Cycling phenom Lance Armstrong has finally come clean about why he and singer Sheryl Crow broke up three years ago — and sure enough, it was because of her biological clock.

In his book Lance, due out in July, Armstrong says that after the couple was engaged in 2005, things got rocky. Crow was 44 and wanted to get married — but he wanted to move slower, since he had three kids from a previous marriage.

"Yet we’re up against her biological clock — that pressure is what cracked it," he said, reports The New York Post. "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."

Since then, Crow adopted a little boy, Wyatt Steven Crow, while Lance has dated the likes of college-age Olsen twin Ashley. His current girlfriend, Anna Hansen, is pregnant and due next month.

As for Crow, in the May issue of WebMD magazine, the breast-cancer survivor dishes on what it’s like being an older mom without a husband, battling her disease and her breakup with Armstrong — another cancer survivor.

"I always saw myself with the traditional husband and the kids and the dog," says the 47-year-old, adding that she had to let go of that ideal to make her dreams of motherhood a reality. "I’ve had maternal instincts since I was really young," she explains. "The best thing I could do was to open that door."