And I would send the babies back to the pet shop if I were you when they get big enough.

When they started getting fat I was real confused- were they pregnant, or were they just fat? I’ve owned many rodents before, mainly hamsters, but never mice- so I thought ‘maybe unlike hamsters who just eat everything and store it away, they just eat and eat and EAT!’ SO I gave them less food. Low and behold, about a week later when I went to compare a picture of a pregnant mouse with my mice, I opened my loft (which is the same product as yours) and discovered both my mice were now lean, and I had a pile of pink fleshy things. Before the birth I didn’t want them to be pregnant, but now I’m glad- it’s been really fun. I have 15 pups- 16, but one died. 3 from my black and white mama (two turning out grey and white with red eyes instead of black and white with black eyes like their sister since they’re albino-I thought that was pretty cool) and 12 from my tan and white mama.
The point of the cage? Oh, I’m building a custom cage so I can have a cage for the males and a cage for the females- and esp. for the males, have 8 seperate floors for each with the ramp to each acting as a door so I can seperate them if they ever fight. (American male mice are MEAN to each other in most cases, mine should be better off since they’ve known each other their whole lives, but sometimes even those ‘clans’ fall apart and may fight to the death.)

Oh, and just to say it because it bugs me (lol)- mice DO NOT eat their pups if you disturb them- the only reason why they may ever eat their pups is if they don’t have enough food to produce enough milk for their pups. People tend to think the whole eating their young is for all rodents, but that’s not true- that’s what the hamster is known for. I’ve been holding my pups since day 3 twice a day. Now that they’re supposed to be in the ‘flea stage’ when their eyes open (Due to being afraid of even you because they can now see you and you’re big and scary) they aren’t afraid of me because they’re familiar with my scent since I’ve been handling them for that long. I’d say if you ever have any more pups (Though I’m sure you’re probably going to try to avoid that now haha), hold them from an early time- it makes them more domesticated and better pets. Mice from petstores are… Okay, but it takes longer to domesticate them since they haven’t been held and given attention during any time of their lives. Most are just feeder mice(AKA: snake food) , that’s what mine are and that’s why I won’t return them to the store once they’re old enough just because I’m attached to them all now and would have a bad concious knowing what their fate would be. But yeah there are ways to tell between genders early on- I knew before 2 weeks how many boys and how many girls I had. Petstore people are honestly usually very poor at their jobs when it comes to rodents unless they themselves specifically have experience with that creature.