The change, directed by Navy Secretary Ray Mabus, makes the Navy the first of the military services to provide more than the current six weeks of leave.
According to the Navy, about 5,000 women would be eligible for the benefit each year, including 500 officers.
The actual order extends convalescent leave for maternity cases only, and is effective immediately. Service secretaries have the authority to make the change themselves, and no change of law is required.
In a statement, Mabus said that the Navy and Marine Corps are routinely looking for ways to recruit and retain good people.
"We have incredibly talented women who want to serve, and they also want to be mothers and have the time to fulfill that important role the right way," said Mabus. "Meaningful maternity leave when it matters most is one of the best ways that we can support the women who serve our county."
The Navy said there shouldn't be any significant personnel costs associated with the change.
According to a Navy study, about 9 percent of enlisted women and 5 percent of women officers are pregnant at any one time.