I was on pre term labour watch last year and spent 12 weeks on bed rest in hospital. While the drs never knew the reason for why I was at risk (body showed early labour signs) I felt it was likely stress related. Spending 3 months in bed with all my meals provided and my basic needs met enabled me to relax and get to term. Old school treatment but it proved to work. I'd say stress correlates directly to pre term births.