Martha Ballard was a real midwife in the late 1700s who delivered more than 1,000 babies without ever losing a mother. Ballard kept a diary of her life and the town secrets she learned thanks to her profession — and she's at the center of Ariel Lawhon's new novel, The Frozen River. In today's episode, Lawhon tells NPR's Scott Simon how she stumbled upon Ballard's story while pregnant with her own child, and why it was important for her to make a 54-year-old woman the hero of her book.