Real men put their careers on hold for babies. At some of the most profitable and hard-driving law firms in the nation, paid paternity leave for new dads-ranging from two to ten weeks-is now the norm, a perk as basic as a shiny BlackBerry.

More striking is that men under 40-even those with partnership aspirations-aren’t hesitating to take the leave, according to interviews with lawyers at nearly a dozen firms. At Kirkland & Ellis, 78 men at the 1,300-lawyer firm took paternity leave in 2007. About 50 of them took four-and-a-half weeks, says partner Jay Lefkowitz, a member of the firm’s management committee. He notes that the firm now offers six weeks of paid paternity leave and that “the numbers [of people taking the leave] have been increasing.”

It’s the same story at White & Case, where 16 men took paternity leave last year. “It started with one, then five, then ten,” says Susanne Wamba, White & Case’s director of health and welfare benefits. The numbers were similar at the firm’s competitors. Last year 24 men (including two partners) took paid paternity leave at Sullivan & Cromwell; 15 at Cravath, Swaine & Moore; 25 at Weil, Gotshal & Manges; 20 at Morrison & Forester; and nine at Baker Botts.

Though paid maternity leave has been a big-firm staple since women started entering the law in significant numbers in the 1980s, paid paternity leave is a relatively recent invention at Am Law 100 firms. For example, White & Case just started paid leave for fathers six years ago, while MoFo launched it eight years ago.

Men taking time off to help care for newborns say they are no longer stigmatized. “Being a father is the single most important aspect of life,” says White & Case litigation associate Brian Bank, who recently returned from a five-week leave (four weeks’ paid paternity, plus one week of vacation). Once he found out that he and his wife were expecting twins, he says he didn’t hesitate to take time off. “It’s part of our compensation and benefits,” he says matter-of-factly.

Unquestionably, that attitude reflects a generational shift. “Male parents are more outspoken, and family life is more important to younger lawyers,” says White & Case’s Wamba, who has worked in human resources at the firm for 18 years. White & Case offered only a one-week paid paternity leave until three-and-a-half years ago, she says, when “one brave soul,” an associate expecting triplets, lobbied to increase the time.

Jason Bartlett, a recently elected partner at MoFo, took the then standard three-week paternity leave (it is now four weeks) in 2003, then again in 2005, when he and his wife adopted children while he was based in the firm’s Tokyo office. Though he was an associate at the time, he says it never crossed his mind that taking time off would be held against him. “I don’t see nurturing and being a tough litigator as being conflicting,” he says. (The firm says 21 lawyers who took paternity leave have made partner since 2002.)

Generally, associates seem confident that time off for fatherhood won’t derail their careers. Nicolas Commandeur, an eighth-year associate at Patterson Belknap Webb & Tyler and a veteran of two paternity leaves (four weeks each), says, “I’m coming up for partnership, and no one has mentioned that my taking time off would have a negative impact.”

Some male lawyers are even taking primary caretaker leave on top of paternity leave. Malick Ghachem, a fifth-year litigation associate at Weil, Gotshal’s Boston office, made firm history last fall when he took an additional six-week leave to stay home with his second child while his wife worked on a book. In all, he garnered ten weeks of paid leave. “My first thought was, I couldn’t do this because I’m a man,” recalls Ghachem, “and men aren’t usually the primary caretaker.” But he says his supervisors “understood it’s firm policy and supported it.” (The firm says that three male associates have taken the full ten-week leave.)

All this fuss over paternity leave puzzles partners at Cravath and Sullivan & Cromwell-and not because they oppose it. Rather, both firms established paid paternity leave long before it became part of the lexicon. S&C partner Ann Bailen Fisher says the firm has had paid paternity leave since 1997, and that “the majority [of fathers now] take the whole four weeks.” And Robert Townsend, managing partner of the corporate department at Cravath, says the paternity policy has been in place so long there that he doesn’t remember when the firm didn’t have it. He personally took the four-week leave as a first-year associate in 1991, he says, when his first child was born. Now the father of four, he says he took time off for all his kids. “More men than not take it,” he says.

These policies, it appears, are here to say. “There’s no turning back [on any kind of family leave],” says Kirkland’s Lefkowitz, whose youngest child was born ten years ago, before paternity leave was on the books at his firm. Lefkowitz says he encourages associates to take paternity leave because “it is good for the marriage, the family, and morale.” The reality, he adds, is “that the workplace in America has changed.”