Underwood was born on October 4, 1986, in Mayfield Heights, Ohio. At age three, she moved with her family to Naperville, Illinois, where she grew up and attended Neuqua Valley High School, graduating in 2004. She began as a Girl Scout in kindergarten and is a lifetime member. At eight years old, Underwood was diagnosed with supraventricular tachycardia, a pre-existing condition that later shaped her views on health policy.
