The Pentagon has cut Marines' maternity leave by one-third, but women who become pregnant by March 3 are still eligible for the full 18 weeks of leave.

The new 12-week maternity leave policy was announced Thursday; details can be found in Marine administrative message 102/16. A privileged health care provider must determine the estimated date of conception on or before March 3 for a Marine to qualify for 18 weeks of leave. Waivers will not be considered.

The Department of the Navy in August expanded maternity leave to 18 weeks, which was the Defense Department's most generous policy. The Air Force and Army had also been considering expanding their policies to 18 weeks, but Defense Secretary Ash Carter ordered all services to adopt a standard 12-week policy to bring the military in line with private industry. It was a setback to Navy Secretary Ray Mabus, who tripled the service's traditional six weeks of paid leave last year to bring it in line with progressive companies like Google.

Mabus' policy allowed new mothers to spread maternity leave over the year following their child's birth, allowing more flexibility to switch off with a spouse's parental leave, for example. The new rules require women to take all 12 weeks of maternity in one block immediately following the birth or the release of the Marine from the hospital, whichever is later. Multiple births from a single pregnancy do not increase the amount of maternity leave.

Rules governing adoption and paternity leave remain unchanged, though the services continue to discuss a possible expansion, which would require congressional approval.

"Besides the changes to maternity leave, DoD is working to expand both paternity leave and adoptive leave for service members," the Navy said in Thursday's release.

Maternity leave was expanded from six weeks in the hopes of retaining women in their late 20s to mid-30s, who are twice as likely to leave military service twice as often as their male peers.

"Women at peak ages for starting a family leave the military at the highest rates," Carter said in January.

While many women lauded the expansion of maternity leave last year, some pointed out that women who take such a long time away from a command could see their assignment and promotion chances affected. The new maternity leave policy addresses this issue specifically.

"No member shall be disadvantaged in her career, including limitations in her assignments (except in the case where she voluntarily agrees to accept an assignment limitation), performance appraisals, or selection for professional military education or training, solely because she has taken maternity leave," according to the message.