The number of births in the United States increased so dramatically in the first seven months of 1990 that even scientists and social planners who expected an increase are being caught by surprise.
"The magnitude of the increase is greater than we've seen in recent years," said Richard Klein, a statistician at the National Center for Health Statistics.
The numbers of births reported to the center by the states for the seven months ending in July 1990 rose 4.5 percent over the same period in July 1989. It had risen 1.5 percent from the same period in 1988 to 1989, which was thought to be quite an increase at the time.
Demographers are hard pressed to explain the jump, which was revealed in preliminary statistics from every region of the country, although not in every state. Maryland showed a high rate of increase -- 13 percent -- but state officials said they had not yet had a chance to interpret the figures.
Ronald Rindfuss, a sociologist at the University of North Carolina at Chapel Hill who specializes in fertility rates, suggested that childbearing at opposite ends of the mammoth baby-boom generation -- women in their 20s and in their late 30s and early 40s -- may be overlapping. At the same time as more women are deciding to have children, some women may be deciding to have more children.
It is impossible, demographers said, to know if this signals the start of a new trend.
The recent change, while impressive, does not approach the famous baby boom of 1946 to 1964. From 1945 to 1947, annual births jumped 30 percent or 35 percent.
"I don't think any demographers expect a sustained and dramatic baby boom of that magnitude," said demographer Michael Teitelbaum of the Alfred P. Sloan Foundation in New York. At the height of the boom in the 1950s, women of child-bearing age bore an average of 3.7 children. The average now is estimated at about 2.1 children.
In some states, such as California and Texas, the increase may be influenced, demographers say, by large and varied ethnic and immigrant populations, some with traditionally higher fertility and with largely younger populations than the United States as a whole.
In Texas, births for the first seven months of 1990 were up 17 percent from 1989. In California, they were up 11 percent. That's fast, even for the fast-growing West.
But several social scientists said that differences in the fertility among ethnic groups can't account for startling increases in states with less dynamic populations. Besides Maryland, they listed Michigan, 16 percent; Connecticut, 10 percent; and Ohio, 6 percent.
"That is a striking increase. It sounds beyond belief," said Peter Morrison, a demographic analyst at the Rand Corp. "You don't see percentage increases like that in a one-year period. Maybe a three- or four-year period."
Like Mr. Morrison, other analysts said they wished they could get their hands on the accompanying details -- the mothers' race, ethnicity, marital status, age and income -- that could explain the change but won't be available from the health statistics center for another year and a half. There can be -- he and other demographers said -- only two possible causes: either fertility rates have taken off; or more women are becoming parents.
It is unlikely, Mr. Rindfuss said, that lots of families suddenly have decided to have a lot more babies. So that leaves the behavior of baby boomers to explain the change.
The reproductive history of the baby boom generation mirrors the history of the postwar years. The group was born following a period of extremely low births in the Depression and World War II.
Because of its sheer size, any behavioral change by the baby boom generation has an oversize impact.
When baby boomers first entered their 20s -- traditionally prime childbearing years -- the generation failed to fulfill its enormous reproductive potential.
Many boomers postponed childbearing. They waited longer and longer until it began to appear they would age into a post-reproductive period as a low-fertility generation.
Then, more change: In the 1980s, although most babies were born to women in their 20s, boomers began breaking records for babies born to women in their 30s and 40s.
Now, Mr. Rindfuss said, it is probably an amplification of that late childbearing, coupled with younger mothers giving birth in their 20s, that helps explain the recent jump.