How do you find source of septic tank (sewer) gas and how do you get rid of it?

We have been having problems with smelling sewer gas in a seldom used bathroom. We just got back home (having been gone for about a week) and the smell is out side AND inside the house. We live in the country and are on a septic system (house built in 1978). Is this dangerous (until we can get this fixed)? My 8 month pregnant daughter is due to come Saturday and stay the week-end. Is this a danger to her or her baby (or other chldren)? I've never had a septic system and am concerned. Thank you.