With my first my water broke at 32 weeks without labor. I was actually part of the trial for the magnesium. It has an odd feeling, you get suddenly very hot and get a metallic taste in your mouth, but its temporary. I believe the studies found that the magnesium does delay labor. I stayed in the hospital for 2 weeks on bed rest until I was induced at 34 weeks when they determined the risk of infection to the baby was worse than the risk of him being born early.
I wasn't pregnant with twins though.