The champion cyclist was engaged to the Grammy Award-winning artist but the relationship took a turn just before her 44th birthday when things became "kind of a struggle," Armstrong told John Wilcockson, author of the biography "Lance," the New York Post reported.
"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," Armstrong, 37, said. "Yep we're up against her biological clock -- that pressure is what cracked it."
The two went to a relationship counselor to try to smooth things over "but really there's no way to counsel that situation," he said.
"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. ... I would have been in the future but not then."
The pair finally called it quits after about two years of dating and scrapped plans for a Texas wedding.
Crow adopted a 2-week old child in May 2007 and now lives on a sprawling farm outside of Nashville.
Armstrong, already a father of three, announced last December that girlfriend Anna Hansen is pregnant.