Just to say that my ds was born after PROM and spontaneous labour at 30+3. I'm currently 37+3 with baby no 2!!
Because of my history with ds I have been closely monitored during this pregnancy from 20 weeks onwards. Basically the consultant told me that there was no obvious reason for my PROM (yours sounds the same) and to be reassured that labour started because of PROM. If labour had started first then waters gone that might have indicated an inability for my body to get to term. But that wasn't the case
So try not to worry too much and talk over with your midwife at booking in what kind of care you can expect. I was flagged as high risk straight away and put under consultant led care til past 36 weeks. Good luck!! xx