Relax - here's a "First Aid" kit of information:
The gestation period of mice is roughly 3 weeks. Unless she is having a very small litter, she will get pretty round around her belly during the late pregnancy. When her stomach starts to get bulky, be extra careful when handling her. Too rough handling, even by mistake, can hurt or kill the babies inside of her.
If your pregnant mouse is living with other mice at the moment, give her a tank of her own. If you have a spare cage with very small space between the bars (a couple of millimeters, in inches... 0,6"), it could be suitable. Babies can get through very small openings and you don't want to risk them falling off the cage. Levels in a cage are better taken off. This is a safety issue, everything can go perfectly well, but it's better no to take risks. The mother may drag her babies up on a shelf, or the babies can be dragged there while nursing keenly. It could happen that the babies get stuck between the shelf bar, or get chilled.
In this tank, give the female a nesting box and lots of shredded tissue paper or high quality dry hay for nesting material. High quality hay is green, smells good and has no mold is dusty. The female's tank may be a little boring for her, but several boxes, tubes and such can make the female change her nest around a bit too often, especially when she is having her first litter :-) She can be nervous herself, you know. When the babies start to move around by themselves, then it's time to put in all kinds of fun toys. Of course, you have to have a water bottle and food bowl for her.
If you have any idea when your mouse is going to give birth, clean up the tank a couple of days earlier. If she has already built a nest, do not disturb it, just clean up around it.
For diet, give her much richer diet than you normally feed her. Along with the normal basic diet, whether it is seed mix or lab blocks, give her daily a meal of high nutritional value. Good bases for this meal are porridge, cooked rice or potato mash. For protein, add dry dog food (kibble), cat food is also suitable. You can get a small packet of kitten diet - some of them have very small kibbles, which are easy for a mouse to eat. For calcium, mix in with the porridge or similar live culture natural yoghourt. This is good for her digestion as well, much better than plain milk. I don't know what kinds of milk products you have available, but the best option of all would be something my dictionary calls "processed sour milk". It looks like yogurt, but is a bit less liquid. Sour milk products are in general better for the belly than plain milk. For fresh food, mix in small bits and pieces of some fruit or vegetable. Peas are also a favorite treat for mice.
Give this meal once a day. It can be in the late evening or early morning. Evening is better as mice are usually more active at night time. You know your mouse's waking cycle better, so adjust the feeding to suit it. Always remove what has been left over of the meal the next day (if you feed in the evening) or the same evening (if you feed in the morning). Otherwise it will go bad.
Continue giving this meal through her pregnancy and the nursing period. Young, weaned mice need a richer diet than adults as well.
Mice usually have rather large litters, but the size of the litter can vary from just one to over twenty. 22 is the largest litter I've heard about around here. Usually, there are something around 10 babies.
Once the babies are born, do not disturb the nest for the first three days. This is because you don't want to risk the doe rejecting or even eating her young.
When it's time to check the babies for the first time, take the mother out of the tank. She doesn't have to see you checking the babies and get upset about it. Then, rub your fingers with the beddings of the tank to mask your own scent. Gently take a peek in the nest to see that the babies are all right. You can pick them up, but be extremely gentle about it. After that, put the nest back in order and let the doe back in with her babies. Check the babies daily, as human contact even before they have their eyes open, will help making them tame.
The babies grow pretty fast. They are born pink, hairless, eyes and ears closed. The pigment starts to show from day 3 forward. Darker colored babies will have darker skin, lighter colored pink. Markings will also start to show. You can see the color of the babies eyes at a very young age as well; dark eyes can be seen trough the skin as dark little spots, pink eyed babies don't have these. By 10 days, the babies will have their first coat on. Eyes will be open at two weeks.
Starting pretty soon after the eyes open, the babies will get into "flea age". This may last up to 3,5 weeks. During this period, the babies will be very jumpy and rather difficult to handle. The babies will jump around blindly and can even bite. It is better to handle the babies with your hands inside the tank. This way you can prevent them literally flying off your hands, falling to the ground and hurting - even killing - themselves. However difficult the flea-aged young are, do handle them daily. Note, that the flea age is different from litter to litter.
The mother nurses her young for approximately 3 - 3,5 weeks, then she weans them herself. Mice get sexually mature, depending on the strain and size of the mice, at about 3-7 weeks old. It is better to separate the boys from mother and other girls at 3,5 weeks old. The girls can be left with the mother.
Male mice usually don't get along with each other and start fighting eventually. There are always exceptions to the rule, but most males don't tolerate other males in their territory. It depends on the mice, but usually mine can be housed together until an interesting crucial age of 11 weeks. Those who don't start fighting then, can be housed together for longer without problems and usually are until one in the group gets separated to mate with a female. In my experience returning in the group after that doesn't work out well.
If fight occur in the male group, always react as fast as possible to avoid injuries. Remove the bully, not the one who is attacked, and give him a tank / cage of his own to rule.
Text by: Satu Karhumaa. First published as an answer to "Pregnant mouse - please advice" message in alt.pets.mice as two part message, dated 6th of November, 1998.