A pregnant mouse has a noticeable bulge in the center of its body that causes a pear-shaped appearance. The number of babies the mouse is carrying determines when the growing abdomen is noticeable. By the 14th day of pregnancy, pronounced nipples develop on the mouse's underbelly.
The abdomen of an older pregnant mouse that has previously given birth may not be as apparent. This is especially true if the mouse is carrying a smaller litter. Gestation takes 19 to 21 days, and the mouse typically begins building a nest to prepare for the birth five to 14 days before the babies arrive.