Most federal employees will become eligible Thursday for paid parental leave, a benefit valued at about $1 billion a year and one of the most significant expansions of their benefits since the creation of unpaid parental leave more than 25 years ago.

The new entitlement will allow employees to take paid time off for part or all of 12 weeks over a 12-month period, effective with births, adoptions or foster placements that occur Thursday and after. Previously, employees could take 12 weeks of unpaid time available under the Family and Medical Leave Act.

Enacted last December with a delayed effective date, the authority has been years in the making.

“I’m thrilled that this long overdue policy is finally effective for working families in public service,” Rep. Carolyn B. Maloney (D-N.Y.), a longtime advocate of the benefit, said in an emailed comment. “For too long the federal government has been behind the private sector when it comes to family friendly work policies. . . . The federal government will have set a high bar for the private sector to follow.”

According to the Labor Department, 21 percent of private-sector workers are eligible for some amount of paid parental leave and 40 percent have access to short-term disability insurance, a benefit sometimes used by birth mothers but which the government does not offer.

“This new benefit will likely improve the desirability of Federal employment, and likely increase the quality of Federal employees, leading to improved services for the general public,” the Office of Personnel Management said in issuing rules in August to carry out the benefit.

“For too long, federal employees have had to rush back to work because they couldn’t afford unpaid leave at the precise time that their families needed them at home,” National Treasury Employees President Tony Reardon said in a statement.

Still, tens of thousands of the 2.1 million executive branch employees will be left out since Congress has not yet plugged an unintended gap in the law that was discovered soon after it was passed. The House passed a fix in July, but it remains pending in a conference with the Senate.

Also excluded is the semi-corporate U.S. Postal Service, with its approximately 630,000 employees; paid parental leave could be created in the Postal Service only through labor negotiations, spokesman David Partenheimer said in an email.

The House last year initially accepted, as an amendment to a defense budget bill, a provision for paid leave for all purposes for which the FMLA allows unpaid leave, including for various personal and family needs in addition to parental purposes.

The Senate at first rejected that language but ultimately agreed to paid leave — although only for parental purposes — in exchange for the House’s accepting the Trump administration proposal to create a Space Force.

Maloney at the time called it “a tremendous victory for 2.1 million federal employees who will no longer need to choose between being home with their new child or their paychecks.” President Trump tweeted that “after decades of empty promises and inaction . . . I am proud to say that our Government is leading by example in changing the culture of how we support working families.”

However, a closer look at the law soon revealed that it applied only to federal employees under standard civil service laws, called Title 5 of the U.S. Code. Employees of some agencies, or portions of them, fall under different laws.

Among those excluded, along with postal employees, were the 44,000-employee Federal Aviation Administration, 109,000 “Title 38” medical personnel of the Department of Veterans Affairs, and Transportation Security Administration employees other than screeners — who were included in the law and who account for about two-thirds of that agency’s 64,000 employees.

Legislation to extend the benefit to agencies left out, other than the Postal Service, was introduced soon after the problem was discovered but did not advance through the early months of this year. Sponsors then turned again to the annual defense bill as a vehicle, adding the expansion to the House version of that bill.

However, that measure remains in a conference with the Senate, which passed its own version without such language. Maloney, who chairs the House Oversight and Reform Committee, said that enacting the fix is a priority for her.

“Every federal employee, regardless of their title or status, should be able to use this new paid parental leave — and no federal employee should be forced to choose between a paycheck and caring for their child,” American Federation of Government Employees President Everett Kelley said in an emailed comment.

In the absence of action by Congress, VA is extending the benefit to its Title 38 employees, director of media relations Susan Carter said. At the Postal Service, “there is no bargaining currently in process concerning this provision,” Partenheimer said.

Two other agencies where bargaining over benefits is allowed, the Federal Deposit Insurance Corp. and the Securities and Exchange Commission, previously had allowed six weeks of paid leave after negotiations with the National Treasury Employees Union. Those programs have since been expanded to 12 weeks, the union said, adding that a separately negotiated 12-week benefit at the Office of the Comptroller of the Currency took effect in April.

On the basis of estimates of 51,000 births and 3,600 adoptions or foster placements annually in the federal workforce, the OPM projected a cost to the government — and therefore the value to employees — in pay and benefits of “between $890 million and $1.3 billion, with a mean estimate of $1.1 billion.”

That represents one of the most substantial expansions of federal benefits since the FLMA was passed in 1993. In the following years, the government allowed federal employees to pay health-care premiums with pretax money and started offering tax-favored accounts for health-care and dependent-care costs — benefits also widely introduced in the private sector in that period.

It also added insurance programs for long-term care and for vision and dental care, although with no employer contribution toward premiums.

Meanwhile, proposals have been raised year after year — on the basis of fiscal concerns, ideology or both — to limit the value of federal employment benefits or shift more of the cost to employees. Most of the proposals stalled in Congress, but federal employees hired after 2012 were made to pay more toward retirement than those hired previously, for example.

That pressure has continued; less than three months after lauding paid parental leave for federal employees, President Trump proposed to reduce the value of their retirement benefits in several ways, along with requiring most to pay more toward those benefits. Those proposals, too, have stalled.

The OPM notice meanwhile stressed the limits of the new paid leave entitlement:

•As with unpaid FMLA leave, it will be available only after an employee has worked for the government for 12 months and will not be available to those working on a temporary or intermittent basis.

•While the FMLA allows the taking of unpaid leave both before and after the birth or placement of a child, paid leave will be available only afterward; employees still could take other forms of paid leave instead of unpaid time before the event, though.

•Any unused amount of paid time will not carry over beyond 12 months.

•Employees taking paid leave must agree to return to work for the government for at least 12 weeks or else repay the government’s contribution toward their health insurance for that time.