Q.On two recent occasions that involved different age groups, I was taken aback by a young mother either raising her sweater or unbuttoning her blouse to nurse her baby. The first occasion was the least embarrassing for all concerned because only family members were there.

The other occasion was far more shocking, in that it involved at least 15 unrelated people, from young children to a 90-year-old woman. In between were adolescents, young singles and young and middle-aged married couples. I was so stunned I didn't know where to look as she nursed with her breast in full view.

Without wishing to criticize today's cultural mores, I still believe each of us must respect the customs held by people of different age groups. What has happened to manners in general?

A. Sometimes a mother nurses A. openly to show how liberated she is -- which is really a political statement -- but she usually goes public for other reasons.

A new mother is generally quite discreet at first, but soon finds it hard to think aboput modesty when she -- and the baby -- spend so much time at home in a state of undress and semidress. It isn't that she's so conscious of her body; it's that she becomes unconscious of it. She's just too tired, and too focused on her child, to think much about anyone else, even when she's visiting.

This is because she sees everything in relation to her child, not the child in relation to the world. When she's with others, she doesn't wonder if the baby will disturb them, but if they will disturb the baby. You may think she's egocentric, but she's not; she's babycentric. She finds it hard to understand another point of view when she's holding the most important, beautiful, intelligent child the world has ever known. How can anyone possibly object to anything that's done for such an amazing person?

But they do, and your letter is a good example.

Breast-feeding hasn't been a public custom among the middle and upper classes for centuries -- if only because clothes were so restricting -- and even breast-feeding in private became rare between the 1940s and the '70s. This made our society that much more opposed to any public display, despite our new styles and easy ways.

While many people think the sight of a nursing babe is the ultimate in beauty, others get embarrassed. A 16-year-old boy is sure to appreciate the breast more than the baby -- which will make him feel ashamed -- and even older men, trained on Playboy, may be uneasy. A nursing mother also may make an older mother feel guilty if she didn't breast-feed her own child. It shouldn't be that way, but it is.

That's why a nursing mother should be discreet, wearing a blouse she can open with modesty and covering her breast with a light shawl when she's nursing. She even might offer to go into another room if she thinks the hostess or one of her guests would be offended.

This may sound terribly old-fashioned, but good manners are no more than kindness, caring and sensitivity. This doesn't mean, however, that others have the right to ask her to leave the room when she nurses. That would be at least as unkind, uncaring and insensitive, for new mothers are so psychologically vulnerable.

It also would take away much of the gaiety a party brings. An adult gathering is a big event to a housebound mother, even though she doesn't talk about anything else but the baby (and resents it a little when anyone else does). To leave the party unwillingly would only add to the occasional sense of martyrdom that comes with parenthood.

At the most, the hostess might tell her -- before the fact -- that she is welcome to use the bedroom when she wants to nurse.

A little seclusion isn't such a bad idea in the early months. Both she and the baby would profit by 15 minutes of quiet nursing. Lying down together or cuddling in a chair is a precious time. The more relaxed the mother, the more milk she'll produce, and that's what is important.

Breast milk is almost always better for a child than formula. It not only gives the mother time to hold the baby close -- something parents may forget to do when they give their baby a bottle -- but it contains more nutrients and it gives the baby more immunity to allergies and illnesses. Above all, it is 'species-specific,' as they say in veterinary circles. Human milk is meant for little children and cow's milk is meant for calves.

Even though it may be indiscreet of a mother to nurse in front of teen-agers, singles and the elderly, she still should be appreciated for doing her job so well. A well-nourished child is a healthy child and a gift to all of us.