And she calls him that for good reason.
In between the births of their first son, 5-year-old Isaiah, and Jacob's arrival in 2019, Underwood suffered three miscarriages that left her feeling pained and out of control.
"For my body to not be doing something it was 'supposed to do' was a tough pill to swallow," the country music hit-maker revealed for a cover interview with Women's Health magazine. "It reminded me I'm not in control of everything."
And speaking about it publicly, as she did for the first time in 2018, reminded her that she wasn't alone.
Underwood, 36, told the magazine that she felt like "a weight lifted" off her shoulders when she revealed what she'd been through, and it led to others sharing their own experiences.
"It's not a dirty secret," she said. "It's something many women go through."
That openness helped her heal, as did the addition of Jacob to the family, but those weren't the only keys to her self-care.
According to Women's Health, the "Cry Pretty" singer considers exercise so essential to her well-being, she thinks of it as her antidepressant and anti-anxiety medications.
"Physical fitness makes everything else possible," she said. "I'd love to sit in a bubble bath, but that's not going to happen. My self-care is my gym time, and that's a stress reliever for me."
Underwood, who has her own fitness wear line called CALIA, actually credits haters with helping her fall in love with fitness. During her 2005 stint on "American Idol," she turned to working out to work through criticism about her body.
"I shouldn't care what other people think about me," she admitted, but added, "I was tired, and I kept buying bigger clothes. I knew I could be better for myself, and I let my haters be my motivators."
But for a while, she allowed them to motivate her too far. After discovering she was sleeping better and had more energy for her "grueling schedule" post-"Idol" when she worked out and cut calories, she thought working out more and reducing her calories to as low as 800 per day would leave her feeling even better.
It didn't work.
"Your body is screaming out, "I need more calories, I need more carbs!" she said, and so that's what she gave her body — and a more balanced approach going forward.