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.
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.