On her first day back at work after giving birth, Tricia Olson drank copious amounts of coffee, stuffed tissues in her pocket, and tried not to cry. After all, her son Gus was just 3 weeks old.

Olson, 32, works for a small towing company and U-Haul franchise in Rock Springs, Wyo., and she could not afford to be away from work any longer.

"The house bill's not going to pay itself," she says, her voice breaking in an audio diary she kept as part of a series on the challenges facing working parents airing on NPR's All Things Considered.

Olson is one of just four employees she says are "like family," and like many U.S. workers, she has no paid leave at all: not for vacation, not if she gets sick, and certainly not for parental leave.

Normally, she's the only one in the office to take calls. Her boss agreed to fill in for her for three weeks after the delivery, but she says "even just that ... makes me feel guilty."

Olson is hardly alone in returning to work so early. But this is a uniquely American problem.

"The U.S. is absolutely the only high-income country that doesn't" provide paid maternity leave, says Jody Heymann of UCLA's Fielding School of Public Health.

In fact, of the 193 countries in the United Nations, Heymann says, 185 have national paid leave laws. Those that don't are Papua New Guinea, Suriname, a few small South Pacific Island states — and the United States.

Even many low- and middle-income countries provide at least 14 weeks of paid leave, Heymannn says, and have done so for decades. Fifty nations provide six months of leave or more.

The International Labor Organization made paid maternity leave a priority a century ago. But Heymann says "most countries, just seeing what it was like for women to try to balance work and caregiving, knew they had to do it for the sake of families, for the sake of kids and for the success of their economies."

Why didn't the U.S. join that trend? Family law experts point back to the aftermath of World War II. European nations needed women in the workplace to help with reconstruction. But in the U.S., untouched by conflict, women who had been working in factories went back into the home after returning soldiers reclaimed their jobs. Analysts also credit a powerful business lobby, weak unions and a national mindset that relies more on the individual than the state.

Given all that, it was a tough political battle in the U.S. just to pass unpaid leave, and that didn't happen until 1993. The Family and Medical Leave Act, or FMLA, guarantees up to 12 weeks of time off, without pay. It applies to both women and men, and can be used not just for a new child, but also to recover from an illness or care for a loved one.

Yet the law doesn't cover some 40 percent of the workforce, including those — like Tricia Olson — who work for companies with fewer than 50 employees, or those who work too few hours. Many others who do qualify for FMLA say they just can't afford to go without a paycheck.

But pressure for paid leave is growing. Today, in a large majority of families with young children, both parents are in the workforce, and more families are headed by a single parent. In recent years, this has helped push a handful of states to pass their own paid leave measures.

California, Rhode Island and New Jersey provide four to six weeks of leave with partial pay, funded by employees through a disability insurance program. This year, New York passed the nation's most generous paid family leave law, to take effect in 2018. It will start with a guarantee of eight weeks off, eventually rising to 12 weeks as early as 2021.

A number of cities have also passed paid leave laws for municipal employees, and this year San Francisco passed the first such law that covers private workers as well.

Local business groups have largely opposed such laws, arguing they are burdensome to administer and hurt the business climate. In a memo to lawmakers in Albany, N.Y., the Business Council of New York State said companies will face "increased personnel costs through overtime and diminished morale for those who pick up the slack from these additional absences."

There's also congressional legislation that would mandate a national program of paid family leave, but it's failed to pass for several years. In the past, the U.S. Chamber of Commerce has argued that a one-size-fits-all mandate is unworkable, and that individual companies should be able to decide whether to offer paid family leave, and how much. But there are signs that business opposition to paid leave laws is softening.

In California, five years after the nation's first paid family leave law took effect, 90 percent of businesses surveyed reported a positive or neutral impact on their establishments. Aparna Mathur of the conservative American Enterprise Institute tells NPR that paid leave laws have the potential to be beneficial and help employers retain workers.

Advocates say that saves money.

"Look at the cost of losing loyal talent, losing workers with some institutional knowledge," says Vicki Shabo of the National Partnership for Women and Families.

In recent years, a wave of companies have adopted paid family leave, including big names such as Nike, Coca-Cola and Hilton. Silicon Valley, especially, has seen something of a benefits race, with businesses offering ever more generous terms as they compete for highly skilled tech workers, and try to attract and retain more women to an overwhelmingly male industry.

In just one example, Vodafone this year announced a new policy of four months of paid maternity leave. It also said that after new mothers return from leave, it will pay their full salary for just 30 hours of work, for up to six months. A number of other companies this year extended the duration of their existing paid parental leave programs.

Still, such policies cover only 13 percent of the workforce, and largely leave out hourly workers.

Paid leave has strong public support, especially among so-called millennials, young adults under 35. Surveys find both women and men care deeply about it, and are demanding it from employers. Researchers applaud this, and say the more time a father spends with a child in those early weeks and months, the more engaged he's likely to be for years to come. That, in turn, can help the mother maintain her own career.

In a sign of this remarkable shift in attitudes, both presidential candidates are proposing mandatory paid time off for new parents. Donald Trump calls for six weeks, paid through unemployment insurance. Hillary Clinton wants 12 weeks, for both men and women, funded by a tax increase.

"Paid family and medical leave is a public good as well as private benefit," says Shabo, the women and families advocate.

There's also evidence that paid maternity leave has health benefits for both mother and infant. Dr. Benard Dreyer, president of the American Academy of Pediatrics, says women with paid leave are more likely to breastfeed, and to do so for longer, and that has "very strong long-term effects on child health."

He says research also links paid leave with decreased maternal depression, a condition that "prevents bonding and has negative effects on child social and emotional development."

In Wyoming, Tricia Olson says she decided not to breastfeed her son in part because of her short time off. Plus, she couldn't imagine keeping it up at work, given the large window in her office.

Olson and her husband discussed the possibility of her switching jobs. But they decided that while she doesn't get paid leave or other traditional benefits at her current workplace, she values the flexibility she has to be able to take time off — albeit unpaid — to take care of things at home.

Of course, Olson wishes she could have stayed home longer. But she's grateful to her co-workers for the few weeks she had, and determined to get through the next few months balancing her job and her baby.

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