I'm 27 wks. pregnant have a gbs uti colony of 100,000 (again). ob prescribed clindamycin without testing it, and ignored my concern of c-diff. ?

No urgency to treat: Unless you have symptoms of UTI(frequent urination, pain or urgency to urinate), you do not have to rush into taking antibiotics. Clindmycin is rarely used for UTI. You may be better of seeking a second opinion from a primary care doctor(Family Practice or Internist) who can be more careful in making a decision whether you have UTI or a contamination and which antibiotic is safer in pregnancy.