Unplanned Mouse Litter

Question
My mouse had recently had a litter that I knew was coming but I hadn't removed the father from the cage because I had thought I had a week or so left until she gave birth. She gave birth a few weeks ago, but I didn't find out until a day or so after she did. She is pregnant again while nursing. Do I need to remove the older litter when she does give birth? The current litter is almost 3 weeks old.

Answer
Hi Jennifer,

That's unfortunate, but it does happen.  There are two risks to keeping both litters in at the same time - first, the older babies will be a lot bigger than the newborns, and run the risk of trampling them while trying to nurse alongside them.  Second, the older ones will drink a lot more of mom's milk, leaving less for the newborns, which may cause them to develop much more slowly and with less of an advantage.

I would not remove them before 21 days old, but after that, you just need to be sure they know how to drink from the water bottle.  Usually, once they see one sibling do it, they all catch on pretty quickly.  They usually discover solid food first, so once they've got the bottle down they can be moved.  If you have to separate them sooner, you can supplement anyone you don't see using the bottle (or anyone you fear is becoming dehydrated - dull eyes, low energy, loose skin, messy fur) with kitten milk replacement twice a day until you know they can keep themselves hydrated.  You can get milk replacement from your local pet or feed store, and the powder is definitely a great thing to have on hand for emergencies anyways.

Best of luck, and congratulations on your two litters (even if one was on accident)!
-Tam