Mouse litters can range in size from one to 32 depending on the size and age of the doe (female mouse). A doe is more likely to have a larger litter if its mother had a large litter.
A doe comes into heat every three to five days and typically has a gestation period from 18 to 21 days. A doe's first and last litters tend to be the smallest. Mice can have litters when they are as young as 5 weeks old. It is important to separate mice by sex by the time they are 4 weeks old to prevent unexpected and unwanted pregnancies.