Female rats are like pretty much all mammals, I think. Their hormones cycle, and the changes can cause changes in temperament and/or behavior. The biggest hormonal swings are during pregnancy and the period following childbirth. Part of what their hormones tell them to do is to protect those babies, and they do!

I think how likely a rat is to bite, both as part of the mothering thing and just in general, is part genetics and part socialization. Only 1 out of the females I've raised from birth has ever bitten me. Out of the 3 females I used to start my colony, 1 of them bit me, so that's a much higher percentage, and I think it has a lot to do with the fact that those 3 were not handled much as babies cuz they were quite skittish when we got them. If you socialize the females you are holding back to be breeders, and also eliminate biters, you should get to a point where you no longer need to worry much about being bit, as long as you are reasonable and don't harass the mothers with newborn litters and also take care to offer proper support when handling a visibly preggo one.

I also get nibbled by the rats at times. I think it is an exploring thing, and not really a bite. I can feel it is their mouth and their teeth touching me, but there is no pressure at all. I have one that would "taste" me pretty much every time I put my hand in her tub, but never bit. I decided that was getting ridiculous, so she is now no longer allowed to see that treats come from human hands, and that seems to have solved the issue.