Aug. 16, 2005  -- This is the time of year when the storks get a workout and the grim reaper can let his minions schedule a vacation.

It's August -- the height of baby season, and the time of year when Americans are least likely to die.

"There's no if, ands or buts," said Beth Ann Roberts, nurse manager of labor and delivery at Baptist Hospital in Miami. "We have a definite influx [of babies] when it comes -- July, August, September."

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In most recent years, August and September are the top months for births in the United States, according to federal statistics.

August had the most total births every year from 1990 to 2002 except for four -- 1992, 1993, 1997 and 1998 -- when it was edged out by July.

September, a day shorter than July and August, averaged the most births per day over the period from 1990 to 2002. (Click HERE for a table of month-by-month births.)

Demographers and sociologists long have puzzled over the mysteries of America's birth seasons: Why do births peak in the fall almost every year, and then dip in January and again in April? Why do other parts of the world have their own repetitive patterns, in some cases practically the opposite of America's? And why do birth seasons occasionally change over time?

"There is no consensus that I'm aware of as far as a factor, or the factors, that contribute to this," said Brady Hamilton, a statistician and demographer for the reproductive statistics branch of the National Center for Health Statistics. "It seems to be a myriad of different hypotheses."

Death Trends More Predictable

Officials are far more settled on why deaths tend to dip at this time of year and surge again in the winter months. On a day-by-day basis, August has the fewest deaths, while January has the most. (Click above for a table of month-by-month deaths.)

"This is consistent with what we know about the effects of winter and the flu season on those that are sick and frail," said Robert N. Anderson, chief of the NCHS's mortality statistics branch.

Deaths are fairly evenly distributed over the days of the week, Anderson said, with a barely perceptible increase on weekends, perhaps attributable to factors like alcohol and accidents.

Births, on the other hand, vary more by day of the week. They're most common on Tuesdays, and least common on weekends. The weekly pattern seems to be caused by doctors scheduling induced births and ceasareans early in the week, and clearing the slate for weekends. The experts tend to agree on that.

But despite several studies over the years, theories abound on why some months consistently have more births than others.

On a hospital-by-hospital level, some barely even see a pattern.

"If we see an increase in the number of births or have to schedule a C-section, we have to staff up," said Ellen Painter, director of marketing at Presbyterian Hospital of Denton in Texas. "But as far as seeing September and saying we have to staff up, we don't do that."

Not Universal

The birth rate seems to be more than just a natural human rhythm. After all, the modern American birth pattern, though common in some other parts of the world, does not prevail everywhere.

Births in parts of Europe tend to peak in spring -- just as U.S. births dip -- and generally decline through the rest of the year. Some studies found a secondary peak in September in parts of Europe.

Globally, the European-style spring peak in births "has been considered to be a basic rhythm of human reproduction," according to an online summary of a 1987 Japanese study.

But patterns in specific cultures have changed over time. The 1987 Japanese study of birthrates in Japan, the United States and other countries found births surging in different months and seasons during different historical eras. It even found cases of contrasting birthrate patterns for separate ethnic groups within a single region.

In the United States, there once was a large secondary surge in births from January to March. It largely subsided between 1933 and 1963, the span examined in a 1966 NCHS study of birth data. However, even then, August and September clearly were the peak months for births.

Dividing the United States into four regions, the NCHS study found the South had the greatest monthly fluctuation and the Northeast had the least. It found "non-white" mothers exhibited greater birth seasonality than whites. And comparing U.S. birthrate patterns from 1957 to 1959 to other countries' patterns over the same period, the U.S. appeared to differ from its neighbors -- Canada and Mexico -- and most closely to resemble Southern Hemisphere countries, particularly the southern part of Australia.

Incidentally, the rate of births may not be the only thing subject to cycles. The timing of births also may count, according to the Japanese study. For instance, Japanese children born at different times of year in the mid-20th century showed tendencies toward differences in birth weight, health and birth defects, as well as later athleticism and physical development. Studies of different locations and eras around the world revealed varying patterns of twinning rates, and changing seasonal ratios of boys to girls.

Some experts suspect human biology may contribute to birthrate surges.

For example, the Japanese study suggested a mother's own birth month might play a role, perhaps by giving her biological immunity to "epidemic seasonal infertility" that affects women born in other months.

Daniel A. Seiver, a visiting professor of economics at the University of San Diego, has theorized that human sex hormones may fluctuate by season, possibly affecting birth rates.

Too Hot?

Perhaps more commonly, studies have speculated that climatic or seasonal factors -- such as responses to heat, cold, sunlight, rainfall or agricultural cycles -- may contribute to fluctuating birthrates.

A Brown University study of Senegal's birth patterns found rural mothers had more pronounced birth seasonality than urban dwellers, with a tendency to have babies starting in February, in the dry season after the harvest was complete.

Seiver believes the growing ubiquitousness of air conditioning may be altering birth seasonality in the United States. Seiver's studies at Miami University of Ohio noted that the traditional "April trough" in births, presumably caused by fewer conceptions in summertime, has been falling in recent decades, particularly in the hot South, where it was most pronounced. Nationally, April births once fell 20 percent below the fall peak, he said, but for 1995-2002, the span was just 7.4 percent, and January had displaced April as the slowest month for births.

Must Be a Full Moon

Though heat may be a factor, another usual suspect definitely is not, according to Dan Caton, professor of physics and astronomy at Appalachian State University in West Virginia. He tested the old wives' tale (or old midwives' tale?) that babies come during full moons and found no correlation between the phases of the moon and births.

"If there were any true effect, it would have come out, and there's just nothing there," Caton said. "But the myth will outlive me."

Indeed, Roberts, the nurse manager for labor and delivery at Baptist Hospital in Miami, said her nurses still believe the legend: "When there's a full moon, when there's a storm brewing, we feel there are more people coming. Is it a proven fact? Of course not. … But if you ask most labor room nurses, they'll swear by it."

Caton thinks he knows why the perception defies the statistics.

"The moon you're most likely to see is the full moon because it rises at sunset and sets at sunrise," he said. If you're a maternity ward nurse, "you're going to remember those occasions, but you're not going to remember others -- the equally busy nights when you came out and you weren't distracted by the full moon. So the myth will live on."

'Elder Fire Horse' Effect?

Some suspect culture may contribute to the different birthrate cycles in different countries.

In religious Christian countries, for instance, the Christmas holidays might spur conception -- and, by extension, a fall birth surge like the one seen in the United States. However, at least one study noted fall birth surges in India and Israel, two generally non-Christian countries.

Perhaps Europe's cycle could be influenced by a greater amount of vacation and leisure time in the summer -- causing more summer conceptions and accounting for the spring surge in births.

Some have speculated people in certain countries may try to time their births for culturally auspicious times, to avoid inauspicious times. For instance, Japan saw a sudden dip in registered births in 1966, the year of the "elder fire horse" in that country's traditional 60-year cycle.

"There is a superstition that girls born in the year of the elder fire horse, 'Hinoe-Uma,' will kill their husbands," said Ron Rindfuss, a professor of sociology at the University of North Carolina-Chapel Hill, and a fellow at the Carolina Population Center. "Prospective parents, fearful about the marriageability of their daughters, prefer to avoid having a daughter born in the year of the elder fire horse."

Baby Booms and Busts

There may even be evidence of a cultural event's short-term impact on births in America: Births of Southern white babies dipped for a time following Brown v. Board of Education, the landmark 1954 Supreme Court decision that racially integrated America's schools, according to a study by Rindfuss.

"That one, to me, at least makes good intuitive sense," Rindfuss said. "The world in which their [segregation-era Southern whites'] children would go to school potentially was radically altered."

Similarly, a Penn State University study found an increase in marriages, births and divorces in regions hardest hit by 1989's Hurricane Hugo, and Roberts believes her Miami maternity ward sees the effects of hurricanes nine months later.

But demographers voice skepticism about most claims of baby booms or busts following disasters and landmark events -- because the media often tout them using anecdotal evidence, but they seem rarely to pan out upon scientific examination.

For example, several experts said the numbers show no indication of a post-9/11 baby boom, despite widespread belief following the attacks that such a boom would occur.

J. Richard Udry, a professor emeritus of sociology and public health at the University of North Carolina-Chapel Hill and fellow at the Carolina Population Center, likewise studied the statistics to debunk reports of a baby boom following the 1965 New York blackout.

"It's easy to think them up, and it's pretty easy to shoot them down if you take the time," Udry said. "We've all leaned to be more cautious on our pronouncements."

He paused, and then added, "Well, some people have learned, and some people haven't."