Pregnant mice

Question
Hi I believe i have two pregnant mice. I had the male in with them for about two weeks before removing him and it has now been 38 days since I removed him and there is still no babies! But my females both look and act pregnant. They have been nesting for over a week and the one will always try to bite if I get to close. There bellies are pear shaped and ones is bulged more then the other. I think they are pregnant, but I just don't understand why they haven't given birth yet. Any suggestions?

Answer
Hi Amanda,

If you were breeding them intentionally, it sounds like it is time to try again!  As you probably know, the gestation period for mice is 21 days, give or take a couple.  Even if they were late, if it has been 38 days since his removal, it sounds like the mating was unsuccessful.

There are some normal mousey behaviors that can appear like pregnancy ones, such as nesting.  Mice naturally love to dig tunnels and arrange a nest or house in their home, so even though it may seem like they are preparing for new arrivals, they might just be getting cozy in their newly male-free home.  Young mice that have just come home with you might also still have some weight to gain - a 2 month old mouse is certainly an adult, but will continue to grow and round out until 3 to 4 months, when the best breeding age begins.  Lastly, this skittish behavior could also be a product of them getting used to you.  If you have had them a long while, they might just be picking up on your excitement and getting nervous, but being with a male can be stressful to females even if for a short period.

For now I would work on getting them used to you so they don't get too anxious when they do successfully become pregnant.  You were spot on with two weeks of being with the male, so I would just try it again, as it can take a few tries.  If you still have no babies come along you might consider a new male, but I think he probably just needs another try.

Best of luck!
-Tam