I'm 30 weeks pregnant and have had a rash in the inside of my elbows for over a month. Is it possible that it is a yeast infection?

Possible, yes. I'm assuming by "inside of elbows", you mean the opposite side, where they bend. If so, yeast infections do sometimes occur in those areas - but so do other rashes, such as eczema. Most rashes that occur during pregnancy have nothing to do with the pregnancy; but there are several pregnancy-specific rashes. If you've had it for over a month, it's probably time to visit a dermatologist.
Pregnant with a rash. There are many types of rashes and several are specifically associated with pregnancy. You should see your ob/gyn or dermatologist as soon as possible.