Could you explain exactly why your asking? Are you wanting to concieve or trying to steer clear of a pregnancy??
I agree with the poster above. Also, if you have to ask someone that question, you don't need to have unprotected sex at all. If you're trying to have a baby, you should already know the answer. If you're not trying to have a baby, don't have unprotected sex.
Never unless you are in a committed relatiomship there a lot of std and un planned pregnacies