Sometimes a pet mouse will have unknowingly been bred at the pet store and you may be surprised to find a litter of babies in your mouse's cage shortly after bringing her home. Also, if you take home two mice who were incorrectly sexed, you may be surprised by a litter. Not to worry, though, as the new mom mouse will likely cope fine, with a little help from you!
What to Do When Your Mouse Has Babies
A mouse that has given birth should be disturbed as little as possible or she may become agitated and abandon or neglect the babies. This means other than providing food and water, you should leave the mice and their cage alone as much as possible for the first couple of weeks. While it is tempting to look at the babies and make sure everything is okay, it is best to leave the mom to do her thing.
Place strips of toilet paper or facial tissue in the cage so mom can build a soft nest. Ideally, you would do this once you realize your mouse is pregnant (it is usually pretty obvious by a couple of days before birth, simply by the sheer size of her belly).
If you realize your mouse is pregnant, do a thorough cleaning to prepare for the new arrivals. However, once the babies are born, don't clean the cage for a while. Leave it alone for the first 10 to 14 days (you can spot clean really wet spots if absolutely necessary, but avoid disturbing the nest).
Make sure to provide plenty of food and freshwater, but make feeding and watering as efficient and calm as possible.
While it is okay to have a quick peek in the cage on occasion, avoid sitting and watching for long periods.
Keep in mind that the mom will be very protective and may act more aggressive than usual; this is natural and no cause for concern.
Though interference should be avoided if at all possible if for some rare reason you must move a baby, use a spoon so you do not get your scent on the baby. It is rarely necessary to move a baby though even if they are out of the nest; the mom will usually retrieve any stray pups and return them to the nest.
Caring for the Litter
If you have other female mice in the cage, it may be fine to leave them in the cage, as they will often help care for a litter. However, sometimes another female will try to take over a litter or cause other interference, so make sure other females are helping and not hindering the new mom. If you have a male mouse, though, now is the time to move him out, both to prevent both another pregnancy (which can happen shortly after birth) and to make sure he doesn't interfere with mom raising the litter (which is uncommon but possible).
The baby mice will be ready for weaning at about 3 weeks of age, and it is best to separate the male pups into one group and the females into another at this point.
Make sure you are feeding a high-quality diet with lots of protein. In addition to a good-quality diet, you can offer small amounts of hard-boiled egg, bits of cooked chicken, cheese, wheat germ or dog biscuits to give the mom a nutritional boost.
Unfortunately, if you got a pregnant mouse from the pet store she may be quite young and may not have the best mothering abilities. She may be more likely to abandon or even cannibalize her babies. If she doesn't care for them and they are less than 10 days of age, it is extremely difficult to save the babies. Don't try to handle the babies at all for at least two weeks. It is best to make sure you get your mouse from a store that separates males from females at a young age to avoid surprises like this.