Lance Armstrong is blaming the constant tick, tick, ticking of Sheryl Crow's biological clock as the key factor in their break-up three years ago.

In "Lance," out in July from Da Capo, the Tour de France legend tells author John Wilcockson that after he and the sexy rocker were engaged in 2005, two weeks before her 44th birthday, their relationship became "kind of a struggle."

"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, Grace and Bella]," Armstrong, 37, reveals. "Yet we're up against her biological clock — that pressure is what cracked it."

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

Meanwhile, Armstrong — who then dated Tory Burch, Ashley Olsen, Lisa Shields and Kate Hudson — is ready to be a father again. Last December, he announced that his girlfriend, Anna Hansen, is pregnant. Their baby was conceived naturally, defying doctors' concerns that Armstrong's bout with testicular cancer had rendered him sterile. The baby is due next month.

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