How to Identifying male mice from female mice babies?

Until the babies open their eyes, it's difficult to tell gender, but not impossible. Simply lift the tail and take a good look at the two openings. Compare the babies to each other to get a feel for the distances between the openigns. The mice with closer openings are female, and farther openings are male.

Around the time the fur is fully grown in and they start opening their eyes, a female mouse's nipples will become visible, but then disappear again for a few weeks until it is fully mature. As for boys, their scrotum become large and very noticeable around 3 and a half to four weeks.