Brady, 41, had just split from actress and model Bridget Moynahan, who was pregnant with his child at the time.
In her new book, "Lessons: My Path to a Meaningful Life," Bundchen recalls the moment she learned that Brady was having a child with his ex.
"Two months into our relationship, Tom told me is ex-girlfriend was pregnant,” the 38-year-old supermodel writes. "The very next day the news was everywhere, and I felt my world had been turned upside down.”
Moynahan and Brady welcomed son, Jack, now 11, in 2007. Bundchen and Brady married in 2009 and are parents to Benjamin, 8, and Vivian, 5.
Bundchen also recently opened up to People about her relationship with Moynahan, 47, saying, "I’m so grateful for [Bridget]. I know this was hard, but I couldn’t imagine my life without [Jack]. I call him my bonus child.”
She additionally praised Brady’s eldest son, saying, "He opened my heart in ways I didn’t even know my heart could expand.”
Bundchen opens up further about very personal topics in her new book. In a recent "Good Morning America" interview about the biography, she broke down in tears while talking about her panic attacks in her 20s.