One way to determine a mouse’s/rats gender before they are sexually mature is to compare the between their anus and genitals. Males have a much larger space than females between these two areas. Eventually, the male’s testicles will descend between this area. It is nearly impossible to tell gender of newborns. It is best to wait a week or two before determining gender. The same is true with rats only it seems more difficult to determine gender if you do not have rats of different genders to compare to one another. All rats have large spaces between genitals and anus only the males have an even larger space than females.

Rat/Mouse pregnancy = approximately 21 days

Any time after her birthing a doe (female) can be impregnated again by the male. Some breeders have a doe both pregnant and nursing but this causes extreme stress for the doe. It is my observation that the higher stressed the mother is, the smaller/weaker the offspring are.
For some reason a litter may be born that is extremely small and barely alive. If the mother cannibalizes one, the odds are that she will cannibalize the rest and in my opinion it is best to cut your losses. Their is no need for the mother to become further stressed trying to nurse a lost cause. At this point I "call" the litter and feed them to my young kingnakes then freeze the extras for further meals.

Males will sometimes fail to mate if they are too overweight. The best thing to do with an overweight male is to temporarily replace him with what I call a "floater" male. A floater male does not have a designated harem, he is simply an extra sexually mature male that can mate with females who do not have a harem at the time or step in for another harem male. I have more than 5 females per harem but at any given time there are many females in birthing cages. If it so happens that all the harems are full (5 females, 1 male) the extra females can mate with the floater male. While the floater male is taking care of business, the other male needs to be on a strict diet and given lots of exercise on an exercise wheel before he may be returned to his harem.

Females may fail to mate if they are extremely underweight. Therefore I recommend the female be removed from the harem and given somewhat high-fat foods along with normal fiber and protein. Here are some good foods to give an underweight mouse:
Avocado on bread or crackers
Peanuts
Almonds
Sunflower seeds
Low lactose cheese like Asiago or provolone
Peanut butter spread on bread or crackers (If peanut butter is too thick the mouse may choke)

I started breeding rats when the mice could not catch up with my monitor's food demands. Mice take over 6 weeks to mature and in that time you must feed them, water them, have extra cages for them and ...smell them. I only breed mice for my young snakes now. Rats are far better to feed larger monitors because they take merely 1 1/2 - 2 weeks to become the size of an adult mouse. You do not need any extra cages for them beside the birthing cage and the mother does all the feeding (until 2 1/2 weeks of age when they start to nibble on whole foods.) My monitor Loki is 34" and currently eating 4 week old rats and only needs 1 - 2 per meal. 

Keeping your rodents in a storage shed/loft/covered porch is more convenient (and less smelly)than inside your house.

Laboratory Mice breeding cages can be purchased at Reptile shows for about $10