Mice become fertlie 28 days after they are born. Some have been known to breed a little earlier at 21 days, however that is a rare case. The pregnancy term can last 18 to 21 days. At 14 days in the pregnancy you should be able to notice a very round mouse or your mouse has become "pear shaped".

First of all if there was a male in the tank he needs to be taken out. Mice can get pregnant right after giving birth and that is very stressful for the mouse. If there are other mice that are female in the tank (meaning 1 to 3 mice) then it is personal choice wheter you leave the mice in the tank with the pregnant one or not. Some mice may see the cage mates as a threat and that can cause them to abandon the litter or consume them. Other problems may arise in that one of the nannies will try to steal the babies. But other than that usually the nanny system works well.