My sis did some nipple stimulation 6 weeks before my niece was due, it brought on early contractions which nurses managed to stop. (she was also in hospital for a couple of days being given steroid injections to get bub's lungs ready). I don't think she intended to do it to bring early labour on, it was more to try and relieve the pain of the full boobs. (she'll probably kill me for telling you all this)