Centenarians are different from the rest of us, and it's not just that they are a lot older. They are a select group, having persisted through wars, diseases, disasters and accidents that kill tens of millions of ordinary mortals every year.

In looking at what makes a 100-year-old so special -- an extremely small percentage live to that age or older -- those who study aging cite factors such as genetics (particularly having two X chromosomes, as 85 percent of centenarians are women) and environmental influences, including good nutrition and health habits.

But a statistical study of centenarians by researchers at the University of Chicago has found other potential predictors of extreme longevity.

Women and men who were the first born in large families, the study found, were two to three times more likely to make it to 100 than later-born children. Those raised in the rural West had a better chance of reaching 100. And those born in October and November had longer life expectancies than those born in April through June.

So if you are a fall baby, the first child of a farming couple from Boise, are you a safe bet to make it to 100?

Hardly, experts say. Factors such as birth month play a small role, at best, in the likelihood of reaching advanced age and are overwhelmed by others. Still, the odd predictors may say interesting things about what makes one person live so long.

"In the case of birth order, we were really surprised," said Leonid Gavrilov of the National Opinion Research Center at the University of Chicago.

Gavrilov and his wife, Natalia Gavrilova, sifted 75 million computerized genealogical records to identify centenarians born from 1875 to 1899, then consulted Social Security death records and census data to find more about the circumstances of their lives. The study was prepared for the Society of Actuaries.

What may improve the chances of first-borns reaching 100 is that, in general, they are born of relatively young parents. Sperm and eggs can become damaged over time, so children of older parents may have more health problems related to genetics.

Similarly, birth month -- which has been correlated to life expectancy in other studies -- may be an indication of seasonal vitamin deficiencies or seasonal illnesses such as flu that affect childhood health, with an impact later in life.

In the United States, those health problems were more serious a century ago, when current centenarians were born, than now, when foods are fortified, fresh vegetables are available year round, vaccines are commonplace, and sanitation is better.

"Those are things that we just wouldn't experience these days," said Thomas Perls, a professor at Boston University Medical Center and director of the New England Centenarian Study. "That was a time when people lost a quarter of their kids to childhood illnesses."

So birth month, even if it had some importance in the late 19th century, may no longer have much of a role in longevity.

If so, those who study the very old will have plenty of chances to find out, because the number of centenarians is booming. From the approximately 50,000 centenarians counted in the 2000 census, demographers expect the number to soar; one census estimate is there will be about 800,000 by the middle of this century.