Believe it or not, this one's not entirely an old wives' tale. If you're full-term and have a partially softened and effaced cervix, there's a chance that getting frisky will help induce labor. Just don't rely on it as a surefire way to speed up the process. A lot of people think it's the orgasm, or the rhythmic contraction of your uterus, that may help to get things rolling. But in reality, it's probably the prostaglandins in your man's semen that's the labor-causing culprit, not the orgasm. And while experts aren't sure what triggers labor naturally, they suspect it has a lot to do with hormones produced by both mom and baby — which is why attempts to start the process on your own isn't really a reliable method.
If you aren't full-term yet, sex generally won't cause preterm labor. However, if you’ve been diagnosed as having a greater risk of delivering prematurely, you should talk to your doctor; he may recommend certain precautions or limits on sex.