What is the cause of single umbelical artery and would you recommend to terminate?

See below. Most cases of single umbilical artery are random occurrence and associated anomalies found in genitourinary tract or in gastrointestinal tract are usually not serious in nature.
Most are normal kids. If concerned you can have the NIPP test done. It takes maternal blood and finds fetal cells to test for the rare chromosome defects that can have SUA. There are other less worrisome associations, but I would not terminate just on the basis of that finding.