It's mainly the prostaglandins in sperm that trigger things - when they
induce labour artificially, the first step is usually applying
prostaglandin gel to the cervix. Having sex to hasten labour does the
same thing, so long as sperm gets to the cervix. Uterine contractions
caused by orgasm *might* trigger bigger contractions. Just being in the
mood shouldn't cause labour - but you never know. Being in the mood
might actually be a result of being ready for labour... :-)