Look for the little dark spot on their bum. If they have one, they're a boy (at least, that's how I understand it.) I did all mine today :3. But mine are all coming up black, so if yours are lighter they might not have one.
Oh, the dark spot thing, to clarify. If the mouse has dark pigment, often the boys have a dark spot around their anus, and the girls don't. Or at least, mine do XD. But it could just be my freaky litter.sarahc said:it either had something wrong with it or the mother is not coping with a large litter and has reduced the amount herself .If she was to kill the entire litter at 11 days old I would suspect her to be ill and unable to rear them.It would be unusual ,I should try and resist handling the babies to frequently.I just want to comment on the sexing.I'm not sure what's meant by a black spot on the bum meaning it's male,thats a new one on me.If there is a gap between the anus and the stump it is male,the stump will also be larger,if the stump is small and flat with virtually no space between it and the anus it's female.They can be sexed at birth with a bit of practice and a magnifying glass will help if you are struggling.