My sweetest rat bit the tar out of me about an hour before she delivered. That's how I knew she was close. When her litter was younger, she would give me warning nips. Now that the babies are nearly weaned, she actually welcomes my hand in their cage because that means either she gets some time away from her babies or I take the babies away for a little bit so she can have some peace and quiet. Now, my more hand-shy females are the ones that I can just reach in and mess with the babies with no problems. It just depends on the female.