Carter told a Pentagon news conference that he will double the length of fully paid maternity leave for female service members and work with Congress to boost time off for paternity leave and adoptions.

He said he also intends to expand health care coverage to include more benefits for women trying to get pregnant. And he is directing the military services to expand the hours that military child care facilities are open and the number of children that can be accommodated.

The changes, which will cost an estimated cost of $385 million over the next five years, are part of Carter’s ongoing effort to modernize the military and make it more attractive to job seekers. He’s already pushed past Marine Corps objections to allowing women to apply for combat jobs and has expressed a willingness to consider allowing transgender people serve openly.

The new initiatives, said Carter, “will strengthen our competitive position in the battle for top talent, in turn guaranteeing our competitive position against potential adversaries.”

But, he added, any adjustments had to be balanced against the need to maintain a strong and ready fighting force. “Fairness is important, but always, always the mission effectiveness of our force comes first,” he said. “We are not Google. We are not Walmart. We’re war fighters.”

Pregnancy is a key issue for military women, who often have to deploy for months at a time and try to plan childbirth around their more stable duty assignments. And men worry about injuries during their deployments to warzones that might impair their abilities to father children.

Officials said the military service chiefs and leaders largely endorsed the changes. There were concerns, however, expressed mainly by the Army and Marine Corps about the impact of any significant increase in maternity leave.

The maternity leave issue is complicated by the fact that Navy Secretary Ray Mabus already increased paid time-off for Navy and Marine Corps forces to 18 weeks. His decision last July tripled the current military leave of six weeks. The Marine Corps and Army, however, raised worries about extending leave to 18 weeks, saying it would keep key combat support troops off their jobs for too long and make it difficult to cover their posts by shifting personnel around.

Carter’s decision to settle on 12 weeks would force the Navy to scale back its 18-week leave and make accommodations for those who may already have planned the longer time off. Carter said that any Navy or Marine Corps service members who are pregnant would get the 18 weeks they expected.

The health care coverage proposal is complex, but would involve increased benefits for women seeking more extensive fertility and pregnancy assistance. Specifically he called for a pilot program that would extend health care coverage to active duty women seeking to freeze their eggs and men who wanted to freeze their sperm.

“As a profession of arms, we ask our men and women to make incomparable sacrifices,” Carter said. “We ask them, potentially, to place themselves at risk of sacrificing their ability to have children when they return home. It’s clear that the benefits we offer our troops can better account for this.”

The planned increase for paternity leave would go from 10 days to two full weeks. And officials said Carter wants to expand the current three-week leave for an adoptive parent, and allow the second parent to take two weeks off, if that person is also are in the military. The paternity and adoption leave changes would require approval from Congress.

The child care plan would increase the minimum time the facilities are open to 14 hours, from the current 12. Carter also wants the services to modernize and improve the child care system to reduce waiting lists and increase the number of children who can be served.

One of the more flexible changes unveiled Thursday, Carter said, would allow service members to stay at a particular base or duty station for a longer time, and in exchange they would commit to a longer time in the service. Such decisions would be at the discretion of commanders who would consider the military needs of the unit.

According to Carter, 52 percent of the military’s enlisted force and 70 percent of its officers are married, and there are about 84,000 marriages where both are in the service.

The expansion of family leave benefits is the second phase in a broader campaign by Carter to modernize the military and help troops better balance their family commitments with their desire to serve their country.

Last November he rolled out a series of initiatives aimed at attracting and retaining quality service members. They ranged from increasing internships to changing the retirement system to allow investments in a 401(k)-type retirement plan.

Many of the changes are an effort to align the Pentagon with the corporate world, strengthen ties with high-tech companies and bring the best from that field into the Defense Department.