(Bloomberg)--Cristian Igualt and his wife, Ana Castro, justhad their second child.

Now both parents are on months-long paid parental leaves from their job,making the family extremely lucky and rare in the U.S.workforce.

Read: Business school profs push for paid familyleave legislation

Igualt works at a software company, Optimizely, with a generousparental-leave policy: 17 weeks at full pay for all newparents.

His female coworkers who have children get the same benefit.

Castro is eligible for 12 weeks off at half-pay underCalifornia's parental-leave laws, which are moregenerous than other states.

Unlike the vast majority of new parents, Igualt and Castro don'thave to worry about earning money or keeping their jobs whilecaring for a newborn. "This means that both of us are involved inparenting and creating the norms of being active parents," Igualtsays.

It's unusual for employers in the U.S. to offer paternitybenefits at all, let alone to provide the same amount of paid timeoff for mothers and fathers.

There's not even a legal requirement to offer paidmaternity leave to new mothers.

The Family Medical Leave act allows for up to 12 weeks ofunpaid maternity leave, but it only applies to full-timeworkers at companies with 50 or more employees—a fraction ofthe U.S. workforce.

Read: Why businesses can't afford to ignorework-life perks

A few states, including California, have stronger laws thatstill stop short of covering all parents.

That leaves maternity and paternity benefits up toemployers, and most opt-out. Around 11 percent of workers arecovered by formal paid family leave policies, according to a reportby the White House.

Of the companies that do offer benefits to new parents, mostfocus on moms who need time off to physically recoverfrom giving birth.

Only 17 percent of employers surveyed by the Society forHuman Resource Management provide new dads with paidleave.

In some more elite corners of the workforce, however,there's a parental leave arms-race among employers trying toattract the best talent.

At companies like Google and IBM, paternity leaveis starting to become more commonplace, with themost generous employers offering all new parents the same amount oftime off. "The newest trend is wrapping it all together," saysCarol Sladek, a consultant at Aon Hewitt.

Pressured by its peers, Amazon just added paternityleave to its benefits plan. Microsoft, in addition todoubling maternity leave to 20 weeks of paid time off, recentlytripled paid leave for all other new parents, including adoptive,LGBT, and foster.

Employers with the most generous benefits for new parents appearto be erasing distinctions between mothers and fathers alltogether, as Optimizely has done.

Last month Zendesk, a software developmentcompany, ditched its maternity leave for a new policythat encompasses any parent.

Now all employees who welcome a new child are eligiblefor 16 weeks of paid time off.

Netflix's new unlimited year-long parental leavepolicy includes mother and fathers alike, as do the benefitsoffered at Reddit, Facebook, and Square.

Those employers and states that offer paid time off tend tofocus exclusively on mothers, and this emphasis puts theaccompanying career handicaps of caring for newborns onwomen.

Working moms already face what's called "the wage penalty formotherhood," a phrase used by sociologist MichelleBudig in her research showing that a woman's earningsdecrease by 4 percent with each additional child.

New mothers without paid leave tend to drop out of theworkforce, as many studies have found, and the longerthey remain out of work, the more it hurts their careers.

Yet the average mother takes two months more time off than theaverage dad, who only takes two weeks off, according to a 2014study from the Boston College Center for Work and Family.

A study from last year found 86 percent of men said theywould take paternity leave, as long as they received at least 70percent of their normal salary.

The companies at the forefront of egalitarian leave want tocounteract this imbalance.

Denying equal time off to new fathers "incentivizes the dad tocome back to work quicker," says David Hanrahan, the vicepresident of human resources at Zendesk. "It puts them in front ofopportunities and pay increases and potential promotions."

When men do take parental leave, women benefit.

In Sweden, for example, each additional month of leave taken bythe father increases the mother's earnings by 6.7percent.

Maternity leave laws also only consider the most traditionalfamily arrangements, leaving out parents who are adopting,working with surrogates, or fostering.

"When an LGBT parent says, 'Hey, my spouse and I areexpecting, how does this apply to us? Do we get the 16 or fourweeks?' It's kind of awkward," says Hanrahan.

Of course, there are costs associated with providing long- termleave to more employees.

Optimizely open-sourced the financial model it used tocalculate the incremental costs of having parents out onleave.

In a hypothetical situation in which 20 employees from afictional 400-person company took leave, the total incremental costper employee under the model would be $1,634.

"That’s roughly the cost of a business trip per employee,"says Bill Press, Optimizely's director of engineering and thecreator of its parental-leave financial model. The modelalso doesn't take into account the productivity and retentionsavings.

Having two parents at home makes caring for a newborn easier.Igualt, the Optimizely employee enjoying more than four months ofpaid leave, offers breastfeeding as an example. "It's importantthat she starts to pump and save milk for when she goes back towork," he says of his wife. "That is something that immobilizesher. Meanwhile there are two other kids that need to be taken careof."

If Igualt couldn't take off work, the family might have to payfor additional child care, something not all parents can afford.Plus, he would miss out on bonding time with his children. A2007 study found that dads who take more time off aremore involved in child care once they return towork.

Attitudes about paternity leave are shifting.

When Igualt mentioned his paternity leave to his friendsand family outside of the Bay Area, he found that many advisedagainst taking the entire time offered by his employer.

Only peers in Silicon Valley encouraged him. "It'sworth making clear that [parenting is] really hard,"says Igualt. "With the extra time, there is less stress on thewhole family. We're both able to be our better selves, both at homeand also at work."

(Note: The example cited above from Optimizely's financialmodel for 20 employees taking parental leave is a hypothetical, nota real case.)

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