Mice reach sexual maturity at about 6 to 8 weeks of age. From this age onward, females and males should be housed separately.
If you are planning to breed your mice, you should provide nesting material (tissue paper works well) in one corner of the cage. Females can have up to 15 litters a year and can become pregnant within 24 hours after giving birth. The average gestation time for mice is 19 to 21 days. Baby mice are called pups and are born deaf and blind. The average litter size is 10 to 12 pups. The cage should be kept in a quiet place and the litter should not be disturbed for at least 7 days after birth, especially if this is the female’s first litter. Within 2 weeks, the baby mice will look like small adults.
Breeding and reproduction in mice can decrease due to various factors such as age, malnutrition, abnormal light cycles, cold environments, cysts on the ovaries, tumors, and inadequate nesting material. Pregnant females may abort, abandon, or eat their babies due to inadequate food, lack of water, overcrowded group housing, inadequate nesting materials, sick or deformed pups, or excessive noise.