San Francisco just became the first city to mandate fully paid parental leave

On Tuesday, San Francisco became the first city in the US to require businesses to offer six weeks of fully paid leave for parents to bond with a new baby.

This comes just days after a huge win for paid-leave advocates in New York state, which will start phasing in a 12-week, two-thirds-salary family leave program in 2018. Also on Tuesday, Twitter announced it will offer all new parents at the company 20 weeks of paid leave beginning May 1.

The state of California already offers six weeks of family leave at 55 percent of pay. It's one of only four states (now including New York) to offer a program like this, which is run through the state's disability insurance program and funded by small employee paycheck contributions.

But as of January 1, 2017, San Francisco will require companies with more than 50 employees to make up the 45 percent difference so those six weeks can be fully paid. Companies with more than 20 employees will have to follow suit a year later.

The policy covers mothers and fathers, same-sex couples, and adopted children. It doesn't include sick leave to care for an ailing relative, which a lot of paid family leave programs like New York state's new one do. California does now mandate at least three days of paid sick leave per year, a policy San Francisco passed in 2006 that helped kick off a national movement to pass paid sick leave laws at the local level.

The parental leave policy had broad support, the New York Times reports, with the exception of one group of small-business owners. The San Francisco Chamber of Commerce took a neutral position on the issue.

Local business groups and chambers of commerce often oppose policies like paid leave and raising the minimum wage, claiming that they will kill jobs or otherwise be bad for business.

Research suggests that paid family leave doesn't hurt businesses, though, and can in fact be good for business because it improves worker retention. And a new leaked survey of business executives shows that the vast majority actually support laws requiring paid family leave.

The US doesn't have national paid leave, so some states and businesses try to fill in the gaps

A lot of Silicon Valley companies and other white-collar employers have been aggressively competing to make their parental leave programs more generous over the past few years. That trend may have helped inspire San Francisco's new law.

But for Americans who don't happen to work at companies like these, things can be tough.

The United States does have an unpaid 12-week national family leave policy, but its protections are meaningless for a large chunk of the population. About 40 percent of workers aren't covered under the Family and Medical Leave Act because their company has fewer than 50 employees, or because they've worked too few hours or been there less than a year. And of course, many people just can't afford to take unpaid time off work.

Our lack of national paid leave puts us starkly at odds with the rest of the developed world, as well as with developing nations.

Democrats in Congress have been trying to fix this by pushing the FAMILY (Family and Medical Insurance Leave) Act, which Vox's Matt Yglesias called the "best parental leave proposal in Congress today." It's pretty similar to New York's plan, an insurance model with 12 weeks of leave at two-thirds of pay, except it's funded with paycheck contributions from both employers and employees. Republicans in Congress, unsurprisingly, have been unwilling to even give the bill a hearing.

But 12 weeks at partial pay, or six weeks fully paid, is actually a low bar compared with what other developed nations offer: partial pay for 35 weeks in Canada and 44 weeks in Germany, and up to 56 weeks in Norway depending on the pay scale. Even Saudi Arabia offers 10 weeks of paid maternity leave, which is a lot more than the zero weeks Americans get unless they work for a generous employer.

An investigation by In These Times found that one in four working mothers goes back to work within two weeks of childbirth, usually due to financial pressures. But going back to work so soon can have nightmarish consequences for a new mother's physical and mental health. Lacking paid leave also pushes a lot of women out of the job market and discourages fathers from being equal partners in child rearing.

That's why advocates say we can't afford not to pass paid family leave, and why a growing number of states and cities are stepping up to make that happen even if Congress won't.