRUV draining into the right liver, another type of PRUV was likely to consist of the right UV draining into the left liver. [ABSTRACT FROM AUTHOR]