It is reported that a mature female rat reproduces once in about 20 days. A small litter can give birth to six or seven small mice, and as many as a dozen small mice. And a little mouse can reach sexual maturity and reproductive ability at 1.5 to 2.5 months. Interestingly, scientists discovered that mice have a family planning function. When the total number of rats in an area is large, the rats will slow down their frequency of reproduction, and when there are fewer rats or people are mass extermination of rats, they will speed up reproduction and make the rats multiply rapidly.
The main point of this passage is that there is self-regulation of the number of births in mice.