mouse pup sex differentiation

Question
I want to know that how can we differentiate the sex of mouse pups in their first week of life?

Answer
It's quite difficult but possible. If you compare the distance between anus and genitalia of a male to that of a female, you'll see the distance is longer for the male. This won't be possible if you have an all male litter or all female litter but another way to tell is when they just start getting fur you'll notice nipples on females. Males have no nipples.