One reason for the lack of rules is that women make up only about 4 percent of the nation’s 159,000 certified airline pilots — a number that has been slow to rise over the past decade or so.

There were no female pilots at the biggest airlines until 1973, when American Airlines hired the first, Bonnie Tiburzi Caputo. In a reminder of how times have changed, that news was reported in The Los Angeles Times under the headline, “Airline Pilot to Fly by Seat of Panties.”

“Airline jobs were really reserved for men,” said Captain Caputo, 67, who became something of a minor celebrity when American hired her. She has been retired from the airline for about 18 years. “When we started, there were no maternity leaves, because there were no female pilots.”

More than 40 years later, the major carriers still haven’t resolved this issue. They set their policies for pilots based on the collective bargaining agreements negotiated by the unions. But women of childbearing age account for just a sliver of union membership, so maternity leave and breast-feeding policies have not been at the top of union agendas.

Plus, some members oppose the proposals, citing the costs. One local union leader told several women in an email: “Having a child is a personal choice and asking the rest of us to fund your choice will be a difficult sell to the pilot group.” The leader declined to be interviewed for this article; the union said he was not an authorized spokesman.

Delta’s female pilots still hope to win over a majority of their colleagues. They argue that without paid leave, they’re faced with a choice to either stay home to breast-feed their babies or earn income for their families.