28 days is waaaaay to soon for a female mouse to begin breeding. Sure they are ABLE physically to conceive but that does not mean her body is READY for that kind of task. I mean, technically a 13 year old can conceive, but I bet you her body would not be able to handle it and she'd have all sorts of problems. I say ABSOLUTE minimum is 6 weeks. I have four does that are 6 weeks old tomorrow. They are not as big as their mothers, maybe only 25g at most. To me, they are not ready. I will wait another two weeks and then weigh them again.