THE LATEST INSTALLMENT in the continuing saga of the perils of nursing mothers occurred last week at a regional postal facility when a woman employe was told she could not use the first aid station to pump breast milk for her child. The woman was not trying to nurse her child in public and she was not trying to bring her child into the post office or anything wildly progressive and risque like that. All she wanted to do was to extract the milk she produced while working at night so her husband could feed it to their baby from a bottle when she was gone.

Now you might conclude from this that our heroine had devised a clever, efficient, private way of continuing to feed her four-month-old baby breast milk -- the most digestible, best food you can give a baby -- while returning to work and helping the family budget. You might even think that the woman had found a solution that other corporations could use to accommodate their nursing employes. First aid stations provide more privacy then ladies rooms, and usually have refrigerators where the milk can be kept until the employe goes home. But this is not the way someone at the post office saw it.

The woman used the first aid station once and then was notified by a nurse supervisor that it was not, as The Washington Star neatly put it, the policy of the post office "to allow federal facilities to be used for draining breast milk."

Our heroine, according to the paper, was told she could use her car.

Our heroine was also hopping mad. At one point, she even took two hours of leave without pay to drive home and leave breast milk in the refrigerator and drive back. She also told a neighbor what was going on and, this being Washington, it turned out that the neighbor's husband had worked on the House Post Office and Civil Service Committee. He made the right phone calls, and by the next afternoon, he found out that there was no such policy, that an arbitrary decision had been made by someone at her post office, and that the decision was being reversed: our heroine could use the first aid station for as long as she wanted.

Jeanne O'Neill, a media relations officer for the U.S. Postal Service, said someone at the regional facility arbitrarily decided the first aid station should only be used for injuries and illnesses people get on the job. "It was a problem about understanding this was not going to go on forever, that it was something that would be a convenience for a certain amount of time."

When the decision was appealed, she said, "our people here said our national policy is to accommodate our people in a situation like this and to suit medical needs within reason.

"We value our employes . . . We pay a lot of money to train our employes. This woman is a highly skilled employe. She took her maternity leave and then she came back to work, and it would be foolish not to accommodate this."

Of course, it was foolish. Nursing a child probably doesn't seem terribly important to people who don't have children or haven't been involved in the business of nursing babies, but to those who believe breast feeding is vastly better for their children, this is not a trifling matter.

Studies have repeatedly shown that breast milk provides nutrients and immunities that are particularly beneficial to children during the first year of life. Mothers who insist on breastfeeding are making a decision that has to do with their child's physical and emotional well-being.

Our heroine, for example, was prepared to drive two extra hours a day to home and back, and to take leave without pay, in order to continue feeding her baby breast milk, instead of cow's milk or formula.

She is hardly alone in this. The Journal of the American Academy of Pediatrics reported in December that American mothers are turning to breast feeding in sharply increasing numbers. The percentage of infants who were breast-fed in hospitals jumped from 24.7 percent in 1971 to 46.6 percent in 1978. The number still breastfeeding at five months jumped from 5.5 percent to 20.5 percent. Since working mothers rarely stay home with their babies for six years or even six months anymore, the figures suggest that there are thousands of working mothers who are still nursing babies after they go back to work.

Working mothers who breastfeed is a phenomenon that is not going to go away.

Linda Eaton, the Iowa City firefighter, made breastfeeding a cause celebre when she went to court to win her right to feed her child at the firehouse. Our heroine at the post office was fortunate enough to have a neighbor who knew how to find out what is and what isn't policy in Washington. So for her, the matter was settled rather quickly. But both women had to endure considerable unpleasantness and publicity, and perhaps some embarrassment, simply because they insisted on doing the best thing for their children.

Corporations -- far from inconveniencing their nursing employes and making life hard for them -- ought to understand what the women are doing and why, and ought to appreciate women who have figured out ingenious ways of managing to come back to work while they continue nursing.

Someone at the post office expressed concern that our heroine might set a precedent by using the first aid station, and indeed she might. The next woman postal employe who wants to continue nursing after she comes back to work now knows a way of doing it.

Our heroine had the kind of good idea other corporations might pick up on, too.