there are a few good articles on the BB site:

Labour Signs - Could I Be in Early Labour?

Contractions - What Do Contractions Feel Like?

It's not so much the sex, it's the oxytocin that encourages labour - which is released during happy So a good snog may do the trick just as well. I do know that this morning my DH was a bit frisky and I told him not on his life if he valued his penis.