Mice are social animals and may care better for their young when they are housed with friends. You can house females continuously with the sire, or house pregnant females together, or house a pregnant female with a non-pregnant female. You should not add new mice to a cage just a few days before birth, as this will disturb them. But as these were bought out of the same area in a store then these might be considered 'friends'.
First time mothers and very young females are less likely to raise a litter successfully than experienced mothers and more mature females. So if you lose the litter this might be mother nature's way of letting the mom grow up more.
Mothers shouldn't be disturbed the first day after birth. By the second day, mothers should tolerate disruptions better. Sudden loud noises and bad ventilation can also cause the mother to cannibalize the litter.
Lots of good info available here
http://www.afrma.org/rmindex.htm
Hope this helps you!
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