Mice are fertile animals because female mice come into heat every four to five days. Their reproductive rate depends upon how often males and females actually mate. An average female mouse has 30 to 35 babies per year.
A mouse's pregnancy lasts between 19 and 21 days. However, if the mother is still nursing another litter, the gestation period takes up to one week longer. An average birth yields 7 to 12 babies, or pups. However, it is not abnormal to have litters with up to 20 pups. A female mouse is capable of breeding within about one day of giving birth. Although the babies are born blind, deaf and hairless, this starts to change after three days. Young mice are walking and eating solids within two weeks. When they are 28 to 35 days old, mice are fully mature and able to reproduce.