Military service members will now be eligible for 12 weeks of paid parental leave, the Pentagon said on Wednesday, announcing a change that doubles how much leave is available to parents and does away with distinctions between primary and secondary caregivers.

The Defense Department’s under secretary for personnel and readiness, Gilbert R. Cisneros Jr., issued a memorandum on Wednesday outlining the new parental leave policy for service members who have been on active or reserve duty for at least a year.

“It is important for the development of military families that members be able to care for their newborn, adopted or placed child or children,” Mr. Cisneros wrote in the memorandum.

Under the new policy, a parent who gives birth receives 12 weeks of leave after “a period of convalescence,” to be taken in the first year after the birth of the child. Parents who do not give birth are also eligible for 12 weeks of leave in the first year after their child is born. Parents who adopt a child or provide long-term foster care are also eligible for the 12 weeks of leave. Service members have the option to take the leave in up to 12 increments.

The policy applies retroactively to anyone who had a child, adopted a child or became a long-term foster parent on or after Dec. 27, 2022.

Service members who are deployed must wait to take their parental leave until the deployment ends, except in “exceptional and compelling circumstances.” They can get an extension that allows them to take their leave after the first year.

“Unit commanders must balance the needs of the unit with the needs of the member to maximize opportunity to use parental leave,” Mr. Cisneros wrote.

The changes were mandated by the National Defense Authorization Act of 2022.

About 37 percent of military personnel have children, according to the Department of Defense.

Previously, birth mothers received six weeks of convalescent leave and could receive another six weeks of parental leave as the “primary caregiver.”

In the Air Force and the Army, the “secondary caregiver” previously received three weeks of leave. Last February, the Navy and the Marine Corps increased the amount of parental leave for secondary caregivers to three weeks from two.

The new policy closely mirrors the parental leave policy for federal workers, which in October 2020 was expanded to 12 weeks. The United States is one of the few countries without national paid parental leave.

In May 2021, Senator Tammy Duckworth, a Democrat of Illinois and an Iraq War veteran, and Representative Jackie Speier, a California Democrat who has since retired, introduced legislation to provide 12 weeks of parental leave to military service members. That same month, Representative Chrissy Houlahan, a Democrat of Pennsylvania and an Air Force veteran, introduced a separate bill that would expand services for military parents, including increasing parental leave to 12 weeks and expanding access to postpartum anxiety and depression screenings.

“I can’t understate how monumental this is for our service members and their families,” Ms. Houlahan said in an emailed statement on Thursday. “I had my first child serving active duty in the Air Force, and the wait for child care on base was monthslong. This new policy ensures that families won’t have to choose between a paycheck and taking care of their newborn child.”