WASHINGTON (Reuters) - The U.S. Navy is tripling paid maternity leave for women who serve in the Navy and Marine Corps from six to 18 weeks in the first year after birth.

Secretary of the Navy Ray Mabus said on Thursday the change was effective immediately and would be retroactive to Jan. 1. A mother does not have to take the leave at once but may use it within a year of her child's birth.

"When the women in our Navy and Marine Corps answer the call to serve, they are making the difficult choice to be away from their children - sometimes for prolonged periods of time - so that they can do the demanding jobs that we ask them to do," Mabus said in a statement.

The 84 days, or 18 weeks, of leave replaces the previously authorized 42 days, or six weeks, the Navy said.

The 18 weeks of paid leave is generous by American standards.

U.S. President Barack Obama in January directed federal government agencies to advance six weeks of paid sick leave to workers with a new child and asked Congress to pass legislation giving them an additional six weeks of paid parental leave.

(Editing by Sandra Maler)