Mice give birth about 18-21 days after conception. Some have been known to give birth in a shorter time, but the babies were very small, and it is recommended that handling be minimised for two to three days after a premature birth. Also it is important that the mother have extra nutrients.
Nursing mothers require large amount of protein, fat and fibre. Ours have been particularly partial to Weet-Bix and milk, with a bit of pure olive oil mixed in on occasion for extra fat and to keep the coat shiny and healty. The food dish should always be full, just refilling daily is more than sufficient. As the babies get bigger, increase the extra protein and fat, as bigger babies drink more, and the mother is using more energy feeding them.
This is a bit of a contraversial topic. Some breeders maintain that the mothers do best if left alone, others suggest that another doe (preferrably an experienced mother) be in the breeding box with them to help out. It helps your nursing mother to have some respite, but either seems to work. I have seen both work. Be a bit choosy about any mouse you leave with the mother during birth. Make sure they can get along, and if there is any fear or doubt whatsoever that the 'matron' might harm the babies, don't do it.
Leaving boys in is particularly risky. Some of them are okay. Mice actually make wonderful fathers, taking care of the babies extraordinarily. The risk is that, all of a sudden, he might turn on them and try to kill them. It depends very much on the mouse. Never, under any circumstances, put a boy in with a nursing mother, or one about to give birth, even if he is the father and they were separated. If you do want to leave the father in, make sure you don't separate him from the preganant doe again for longer than an hour or so, from the first time you put them together, until the babies are completely weaned (in total - this is about 6-8 weeks). If he is separated, he will forget the babies she is carrying are his, and will likely kill them as soon as they are born, and begin to mate again with the doe - just to make sure the babies she has are his.
Mice have extremely short memories.
Length of time varies for weaning. We have found that most of our babies wean themselves at three to three-and-a-half weeks. You can leave them in until six weeks (and pet shops mostly prefer the babies are at least five weeks old). After that time, they start to develop social structures within the cage, and older mice are harder to sell. Try to make sure the babies are big enough to take care of themselves, as you never know where they will end up. Advice: don't let them go before they are four weeks old - minimum.
It's a good idea to separate the boys from the girls (including the mother) no later than five weeks. At this time they can become fertile, and the bubs are simply to young to safely carry a litter to term. It won't be a problem to leave the boys together until they're ready to go to the petshop, as litter brothers rarely argue amongst themselves. The exception to this is if you take one boy out later for breeding, and then try to re-integrate him into the group. Usually he will begin attacking the others to assert dominance as his testosterone levels are higher. You might be lucky, and they might be fine. But you never know. See my page On Keeping Pet Mice for tips on integrating boys together.
The Petstore - Selling your Bubs:
I no longer recommend sale to pet stores anywhere. They cannot be trusted not to sell your loving babies as live snake food, and that is an appalling fate for a true-bred companion animal. It's no different from breeding your dog and selling the puppies to labs for experiments. Same extreme inhuman lack of consideration. A pet store sells mice as pets.. companions. If a person has sold (or given) their pet mice to a pet store on the basis that the pet store will sell them as pets, then to not do that, and sell them LIVE to snake breeders is plainly fraudulent. So I keep them all now. And I never regret it. They are my babies, my life.
I will leave the following information here on the website for those of you who simply have no choice but
to sell your mice, the following might be useful....
[Be very choosy about who you sell your bubs to. If you are not happy with the conditions they will have to live in, then go elsewhere. I have found a great number of stores who leave boys in with girls, put them in cages so small that noone can move freely, don't care about such serious fighting that leaves the walls of the cage smeared with blood, don't care that does who are pregnant will have her babies only to lose them when the other males in the cage kill them - this sort of thing. The number of mice (and rats) we have found injured and sick, and simply rescued from pet stores is frightening. The best way to stop this is to not give them any business, and tell others to do the same. Eventually, hopefully, they might get the message]