I recommend using a plastic tub or aquarium to raise babies in – or if you must use a cage with bars, make sure the bar spacing is 5mm or less. Babies, once they’re running around, are excellent escape artists! Also, try and make sure the cage is in a quiet area of the house where she won’t be disturbed too much.
You should separate a pregnant female mouse to raise the litter alone – living in a communal cage is stressful for mothers and companions may attack and kill the babies. I would especially do this if you have a group of 3 or more mice living together, as the situation could prove unpredictable, stressful and dangerous for mum and bubs. If you only have two mice, and they live together happily, separating the mum would leave two lonely girls and may not be necessary. The buddy will often act as a nanny, sharing the duties. If you choose to go this route, keep a close eye on the nanny when mum gives birth and make sure she’s not showing any aggression towards the babies.
You will want to provide mum with plenty of nesting material and a nice safe, dark place to build her nest. I use non-scented, unbleached toilet paper for nesting, or you can also buy a sort of cotton wool designed for small animals from pet shops. Avoid using fabric (unless it’s felt), or anything with strings or threads that could twist around little baby feet or heads.
Pregnant and nursing animals use a great deal of nutrients and energy creating, carrying and caring for little lives. They need a good quality, high protein diet, and lots of it. I tend to double the amount of food I give my pregnant girls, and add things like small bits of cooked chicken or tuna, scrambled egg, bread soaked in soy milk, porridge, baby food, etc. You could also add small animal vitamin drops to the water if you’d like to go the full hog, but I rarely do this as I feed a pretty good diet.