Im guessing you can do it as often as you like...If you can be bothered.....lol!!!! I think it is something in the Semen that can trigger it but not always!! Just don't do the deed if you waters have broken...even if it is a trickle!!
Not sure if it is more effective if you do it close to your due date? I think it would be tho cos you can do it throughout your pregnancy and it's safe!!