Was your baby preterm?
None of mine were. They were all within the accepted range 37-41 weeks for gestation. My mil had four boys, only two of whom lived, prematurely back in the 60's. I believe she had an incomplete cervix, which may be what they call a short cervix now?
This article talks about how they are trying to figure out what causes preterm labor and the fact that preterm labor is decreasing, thankfully.