"It was a surprise, but for me, it wasn't a tragedy. I loved Justin so much," Spears wrote in the book, referring to the pregnancy. "I always expected us to have a family together one day. This would just be much earlier than I'd anticipated. But Justin definitely wasn't happy about the pregnancy. He said we weren't ready to have a baby in our lives, that we were way too young."
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The long-awaited memoir, titled "The Woman in Me," is scheduled to be released Oct. 24.
"If it had been left up to me alone, I never would have done it. And yet Justin was so sure that he didn't want to be a father," Spears wrote, according to People. "To this day, it's one of the most agonizing things I have ever experienced in my life."
Ventura County deputies respond to Britney Spears' home after worried calls over knives video
Ventura County deputies respond to Britney Spears' home after worried calls over knives video
Timberlake did not immediately respond publicly after the excerpt from the book was reported.
The autobiography's publisher, Gallery Books, describes it as "a brave and astonishingly moving story about freedom, fame, motherhood, survival, faith, and hope."
Touching on the time she pleaded to a judge in court about ending her conservatorship, the publishing company noted how that moment revealed "the impact of sharing her voice" and how it "changed the course of her life and countless others."
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Spears' conservatorship was terminated in November 2021 after more than 13 years. A judge placed her father, Jamie Spears, and a court-appointed conservator, attorney Andrew Wallet, in charge of every aspect of the singer's life after Britney Spears was hospitalized twice for mental health issues in early 2008.
ABC News contributed to this report.