Here's why:
* Genitals: By this age, their reproductive organs are developed enough to be visibly different. Males will have testicles that are visible near their rear end, while females will have a small vulva.
* Size: Male pups often grow slightly larger and heavier than female pups.
Important Note: It's best to have an experienced breeder or veterinarian determine the sex of your mice. It can be tricky to differentiate between sexes, especially with very young pups.