BEING CONSIDERATE of one's own mother requires no particular instruction, because everyone does that instinctively. (The exception is that children must be taught that mothers should be assumed to despise breakfast in bed unless they specifically state otherwise.)

But knowing how to treat other people's mothers politely does not seem to come naturally.This is especially noticeable in the way that people treat pregnant women, possibly because pregnant women are especially noticeable. A mother cannot be indentified with such certainty in the post-pregnancy stage - there could be another reason for a woman's keeping a stuffed animal in her purse, or having a habit of saying. "Now, you stop that."

But even then, mistakes are possible. Miss Manners' first rule of respect toward potential mothers is to allow to make their announcements themselves. To inquire of a person whether she is pregnant is to ask for big trouble in this girdle-free era when so many people believe that frozen yogurt has practically no calories.

As a return courtesy, the prospective mother should time the announcement properly, that is to say at four to five months before the birth announcement. Anything earlier only prolongs the period in which she will have to engage in inane chatter with everyone she meets.

Patting the pregnant stomach is in the same category as other bodily contact. Except for the prospective father, who has reason to assume that his advances are acceptable, no one may be certain, without inquiring, that physical attentions would be welcome.

Similarly, onlookers who know where babies come from should be careful not to stress this point when addressing pregnant women. Remarks about women's breats, for example, are as vulgar at this time as at any other, and unsolicited appraisal of the fruitful body is no more adorable than other such critiques.

Naturally, carrying a baby about under one's dress is an open invitation for advice from everyone. If a woman does not want others to tell her how to run her life, she should not be having a baby because it is only a matter of a few years before that baby will consider this one of his or her chief duties.

However, this does not include advice on whether and when to have a baby, which is of less practical value to pregnant women than their friends seem to suppose. The time when people saw fit to ask if the deed were done on purpose may have passed, but only to be replaced by the assumption that information on the problems of over-population would be welcome.

The greatest non-question is to ask prospective parents whether they want a boy or girl - with the hope of using their answer against them later - or perhaps it is to ask the prospective siblings whether they want a brother, a sister or neither.

Nor do stories of difficult labors or birth defects make appropriate conversation to pregnant women. What you may tell them is how lovely your children are, and how wisely you brought them up.

And you may smile at them a great deal. They just have to learn to take that with good grace.