She'll pop soon I would think, though my experience with mice is admittedly very limited. I know some of my momma rats get so huge they can barely rise on their hind legs near the end but when they do manage it I'm sure they will topple over from the weight LOL. A rat with 18 or so good sized babies in her is a pretty funny sight! (hmm wait I've been preggo...nuthin funny about that come to think of it! )