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In his upcoming book, Lance Armstrong reveals that he and ex fiancee Sheryl Crow split in 2006 because she wanted to have children -- and he didn't.

"She wanted marriage, she wanted children; and not that I didn't want that, but I didn't want that at that time because I had just gotten out of a marriage, I'd just had kids [Luke, 9, and twins Grace and Bella, 7]," Armstrong says Lance (out in July), as excerpted by the New York Post. "Yet we're up against her biological clock -- that pressure is what cracked it."

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Armstrong, 37, says that he and Crow, 46, even went to a counselor, "but really there's no way to counsel that situation."

"If somebody wants a child -- man, that's the greatest gift you can give to a woman -- ... who are you to stand there and say I don't want one?" he says. "So we were at different points in our lives. We were not compatible on that issue."

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Adds Armstrong (who announced in December that he is expecting a baby with girlfriend Anna Hansen): "I felt like I wasn't ready ... I would have been in the future, but not then."

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In May 2007, the unwed Crow adopted a two-week-old boy, Wyatt.

Of motherhood, Crow told this past February, "Its great! I love every minute of it."

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"The best part is all of it," she added. "I would love more kids! I don't know about 14 kids, but I'd love more!"