When Mattie visits Minnie after the birth of Minnie's twins, she thinks of a story her brother Lawton told her about a bear. It goes a little something like this:
Louis had caught a bear in one of his steel traps. A mother bear that had two cubs. The trap had broken her front leg. By the time Louis and Lawton got to her, she was mad with fear and pain. She lay on her side, keening. Her other side was gone. There was no fur there, no meat, only a livid mass of gore and bones. Her frantic, starving cubs had eaten her flesh away. (33.gravid.37)
Mattie compares this story to the life that Minnie has with her twins, and as Minnie tries to feed her babies, the violent imagery is striking: the bear cubs physically ate the mother, essentially killing her with their need. It's a super morbid image, and it effectively communicates the death of self that Mattie envisions motherhood bringing about.
Though she hasn't committed to heading off to Barnard instead of marrying Royal at this point, it's pretty clear that since this is the story that comes to mind for her as she watches her friend struggle with motherhood, she would be wise to jump ship before she finds herself with a baby gobbling her—and her life—up. Nom nom nom.