The book, to be published in 2007 by Basic Books, takes as its subject "the new later motherhood" - a phenomenon Gregory noticed when she had her first child at 39. Many women she knew were choosing to have children later.

"Nothing had been written about it. And it's a huge change," said Gregory, who was quoted on the subject in Newsweek this summer. In 1960, she says, only one in 76 first births were to women over 35. Today it's one out of 12.

The impact that sea change is having on those women - and their relationships - is at the heart of the book. While interviewing more than 100 women, she's become "an economist, an anthropologist, a biologist." An epilogue will address the effects the 'new later motherhood' is having on the children.

For now, she's eager to finish the book. "My life is a race," she says wryly. "Life will be calmer once the book is finished."