FORTUNE — Last month, President Barack Obama made a big to-do about taking action to narrow the gender gap in pay since — the White House reasoned — pay inequality shortchanged women and their families.

But if the administration is concerned about making things easier on working women, there’s another giant, gaping hole in federal legislation worthy of attention: paid maternity leave.

According to a survey released Tuesday by the United Nation’s labor agency, of the 185 countries and territories it had data for, all but three provide cash benefits to women during maternity leave. The exceptions to the rule? Oman, Papua New Guinea, and — you guessed it — the United States. To make matters worse, more countries are raising the amount of money new parents receive while on leave and extending the duration of their time off.

In the United States, meanwhile, workers who need time off from work to attend to their own medical conditions or those of family members — including new babies — have the right to 12 weeks of leave under the Family and Medical Leave Act. But that leave isn’t paid. And the law only applies to employees who have worked for at least 12 months at a company with at least 50 employees. A 2012 report from the Department of Labor found that 59% of the employees it surveyed said they were eligible for unpaid family leave; a study released shortly after the FMLA went into effect estimated that just one-fifth of new mothers qualified.

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Employers, of course, can decide to provide paid maternity leave on their own, but not all do. The Labor Department survey found that 48% of employees who took leave received full pay; another 17% receive partial pay.

“What should be a joyous occasion for families frequently becomes a trigger for falling into poverty,” says Ellen Bravo, executive director at Family Values @ Work, a national network of coalitions that advocates for family-friendly workplaces. (The Department of Labor survey found that about 15% of employees who were not paid or received partial pay while on leave turned to public assistance for help. )

In the absence of federal legislation, several states have enacted their own laws that provide pay for new parents. California was the first to make such a move in 2004. The state’s paid family leave insurance law provides eligible employees up to six weeks of wage replacement leave at 55% of their usual weekly earnings. New Jersey passed a similar law in 2008 and Rhode Island did the same last year. Washington state signed a paid family leave insurance law in 2007, but it hasn’t gone into effect because it lacks funding. Paid-leave bills are pending in New York and Massachusetts.

A 2011 case study of California’s paid family leave by the Center for Economic and Policy Research found that 89% of the employers surveyed reported that the state’s policy had either a positive or no noticeable effect on productivity. An even larger portion of employers found that the policy had a positive effect or none at all on profitability and performance, turnover, and employee morale. Just over 90% of employees in jobs that paid less than $20 per hour who took leave to bond with a new child reported that the paid time off had a positive effect on their ability to care for their baby.

“Common sense would tell us, research tells us, surveys all tell us that families really need to take care of each other without taking a big financial hit,” Bravo says.

So why is paid family leave not a federal policy?

A nationwide policy was considered at one point — back in the first half of the 20th century. Federal bills to establish temporary disability insurance were introduced in the U.S. from 1939 through 1951, but because employers had been so vocal opposing the mandatory unemployment insurance that became law in 1935 as part of the Social Security Act, the political climate was less than ideal. The task of establishing temporary disability insurance — and its successor, paid family medical leave — was left to the states.

Nationwide paid family leave is now back on the table. In his budget for fiscal year 2015, President Obama proposed a $5 million “State Paid Leave Fund” that would provide competitive grants to help states cover the start-up costs of implementing their own paid leave programs.

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And in December, Senator Kirsten Gillibrand, a Democrat from New York, and Representative Rosa DeLauro, a Democrat from Connecticut, introduced the Family Act, which would establish paid family and medical leave through an employee and employer-funded trust within the Social Security Administration.

Both proposals are now before a contentious Congress, so, let’s be honest, the chances of either going into effect are slim to none.

“Right now, we’re still battling to win a law for accommodations for pregnant workers,” says Bravo, referring to the Pregnant Workers Fairness Act that was introduced in September 2012. The bill would require employers to — for instance — provide a stool to a pregnant worker on a factory floor.

Since we’re still debating whether employers should be required to allow pregnant employees to drink water on the job, it’s no surprise that when it comes to paid maternity leave, the U.S. ranks near the bottom.