Mice are capable of breeding at an incredible rate. The gestation period for a mouse is between 19 and 21 days on average, and a single female mouse can get pregnant five to ten times every year, giving birth to a litter of 3 to 14 pups each time. Most mice litters consist of six to eight pups, adding up to 32 to 56 per female every year.
Mice reproduce throughout the year, and females can mate immediately after giving birth to a litter. This means that after one litter is born, the next could be along in just three weeks. To make matters worse, every female in the litter is capable of reproduction after only six weeks, at which point they can start contributing to the number of new mice being born.
All of this means that, in theory, two mice that sneak into your home could give birth to 60 pups over the year. Of these pups, roughly half will be female mice that will give birth to their own pups after six weeks. Overall, you could be looking at more than 5,000 mice from a single breeding pair every year.
Of course, the real world is more complicated than this. In reality, a mouse’s reproduction rate is limited by the availability of food and shelter. If mice have access to plenty of both, there is little to stop them from breeding continually throughout the year. When they nest within people’s homes, mice usually have everything they need to sustain themselves and their offspring indefinitely. Mice also have longer lifespans when living comfortably in a person’s home. In nature, mice live for about 12 months on average. In captivity, this increases to two or three years.