Too often new parents feel terrified—and not just about the daunting prospect of raising a child. Many of them must grapple with a gut-wrenching decision: How quickly can they leave their newborn to get back to their job? Few can afford to go without salaries for long or to weaken their future wage-earning potential by leaving their job altogether.

Americans face such dilemmas because 88 percent of the private U.S. workforce is ineligible for paid family leave. As a result, many women and men clock back in to work within weeks. The cost of that decision can be measured in poorer health outcomes for their newborn and for themselves.

Right now the U.S. holds the dubious honor of being the only developed nation without policies guaranteeing parents a temporary paid leave from their job. Even countries with fewer resources offer better policies: in Costa Rica, for example, mothers receive full pay if they take leave during the final month of pregnancy and during the first three months postpartum.

Workers in the U.S., however, get only a chance at unpaid leave for three months after birth of a child. And that narrow benefit, part of the 1993 Family and Medical Leave Act (FMLA), applies only to private companies with more than 50 employees (the unpaid offering also applies to public employees). Almost half of the U.S. population is ineligible. And because there are no federal paid-leave requirements, only about one in eight private-sector employees has a paid option. (Scientific American gives 10 days.)

That needs to change to improve our health and economy. Paid leave will keep families healthier. In countries with such policies, it has reduced infant mortality by as much as 10 percent. It also helps to improve new parents' emotional well-being. And multiple studies indicate that paid maternity leave is associated with increased rate and duration of breast-feeding. That practice, pediatricians agree, promotes better health for a newborn.

Paid leave need not come at the expense of profits, either. During this year's presidential campaign, Donald Trump cited concerns about businesses' bottom line for his reluctance to embrace the paid approach (although ultimately he did propose a six-week paid-leave plan). There is already good evidence that paid leave would not hurt businesses. The experience of a few states that have enacted paid-leave laws shows they produce good results for employers and employees alike. California, Rhode Island and New Jersey all have mandatory paid-leave requirements, ranging from four to six weeks at partial pay. The money comes entirely from employee payroll deductions—not out of employers' pockets. In studies of those states, most businesses subsequently reported that such policies had no negative effect on their profitability or productivity, and employers said employee morale improved. Private companies in the U.S., including Microsoft, Goldman Sachs, Nestlé and Facebook, think it makes good economic sense, too. They say they voluntarily offer their employees paid family leave to increase workplace competitiveness and employee retention.

Scattered state and company support for paid leave, however, is not enough. A national policy would enable employees and companies throughout the country to share in these documented advantages. It would also help eradicate other workforce inequities, including long-standing gender pay gaps that are exacerbated when women exit their job to tend to children. (Paid leave allows women to remain in the workforce, reducing holes in a résumé, which helps to address the differences.) If more people are working, that may also reduce government spending on public assistance.

Such policies also promote shared responsibilities in the household. One study of working fathers in the U.S. found that new fathers who took leave of two weeks or more were much more likely to be involved in their child's care nine months after birth—including feeding, changing diapers and getting up during the night when their child needed a parent.

Now, with a new chief executive and Congress, is the time to take federal action and secure at least a month of paid family leave before ultimately scaling up to several months of paid support. Without paid family leave, we are all paying too steep a price.