What do doctor's check your Urine for ?

When you go for prenatal check ups, what are they checking your urine for? I know they check to make sure you're still pregnant(HCG) but are they checking for UTI's and STDs as well? For the past few days my private part has been irritated, like not badly but every so often I find myself reaching down to scratch or rushing to go take a hot bath to ease it. They'd tell me if I have an STD or UTI right? My partner had an STD way back in September but that was resolved. I'm 34 weeks pregnant and I don't want this affecting my son. Advice please??

-FTM