"This whole process 10 to 20 years ago was driven exclusively by the female. That's not true anymore," says Richard Scott, a reproductive endocrinologist and embryologist in Morristown, N.J.

New Yorker Glen McWilliams is fresh from a five-year relationship that he says ended when he wanted a child. McWilliams, 39, a physician, says all but a few of his friends have children, and those who don't keep wondering how long they'll have to wait.

"Unfortunately, it takes up most of our conversation when we're together," he says.

They're not alone. Just look at the breakup of Hollywood glam couple Brad Pitt and Jennifer Aniston.

Pitt, 41, was twice named People magazine's Sexiest Man Alive, in 1995 and 2000. He had publicly talked about wanting kids almost since the couple married in July 2000. But Aniston, 36, reportedly wanted to focus on her movie career after the 10-year run of her NBC sitcom, "Friends," ended last year.

The differing priorities are understandable, says Michael Kimmel, a sociology professor at the State University of New York-Stony Brook with expertise in gender issues.

"Having children is an enormous career obstacle for women and a career enhancer for men," Kimmel said. "Brad knows that it's no obstacle for guys to have families. It makes them cooler and hipper and sexier. But for women, it could be the kiss of death for their careers."

It was the same story with the breakup in 2001 of Julia Roberts and Benjamin Bratt after four years.

Bratt, then 37, wanted children, and Roberts, then 33, didn't. It didn't take long for Bratt to meet a like-minded woman and marry in April 2002.

Before his daughter's birth that December, Bratt was quoted as saying, "It's been a lifelong dream of mine to be a father, and it's coming to fruition."

Roberts, 37, is now the mother of twins, joining a host of late-thirtyish actresses who have temporarily traded the movie set for the diaper set.

"Women are taking their time today," says Pamela Madsen of the American Fertility Association. "They're waiting longer to have children. They want to have their careers first, and they want to be established first."

Still, Madsen says, that delay "does compromise their fertility," which sometimes worries men who are concerned about waiting and racing the woman's biological clock.

In just over 30 years, the age of first birth for women rose from 21.4 to age 25, according to the latest data from the U.S. Census Bureau.

"Younger men are just as concerned about having a balanced life as well as a career," says Deborah Siegel, director of special projects for the National Council for Research on Women. "It's about time that work/life issues become men's issues. They've traditionally been seen as a woman's issue."

That desire to balance work and family life is supported by research conducted for Catalyst, an independent nonprofit organization aimed at expanding opportunities for women at work.

Men born between 1964 and 1975 place a much higher importance on personal and family goals, says Paulette Gerkovich, senior director of research.

Seventy-nine percent said it was extremely important to have a loving family; 62 percent said it was important to share companionship. Only 27 percent said it was important to earn a great deal of money and become well known, she says.

"Gen X men are less willing to make some of the sacrifices and trade-offs between work and family that their predecessors did," Gerkovich says.

Stanley Teitelbaum, a clinical psychologist who practices in New York and New Jersey, says the overlap of parenting roles has encouraged men to express those daddy desires, which he says sometimes increase with midlife.

"There is definitely more incidence of older men having children willingly and gladly than in past generations," he says."

Edward Stephens, 67, has two children; the first was born nine years into his marriage, when he was 40. The Manhattan psychiatrist had wanted children many years earlier but says his wife wasn't ready. They divorced in 1996.

"A whole decade of my life went down the drain," he says.

Although a list of post-40 and post-50 first-time fathers gets notoriety when those dads are David Letterman or Tony Randall, men are thinking more about what limitations their age might place on their relationship with their children, says dating adviser Rochelle Adams, who offers advice for Yahoo Personals.