The annual number of births in this country has climbed to 4 million for the first time since 1964, the last year of the postwar Baby Boom, amid signs of a surprising increase in childbearing among women of every age group and in all regions of the country.

The National Center for Health Statistics estimates that there were 4.1 million live births in the 12-month period ending last Sept. 30, a jump of 4 percent from the previous year.

"If it were a 1 percent increase, a couple of things could conspire to do that in any year. But it's the 4 percent increase in one year that really caught my attention," said Peter Morrison, a demographer at the Rand Corp. "We don't really know why it's happening."

For some time, birthrates have been rising among women over 30 as they made up for postponed childbearing. But many population experts interpret the steep increase in the number of births last year as an indication that more women in their twenties are having children as well.

If true, that would signal a fundamental change in behavior, a departure from the unprecedented standard for delayed childbirth set by women of the Baby Boom generation.

While demographers are unsure of the cause of the increase, they agree on the implications: The biggest bulge of babies in 26 years is beginning its movement through society, with all the attendant demand for health care services, education and consumer goods. And, unlike the boom of the 1950s and 1960s, this wave comes at a time when most mothers are in the work force, creating a much heightened need for child care providers and facilities.

"A 4 percent increase is not trivial," said Charles Westoff, director of the Office of Population Research at Princeton University. "It means a 4 percent increase in everything that gets sold to children. I'm sure the diaper industry is pleased to hear it."

Demographers also say a small, but disturbing, factor in the rise in births could be an increase in deliveries among teenagers. Those rates had been declining through most of the last two decades, but turned up again in 1987 and 1988, particularly among girls 15 to 17.

"That's not a big increase -- it's just troubling," said Art Campbell, deputy director of the Center for Population Research at the National Institutes of Health. But we "thought that rate was on the way down."

The number of annual births in this country peaked at nearly 4.3 million in the early 1960s -- the height of the Baby Boom -- then fell to 3.1 million in the 1970s. Slight increases over much of the past decade began picking up in 1987 and made their biggest jump last year. The birthrate -- the number of births per 1,000 population -- also rose sharply in the 12 months ending in September, up 3 percent over the previous year.

It will be several more months before detailed information on birthrates by age and race becomes available, forcing experts to rely on two-year-old data for clues on the forces behind the recent, large increase.

"We don't know who's having these babies, except it has to be a lot of people," said Stephanie Ventura, a demographer at the National Center for Health Statistics, which is an agency of the Department of Health and Human Services.

The 1988 data show increases in the number of births in all regions of the country and in birthrates for all age groups.

The issue that divides population researchers is whether the new numbers reflect only the continued phenomenon of delayed childbirth among women in the Baby Boom or a change in behavior for younger women in their 20s.

Because of the size of the Baby Boom generation, even relatively low birthrates among these women can yield large numbers of babies. That explains how the annual number of births can rise when the average number of children per woman, at about 2, is significantly below the 1964 average of 3.2.

"The reason we have 4 million births now is not because of the rate, but because there's more women around," Campbell said.

But others believe there are more human factors at work.

Richard Easterlin, professor of economics at the University of Southern California, theorized that Baby Boomers had fewer children because, as members of a huge generation, they faced more economic competition, felt less financially secure and chose not to have large families.

By comparison, he said, women in their twenties today face less competition, feel more affluent and so are choosing to have children sooner.

Thomas Espenshade, a professor of sociology at Princeton University, speculated that women in their twenties are seeing "unanticipated negative consequences of extreme delays in childbearing," including the risk of infertility.

He said he believes these women may be seeking "a more satisfying balance between children and careers. . . . My guess is that there is underway a return toward a trend of somewhat younger childbearing."

Some experts also speculated that immigration could be driving some of the increase. Martin O'Connell, chief of the fertility statistics branch at the Census Bureau, said fertility is as much as 25 percent higher among foreign-born women than Americans born here.

"It's something that should be taken into account," he said.

O'Connell said he does not believe there is a significant increase in childbearing among women in their twenties and expects that births will decrease by the end of this decade with the aging of the Baby Boom generation.

Morrison searched for a change in the economic or social environment over the past two years that could have prompted more women to become pregnant.

"Somewhere along the line, something caused a lot of people to decide to have a child now," he said. "That force came into play in the last 25 months or so. I can't for the life of me fathom what it is."