How can I tell when my female mouse is pregnant?

According to Fancy Mice, the easiest way to determine if a mouse is pregnant is by looking for a waxy white plug in the vulva. Excess semen from the male creates the plug at the entrance of the vagina to prevent the female from mating with other male mice.

Typically, the plug stays in the female for a period of a couple of days. However, once it falls out, it can often be found in the cage. Another way to determine that a mouse is pregnant is by looking for a fuller stomach and enlarged nipples, according to Paw Nation. Nesting also indicates that a mouse may be pregnant. Generally, pregnant mice begin nesting one to two weeks before giving birth.