Not had one die (knock on wood and hope I'm not tempting fate) but close. Csection (and spay) for primary inertia, went fine other than one pup not alive. When pups were 17 days old, I went to a judge's seminar for the day with my husband in charge of dogs, came home that night to her flat out, 105 degree temp, raging mastitis. He thought she was just sleeping all day. We won't get into that. Anyway, I doubt she'd have made it through the night without treatment, took 4 days of IVs and she survived but it was touch and go at first. She couldn't go back with her puppies, fortunately they were old enough to start lapping formula. As far as prevention? I guess we all do what we can, keep a close eye, check temps regularly, make sure everyone is acting and eating normally. My bitch had been on antibiotics for 2 weeks following the Csection, 3 days after she was done with them, raging infection, about the time you think it's okay to go for a day and leave the second string in charge. During whelp (for the real drama queens, that can be 48 hours before a pup ever makes an appearance), I'm with them 24/7, post-whelp, I sleep next to the box for a couple of nights and they all stay in the house anyway. But, things can still go wrong, ruptured uterus, etc.