Why does Springtime refuse to leave Wallowa until after she gives birth in the novel, Thunder Rolling in the Mountains?

Heavily pregnant at the beginning of the book, Springtime disobeys her husband and refuses to cross the river to leave Wallowa with the Ne-mee-poo until she is able to give birth to her daughter in her comfortable homeland. She fears the repercussions of having a child in an unknown land, where anything might go wrong, so she stalls the Ne-mee-poo's journey until her daughter is born, naming her Bending Willow.