Mouse question

Question
I have a mouse that's recently given birth and is now nursing her
pups. About four days ago I noticed that the mother mouse had a lump the size of a small marble around her hindquarters. It's not affecting her ability to defecate, and she's eating and drinking as normally as a nursing mouse would, I think, but the lump has started to darken from pink to dull purple-red, though it's not grown all that much. Behaviour-wise, she seems to be running about a lot more recently, and scrabbling about in her food bowl.

I would take her to a vet, but I only got her three weeks ago (I
didn't know she was pregnant at the time), and I've been trying to
socialise her and her cagemate (another female) regularly since then. She will take mealworms and other treats from my palm sometimes, but I'm not sure if this is because she really is comfortable with me of if she just needs the extra food. With the babies only a week old, I'm concerned that a vet visit coupled with whatever procedures needed to treat the lump would stress her enough to harm her litter.

Should I wait until the pups are a little older, or until they're weaned, or just go to the vet and hope for the best?

Answer
Hi Grace,

I would take her into the vet once the pups have a full coat of hair in - this is before they start jumping around and might escape on the trip, but after mom is likely to flip out and attack them if she thinks she's in danger.  Pups with fur are also old enough that hand-rearing them wouldn't be as difficult if she decides to ignore them.  Somewhere between 7 and 10 or so days old would be ideal, so if they are a week old, it'd be alright to take them in now.

The only reason I'd recommend taking her in sooner (when you bring them in, just bring in the entire cage undisturbed) would be if something went very wrong - if you notice she is suddenly in serious pain, if she stops behaving normally or quits eating/drinking, if the lump bursts or the skin is broken, or anything else that would be an emergency.  At that point she would not be able to care for her pups properly, so the only thing to do would be to take her in immediately.  Hopefully nothing like that will happen, but I'd keep a close eye on her just to be safe.

Best of luck, and let me know if you have any problems with her or hte pups!
-Tam