My doctor prescribed me sulfameth/trimethoprim 800/160 for a uti. im 15 weeks pregnant. my ob said it would be fine, but i heard it wasn't, is it ok?

I think it's fine: A urinary tract infection is a life-threatening event, though thanks to antibiotics we take recovery for granted. Like many (most?) drugs, somebody noticed a few birth defects in animal studies, perhaps at preposterous doses, but the category c means "take it if you need it" (and you did.) and be realistic -- a common, generally safe isn't going to cause a major birth defect at 15 weeks.

Agree with your OB: Your doctor prescribed a fairly standard course of treatment for your uti.