Small pregnant mouse?

Question
Ok so i bought a male and a female, and i bred them. i have supply's & i know most behaviors & have done LOTS of research. But i am skeptic on a few things. It is the day-maybe 2 or 3 days till she is due (i have been keeping track) She is small with maybe a little stomach. i have read that mice can sometimes hardly look different, but every mouse video or pic i see on like looks like they swallowed a pear. the night i figured she got pregnant her and the male went at it for about 5 hours maybe 6. Today i changed her bedding and washed her cage, she went erratic to make a nest, and she pretty much sits in there awake cleaning her self like mad, so im wondering what pre pregnancy behaviors like a day to 5 hours before she would give birth are & did my mouse actually get pregnant?

PS: male mouse is not in there with her, i know about the possibility's and harms it could maybe do.

Answer
Hi Tasha,

You don't say how long they were together, but a female mouse (a doe) goes into estrus once every five or so days.  This means that even if she "went at it" for five or six hours, she may not actually have been ovulating.  Males (bucks) can be very forceful and it's not always a matter of when she is most receptive to mating.  For this reason, it's best to mate the two for about a week (two if you want to be absolutely certain) so that she will have at least one day of ovulation and likely more.  You can check for a plug, a sign of successful mating (the buck ejaculated) but not of pregnancy, using the method here:  http://jaxmice.jax.org/jaxnotes/archive/501d.html  (skip down to "Checking for Vaginal Plugs").

Luckily for you, nesting is a very good sign of a doe about to give birth.  In addition, the 21 day waiting period can vary by a couple of days in either direction.  Beyond day 23 or 24 after separation I would start to think she wasn't really pregnant.  Before that, nesting is the best way to tell.  Make sure there is plenty of food and water and that her cage is away from drafts and electronics.  Hopefully, you'll have pinkies on the way soon.

Best of luck!
-Tam