What does Frannie remember about the baby that didn't "thrive", and what is another memory of someone using that word?

Frannie says that her mother had been ill and had finally been convinced to go to the hospital. She'd discovered that there was a baby but that it was not thriving. The baby eventually died. Frannie also remembers when her mother had been called to the school because Frannie wasn't doing well. The teacher had said that Frannie wasn't "thriving' in specific subjects, and that the words had upset her mother.