Pregnant mouse from pet store

Question
I got two female mice from Petsmart, and I'm almost positive one of them is pregnant. How would this happen? Should I separate the mice? Can I tell how far along they are? Thank you so much because I've had mice before, but this has NEVER happened and people say I'm crazy. What I know about the relationship between the two mice is that I got them at different times, but from the same pet store a day later, the first night; they fought so I put the one that I had before in another cage for the night so I could return her the next day and in the morning, they loved each other and now they are inseperable... is this why? Are they mates or something?

Answer
Dear Sydney,

Pet stores are equal opportunity employers. They will hire you even if you know nothing about animals. All you have to do is pretend. Baby mice need to be separated at age 4 1/2 weeks, but pet stores are often approximate about this, and they probably designate them as female and male due to the length of their eyelashes. Your problem is extremely common.

You can't tell how far along she is, though once she starts to look like she swallowed a pear she is about ready! Still, since a litter can be anywhere from 1 to 20 babies, you can imagine that different mice in different pregnancies have a great difference in size. Sometimes you don't even know a mouse was pregnant. Give her the benefit of the doubt, and treat her as though she might give birth tomorrow.

Give the two of them lots of soft nesting material like kleenex, and a nice dark place for the nest. Give them some scrambled egg for protein. She is lucky that she has a sweet friend to help her with the babies. They might well have been buddies before.

Have fun with the babies. Don't hold them until they are a week old. If you have questions, research my archives (always start with "natasha mice mouse" or write back.

squeaks n giggles,

Natasha