How do i know if i have pregnant mice?

i have feeder mice and the 3 adult females are pregnant but i also think i have some young females that might be to.i checked them everyday that i had them for signs of them going into their cycle one day of not checking and it was to late and i think they went into their first cycle.they are now seperated.but what are the signs of mice being pregnant and how do you know for sure they are?How do i know if i have pregnant mice?
I have bred mice, I'm afraid there is no real way of knowing until they are almost due. I suggest you try not to handle them, but monitor them. You will soon know.How do i know if i have pregnant mice?
they will suddenly be really fat and not move much