GIJane- my son was born at 29w after my water broke 2 weeks earlier. A cause was never found. But before I starts TTC, I had an appt with my OB/gyn to discuss what could be done to help if I got pregnant again. She also said that the only unusual thing that they could find was that I had a mature placenta in my first pregnancy (more calcified/aged Han it should have been that early on but it wasn't a contributor to preemie)
For this pregnancy, I'm getting weekly progesterone shots starting at 16w and ultrasoubds every 2-3 weeks to check on cervix length. If my cervix starts shortening early again, options are to add a cerclage or pessiary to stop/slow it down. My doc said with these measures, my chance of having a preemie is reduced to the same level as the general population of women who have never had a preemie. I'm hopful...!