Housing pregnant and non pregnant mice together

Question
Good morning...yesterday i put two non pregnant female white mice in together with two late term pregnant females.The pregnant females were quite aggressive against the new additions,even though this behavior subsided over a few hours.is this to stressful on my about to become new mothers.I don't want to stress them out and risk losing the babies,i have another cage but its currently occupied by my boys.and i wont be able to get another one for a few days.the other two females were unwanted so i took them...thank you

Answer
Hi David,

It is stressful, but if they've already calmed down, they won't likely be negatively affected.  It becomes the most important right before they give birth and for the first few days of the pups' lives.  What I'd be most worried about is the new females and their behavior around the upcoming litters.  Is there sufficient room and hiding places in the cage for all four women to have a place to stay?  The two mothers may nest together or may need to nest separately, and depending on the temperaments of all involved, the other two females may need to sleep outside the nests.  Better to have extra sleeping spots than too few.  There's also the possibility that the new females will help the mommas out - there's just no telling until the babies arrive.

Cage or no cage, you will *have* to separate them if trouble arises once the pups are born, or if you notice any boxing (sitting on the back legs and punching at another mouse), biting, or injuries.  Keep a very close eye on them until you are sure the pups are safe, in case anything turns suddenly bad.  If you obtain a new cage in a few days and decide to separate them to be safe, go ahead and watch for signs of anxiety that may result.  Like I mentioned before, though, don't change occupants one way or another during the first few days of life for the pups - this is when the moms show their true parenting styles and may not handle change well.  During this time you should also not change the cage or its surroundings if you can help it.

In the future, it'd be a good idea to quarantine new mice, especially from pregnant females, for a good 3 weeks once you get them in their own cage away from the others.  Moving mice can stress them a bit and weaken their immune systems, and you don't want to accidentally bring in a sick mouse that didn't initially look sick and infect a mom.  Many medications can't be given to nursing mommas, so it's better to play it safe.

Best of luck, and congratulations on your two litters!
-Tam