Well, I sure hope it is not HV!
When my bitch, that had had 2 litters, 6&7 pups was bred last year, we did not get much of a breeding on her, and did not back it up w/AI. She had a litter of 4, one got stuck on delivery, lost the placenta on the way out, and drowned, it was a long delivery to get the dead pup out. Her sister, same stud, 1st litter, had 2 pups get stuck and die w/ long deliveries. The stud was a large dog, and the owner, who bred another bitch of mine to him, said that I have a narrower pelvis in my line, she was able to get all her pups out, but had more insemination and more pups inside. The smaller litters grow too large inside before birth. Pelvis size is a factor. In the case of the sister's breeding the 2 stuck pups were bred days before the others, the 4 that made it out. We bred on Thurs night, late, and Sunday morning, with an unsuccessful tie on the days in between. Some people will argue that, but the dead pups were much more developed than the 4 that survived in that litter, and too large to safely exit the womb.
I suggest that AI's be used, not too many days apart. Hormonal assays are pretty cheap and can pinpoint the ovulation date well.
The sister[Kitty's mom] was retired to a pet home. Bear went on to have a lovely litter of 5, with no difficulties during delivery, a couple weeks ago.