I recently (11/18/12) bought 3 mice from a local independant pet shop that has a good reputation. I told them I needed girls as they were to be housed together & I didn't want babies or males who might end up fighting. One is showing definate signs of pregnancy - the 'I swallowed a golf ball'lool & is busy nesting. Another is also looking like she may also be pregnant. Either they were pregnant when i bought them, or the 3rd mouse, despite assurances, is actually a buck. I have absolutely no idea what i am supposed to do......i have had mice on & off as pets for years, but never a pregnant doe. Any advice would be much appreciated - although I am shocked I desperately want the best possible outcome for both mothers & babies, even though, through choice I would never have willingly been placed in my current situation. Many thanks, Sadie.
Oh no! As much as it shouldn't, this sort of thing definitely happens. But one other thing to consider is that a lot of mice sold at pet shops are sold pretty young, and still have growing to do. While the golf ball bit sounds pretty suspicious to me, maybe the other is just growing up!
First, it's important to identify the gender of the third mouse, because mommas can become pregnant again with another litter the very same night they give birth! It isn't as tough as it might seem at first, and I think it's helpful to go to the pet store and see a wide range of examples. Female mice have sort of a figure eight shape at their genitalia - little to no hair between the genitals and the anus, which are pretty close together. They also have two rows of nipples, but these can be hard to find in adults if they have not nursed before. Male mice have no nipples, and a larger, furred space between their genitalia and the anus - this is the area that houses the testes. The testes can "disappear" into the body if the tail is lifted, but they can't stay in there for long, and the best way to spot if your mouse has male bits is to look closely when Mr. or Mrs. mouse sits on something like a ridge of a food bowl, or the edge of something. You should see a pretty evident sack protrude between the anus and the penis if it is a boy. This is difficult to miss in fully grown adults, but as I mentioned, many pet shops sell mice as juveniles. Here is a link with pictures to make it a bit easier to see what I am talking about: http://www.thefunmouse.com/info/sexing.cfm#adult
If it's a girl - excellent! At least you won't have to worry about future surprise litters. If it's a boy - he will need to be separated as soon as physically possible. Females go into estrus once every five days on average, so if they aren't pregnant now, they may be soon!
The good news is that if they are indeed pregnant, there isn't actually much you need to do except keep their lives as stress-free as possible. Try to reduce handling them, scary sounds, or anything like that, and keep plenty of food and water available. They will do the hard part! Once pups are born, for the first several days it would be a good idea to leave them alone. Handling them would be great if the mommas already knew you, but since they are brand new pets, this could cause a lot of stress, so it is best to let them be. After the first week of the pup's lives you can safely (if the moms don't mind) clean the parts of the cage that aren't the nest. You don't have to take everything out - just replace the worst of the litter. Mom keeps the nest area clean on her own. After 1-2 weeks the pups will start wandering out of the nest and exploring the world on their own - at this point watch out that you don't toss any pups with the bedding! Also be careful that they cannot escape; newly adventurous pups can fit through tinier gaps than adults.
At about 4 weeks old the pups should be pretty independent and you'll probably notice some major crowding. As soon as you see them reliably and frequently using the water bottle and eating food (the bottle is usually the last thing they figure out in my experience), they're ready to be separated from mom and based on gender. It would be a good idea to speak with the pet shop as soon as possible so that you can establish if they will take the mice back from you after they are weaned and ready, otherwise you may be stuck looking for homes. It's at this point that your new gender-determining skills will come in VERY handy, as pups can breed as early as 5 weeks, including with mom, so be careful and don't be afraid to ask for a pro's help (like a vet or another pet shop) in double-checking your work. It's always better safe than sorry!
This is just a quick overview, of course, so if you have any specific questions or if I left something out, just let me know!
Best of luck!