Congress is now considering four weeks of paid family and medical leave, down from the 12 weeks that were initially proposed in the Democrats’ spending plan. If the plan becomes law, the United States will no longer be one of six countries in the world — and the only rich country — without any form of national paid leave.

But it would still be an outlier. Of the 185 countries that offer paid leave for new mothers, only one, Eswatini (once called Swaziland), offers fewer than four weeks. Of the 174 countries that offer paid leave for a personal health problem, just 26 offer four weeks or fewer, according to data from the World Policy Analysis Center at the University of California, Los Angeles.

Four weeks would also be significantly less than the 12 weeks of paid parental leave given to federal workers in the United States, and less than the leave that has been passed in nine states and the District of Columbia.

Paid leave is part of the Democrats’ giant budget proposal, which includes other family policies like child and elder care. They are trying to cut to an amount palatable to Senator Joe Manchin of West Virginia and Senator Kyrsten Sinema of Arizona, whose votes are needed to pass it. For paid leave, decreasing the number of weeks is a simple way to lower the price.

Some leave is better than none, researchers said, but evidence from around the world suggests that four weeks is too little to reap the full benefits.

“When you look at other countries, there is evidence of what people need and what’s feasible,” said Jody Heymann, founding director of the policy center and a U.C.L.A. distinguished professor of public health and public affairs. “And by both of those measures, 12 weeks is a modest amount, and anything less is grossly inadequate. The rest of the world, including low-income countries, have found a way to do this.”

Globally, the average paid maternity leave is 29 weeks, and the average paid paternity leave is 16 weeks, the center’s data shows.

There is one element of the paid leave proposal, however, that would put the United States at the forefront internationally: its very broad definition of family and caregiving. It would cover care for all types of loved ones, including in-laws, domestic partners and people who are the “equivalent” of family.

About half of countries allow caregiving leave for sick children. Thirty-nine percent of nations allow it for adult relatives like spouses or parents. Among countries that have leave for family health needs, only 13 define family broadly, including grandparents or other loved ones. Even then, there are sometimes restrictions, like proving that the person needing care is dying.

Globally, 107 countries have parental leave available to fathers, which is more than half, and 47 offer more than four weeks. Many rich countries offer more than 12 weeks. Twenty, including Japan, Canada and Sweden, have options for more than a year.

For proponents of paid leave in the United States, the broad definition of caregiving for loved ones has been a key demand. “We have always fought for a family definition that reflects what American families really do look like, including chosen family,” said Lelaine Bigelow, vice president for congressional relations at the National Partnership for Women and Families. “This has been a big pillar of ours for a long time.”

Existing family leave policies in the United States already have a broad definition of caregiving, largely because the country came around to offering leave so late. Many countries started paid maternity leave in the 1920s. The American leave program — unpaid and for 12 weeks, for about half of workers — started in 1993. By that point, gender norms had evolved and it was common for mothers to work and men to care for children or relatives. The unpaid leave covers people caring for babies; sick spouses, parents or children; or their own medical conditions.

Paid leave is typically not fully paid anywhere; the pay is usually a percentage of the regular salary up to a certain maximum. Around the world, paid leave is generally financed through social insurance, via taxes or contributions from employers and workers. The states with paid leave have a similar system.

The federal plan in Congress is limited by the rules of a budget process called reconciliation, and would be paid for, along with other safety net spending, with general revenue, perhaps from a tax on billionaires.

Democrats are considering other options for making paid leave less expensive. These include making it expire after five years, hoping that it would be popular enough that lawmakers would renew it. Another idea is to make it permanent but with fewer than four weeks of leave. Also on the table is means testing: Low-wage workers would get paid leave but others would not.

Of people who take unpaid leave in the United States, just over half do so for their own health problems. The United States is one of 11 countries that do not offer paid leave for health problems. Of the countries that do, 132 offer three or more months, and 62 offer a year or more if needed.

Many people need more than four weeks. Recovery from major surgery typically takes at least six weeks; recovery from a heart attack can take several months; and cancer treatment often lasts half a year or more.

Short-term disability insurance pays for medical leave in some cases, but only about half of workers qualify. And women, workers of color, and low-income Americans are less likely to have it.

Research on maternity leave suggests that between three and six months is ideal. For birth mothers, recovery typically takes at least six to eight weeks, and maternal health improves when fathers take leave, too. Research shows babies continue to benefit from being home with a parent for the first half year — for bonding; increasing immunization and breastfeeding rates; and decreasing hospitalizations from infectious diseases.

Besides the United States, the only other countries with no paid maternity leave are the Marshall Islands, Micronesia, Nauru, Palau, Papua New Guinea, Suriname and Tonga.

“Any leave is better than zero leave,” said Maya Rossin-Slater, an economist in the Stanford department of health policy who studies paid leave. “That being said, I think it’s also worth pointing out that some of the health benefits of leave are clearly occurring at higher durations.”