Pregnant Mouse?

Question
I am a first time mouse owner(though I've had my fair share of rabbits, hamsters, guinea pigs, and reptiles in the past)and I was curious as to the earlier signs of pregnancy.
In early August I bought two feeder mice(luckily both were female and neither proved to be pregnant) and I added an additional female fancy mouse two weeks ago. I initially had a pregnancy scare with one of the feeders because of weight gain that soon evened out, but because of that I can't tell if the same is happening to my new girl. She has kept her normal weight until the past day when her belly became slightly more defined. Tonight though I was able to see her more(she normally scampers in the tubes instead of the top surface) and was able to get a good look at her belly, which is slightly plump closer towards her tail.
Are there any key regions to watch during her weight gain to figure out if its pregnancy or fat?
(I'll add a picture if she gives me the opportunity)

Answer
Dear Becca,

It is quite likely that she is pregnant, and due to my unexpected delay in answering this :( I wonder if you already have babies.

Are you on Facebook? I have a group for oops litters. I set you up with a mouse breeder and you have a thread to ask all of your questions in one place. We also have resources to use.

This link will also answer most of your questions:

http://www.fancymice.info/breedfrontpage.htm

If you are on Facebook friend me (Natasha Athena Millikan) and message me and I will get you into the group. Remind me that you have mice because we have more rats than mice so I might space out.

Best case scenario she's just a little fat.

Squeaks n giggles,

Natasha