As Dave said, the odds are stacked heavily against really premature babies.
Causes are unknown, but can include a sick or stressed mother (I had one drop a litter early last year as she fought to save her own life [warning graphic pictures]
http://www.ssnakess.com/forums/epicr...dark-side.html and
http://www.ssnakess.com/forums/epicr...dark-side.html).
If it does happen, care opinions vary, but the best advice seems to be keep them warm and moist, with little to no handling and see what happens. After a few days many tie off the remaining yoke.