
Sad, but fairly natural occurrence as nature has designed for redundancies.
> so possibly a later conception?
Just wanted to point out that no matter when a bitch is mated the conception cannot vary as the ova all are released and ripen and are viable for around 48 hours max.
Once the cells divide and finally reach implantation stage they all implant (or not) and develop together.
What affects the developing embryo is how well the placenta develops in a good spot in the uterine wall, and whether the forming foetus is genetically/physically viable.
Birth then of course is a dangerous and traumatic event, oxygen supply may be cut off with early placental detachment or hold up, or even infection from the dead mummified pup.