Sterile?

Question
Alright so I'm really curious. This all seemed a bit strange and I wanted to assure that my rats were healthy and that this wasn't something I was doing wrong.

Basically. I've had a few 'whoops' moments where my female rats have been had by the male rats. I've had moments where my female rats were LIVING with my male rats due to multiple different reasons. I know my females(two) are not old as I rescued Shorty only about 3 months ago and Baby only about a month. BUT. They've never reproduced babies. EVER. I considered this lucky on many occasions however, a few of my does go through the process of nesting, etc, and then simply never have the young ones.

Also, I've got a pair of mice who are doing the same thing. The male and female mouse are living together for feeder purposes, however, in the month or so that I've had them, the female has not reproduced.

I'm not complaining, I'm just worried.

Is there something wrong?

Answer
Females of either species will frequently nest even without concieving -- its a fairly normal process. With your 'oops' moments, you were probably lucky and caught it between estrus' cycles (every 4 days for 12 hours). Also, a very young female will not be as quick to encourage the boys as an older female (anything under 10 weeks), so unless they're left in with a male for an extended period of time, you can generally avoid pregnancy.

I'm honestly not sure why your mouse hasn't - she may be sterile, he may be sterile.. is she getting fat, at all? It takes mice, I believe, 20 days or so to produce a litter.. but don't quote me.. I'm useless with mice!