BY FAR THE MOST interesting news story during the past week was the report that it is expected that more babies will be born in the United States this year than in any year since 1971. The birth rate will probably be at least 2 percent more than last year. That is not all. The experts say that moderate increases in the number of births will continue for some years.

Now, that's what I call news. Important news. It is not suggested that there will be a repetition of the baby boom of the late 1940s and 1950s. But a sustained rise in the fertility rate could at once transform the mood of this country more than anything else. There is much talk of revitalizing the economy. But here is the hint of a revitalization of its very life.

The experts have been taken by surprise. Social scientists and pollsters and government still cannot get into the bedroom to find out what is going on there. It is the last fortress of the individual. The prying agents of modern society have therefore been reduced to talking about sex as if it is only marginally related to reproduction. But, lo and behold, people have been making babies again, like they used to do, in the rather confusing way which God ordained.

I feel entitled to speak of this with some directness and even asperity, because in 1977 I wrote an essay for this newspaper in which I said that babies were coming back. The skeptical and the incredulous asked me what evidence I had. "My ears," was all I could say. I had been listening to how people, especially young women in their child-bearing years, were talking about having children.

It turns out that I was not far wrong. The rise in births began in 1976, it faltered a bit at the end of 1977, and then it picked up again in 1978. But still the experts were not listening. They said that after 1978 the number of births would again begin to fall. But this has not happened.They had been looking only at their demographic statistics. They had not been listening to conversations.

For there is something much more important than a rise in the absolute number of births which they have now to explain. There is a significant rise in the fertility rate of American women after its long decline in the previous 20 years. It is this "total fertility rate" that is crucial. The fertility rate is a measurement which indicates the average number of children born to women of child-bearing age.

An increase in the absolute number of births could be explained by demographic factors. The continuing immigration of Hispanic aliens is obviously significant. But what has really counted is that the children of the baby boom have been reaching the prime child-bearing years. As long as young women of that generation were reaching 25, their sheer numbers would offset any decline in the fertility rate. One may also put among these more or less mechanical explanations the increased concern of women about the use of oral contraceptives.

But still the demographers have had to face up to something more. And one may say this for demographers: Unlike other statisticians, they are humble when they are proved wrong. When men and women in their beds don't behave as the statistics say that they will behave, the demographers almost shout "Hallelujah!" at the discovery that individuals are beyond their understanding.

It is because they suspect that the increase in the fertility rate will continue that they no longer say that the absolute number of births will now begin to decline again. Something much deeper than the demographic factors -- less fathomable -- has entered the calculations which they try to make. We are here at the profoundest level at which human nature is separated from animal nature.

So what are the demographers now saying as they gaze on this new evidence of human waywardness and willfulness? They are saying a whole mouthful, much of which is deeply significant.

They notice a "now or never" attitude in women who in their early 20s rejected the idea of having babies. The number of women who had their first child between the ages of 30 and 35 increased by 37 percent between 1975 and 1979. Many of those who set out to make only careers are now giving up their careers at least temporarily. As one of the authorities puts it, they are "ahead of the women's magazines."

They are saying that there is a return to "traditional values," a questioning of the social experiments of the last two decades, a highly articulate repudiation of "far-out life styles."

They are saying that this is evident also in the younger women between 20 and 29 years, that perhaps no one can say what is going on, that perhaps it's "just fashionable not to postpone any longer." But whatever it is, it is happening. This is why the rise in the fertility rate, as distinct from the increase in the absolute number of births, has upset the earlier predictions.

Let us leave aside the deep personal reasons why people -- women but also men -- may have decided that they want to have babies after all. Let us even leave aside the interesting desire to return to more traditional family lives. For what matters to us collectively is when significant numbers of individuals make the same decisions at the same time. What concerns us is when the individual decisions add up to a social movement, and a rise or fall in the birth rate is the profoundest of social movements.

As he contemplated the steady decline in 1976, Daniel Patrick Moynihan asked what it meant. "I will offer one guess. We will get glum. A people who don't reproduce themselves are saying something. I'll warrant it's not anything cheerful." He quoted a distinguished professor at John Hopkins who had said that much of the gloom of the democratic nations in the 1930s was a response to the thought that they were dying out.She also said that this explained much of the fury of the totalitarian nations.

One can only add, in this intriguing field in which we can only speculate, that historically it is true that a declining fertility rate has always reflected a nation or society that is glum. A rise in the birth rate has similarly reflected and been accompanied by a return of confidence and even cheerfulness in a society. It suggests above all a recovery of confidence in the future.

One does not want to rest too heavily on the moderate increase in births that is now taking place. But a 2 percent rise in the fertility rate is not insubstantial. It is even more substantial in some cities, like Chicago, and some states, like Louisiana, which leaves one wondering what may happen nationwide. For I now predict not only that the increase in the fertility rate will continue, but that the rate of the increase in that rate will also rise. Perhaps, not a boom, but more than a boomlet.

It is surely significant that what one's own ears can tell one is that it is among the very class and groups which spurned the traditional values and indulged in social experiments in the 1960s that there is now the most articualte decision to have babies again in the prime child-bearing years, for the most traditional reasons and in changed but still traditional family settings.

Careers may not be being put second, but they are being made first equal. It is here that some of the changes of the past few years may now assist couples to have babies instead of not to have them. Changes in the roles of husbands and fathers; more flexible arrangements for working outside the home; forms of day care which are not just improvisations; and, above all, ways of ensuring that careers which are interrupted are not thereafter handicapped. All of these are the creative aspect of the recent experiments.

I wish that Women's Liberation, which has achieved so much that was necessary in changing some attitudes, would think more deeply about its next steps. I wish that it would realize that its real success has mainly been in liberating women to be happier and better wives and mothers and not in preventing them from being wives and mothers. I wish it would begin to ally itself with and not fight against our biological natures as men and women and so also the deepest of our emotional satisfactions.

But I wish also that those who have been most alarmed at the experiments of the last two decades would have more confidence in people's sense of their own natures, and stop going around bemoaning the permissiveness all the time and so welcoming and even encouraging every sign of a backlash. We don't need a backlash, a reaction to the past; we need a reassertion, and that we may be getting.

For although there have indeed been danger signals -- as there are in any revolution -- there has been no reason to expect a Gadarene rush down the slope. The vast majority of individuals have too much self-respect to slither into a life of smut, casual relationships, abortions and willful repudiation of both childbearing and the rewards of a familiar day-to-day compainship.

There is no less true and may even be more true of those who undertake the burden of change. Since they begin by quivering at what needs to be changed, they are likely to quiver at what may in the process be lost. One has only to let pornography have its head, and all but a few are very soon bored by it. One has only to let people indulge for a while in promiscuous and unfeeling sex, and all but a few soon weary of it and protest that they are not just animals.

One has only to give a license to four-letter words, and all but a few soon stop using them automatically. One has only to release people from the rule of manners, and all but a few soon ask that they should be treated with manners. . . . And so one could go on. Of course it has been necessary to have those stood guard while the experiments were tried. But that is not the same as reaching for the censor or an inquisitor. It is not the same as reaching for the mere trappings and not the substance of authority.

Beyond even all that, perhaps our politicians and preachers and solemn men should stop talking of malaise, especially the malaise of America. If the women of America are ahead of women's magazines, then perhaps Americans of both sexes are ahead of their leaders. For it is not out of despair in the future that the conversation of men and women leads them to have children.

It was hardly a surprise to read in the Washington Business" section of this newspaper the other day that baby boutiques are doing a thriving trade. The next step is for the women's sections to catch up. Prepare yourself: They will soon be full of baby talk again. Oh, my, one does not live and learn. One lives, and finds oneself back where one came in. It is pleasant that the babies of the bay boom are now glad that they were born and think that their children will be glad that they were born as well.