Pregnant mice behavour and determination of sex

Question
hello,
I just recently bought 3 mice from a local store on two different occasions and i believe that one is pregnant and the other is male.I bought a small one and a larger one on the first occasion, the smaller being male (i believe, its hard to tell) and the larger one being possibly pregnant, the second time i bought a female from a cage that had two VERY pregnant mice inside as well but she doesn't seem to be showing signs of being pregnant.  the store employee had said that determination of sex was not guaranteed but would take back the mice without compensation if they were pregnant.
I am just a bit curious as to how a pregnant mouse will act or behave? the one i think is pregnant is super jumpy and defensive, and has been hardcore nest building. she is a bit fat but i have been spoiling them with whole wheat bread. the supposed male is very small and super friendly, unfortunately due to his size its hard to see if he has testicles, or if he is still too young to have visible testes?
thank you for your time.
sean

Answer
Dear Sean,

First off, here is a great page for sexing mice, even when they are too young for male testes to be obvious:

http://www.thefunmouse.com/info/sexing.cfm

Pregnant mice may act a little jumpier but because you didn't know her before, there's no way to know if she is just a jumpy mouse.  However, nest-building is a good sign.  Also, the likelihood of a mouse not being pregnant after being in a cage with males is pretty small.  

I wouldn't go to that pet store again. It is very irresponsible to leave males and females in the same cage, and for your workers not to even know which are males.  This might make you wonder what else they do irresponsibly, like not taking sick mice out of the cage before everyone is affected, for instance, or taking/breeding mice with known defects.

Mice are pregnant for three weeks.  During the third week they can get very fat:  They look like they have swallowed a pear.  However, the difference between a mouse carrying up to 18 babies or as few as one baby is of course great.  The usual liter is 8-12 and the mouse is visibly pregnant at least the last 5 days.  This discrepancy might make it difficult for you to get the pregnant mouse back to the store before she gives birth, and you can't do it very well after birth without putting the babies in great danger.  

It's not a good idea to spoil mice with a lot of treats.  A little bread a couple times a week is OK, or other occasional treats, but they should mostly be eating their mouse mix or pellets, and raw vegetables.  Fat mice have the same problems that fat people do-- heart problems, diabetes, etc.

However it works out, whichever mice you end up with, you're going to really enjoy your new pets.  People just don't know what wonderful pets mice make!

squeaks n giggles,

Natasha