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The summer has been the top season for births in the United States, according to records kept by the Centers for Disease Control and Prevention, and August often wins the title as the month during which the most babies are born.

In 2006, the most recent year for which statistics are available, August had the most U.S. births, just as it did in 10 of the previous 16 years, with the CDC estimating that 9 percent of all births occurred then. February had the least.

The number of births for each month of 2006.
The number of births for each month of 2006.
Credit: CDC.

T.J. Mathews, a demographer at the CDC's National Center for Health Statistics, said that while August sometimes has the highest number, in other years it's July or September.

August had the most births each year from 1990 to 2006 except for six years (1992, 1993, 1997, 1998, 2003 and 2004) when it was edged out by July, according to National Center for Health Statistics.

Historically, the sweltering, late-summer months are when obstetricians witness an increase in the arrival of newborns.

"It must have something to do with the time of conception and whether there are timing issues with the outcome of pregnancy, besides just a live birth," Mathews told LifesLittleMysteries.

Why August?

Seasons and weather conditions play a role in birthdays, according to Paul Sutton, another CDC health statistics demographer. He theorizes that as the mercury drops in the late fall and winter, things heat up at home as people spend more time indoors. About nine months later, a summer baby is born.

The most common day of the week to deliver in 2006 in the United States was Wednesday, stealing the No. 1 spot from Tuesday, which had been the most common birth day since 1990, according to the CDC.

Worldwide birthing statistics vary with the climates and cultural factors that influence the timing of conceptions and births in other countries.

For example, a study conducted by the University of Occupational and Environmental Health in Kitakyushu City, Japan, noted that Japan experiences two peaks in birth rates throughout the year one lasting from December to February, and the other in August and September. The researchers speculated that the popularity of springtime marriages in Japan and seasonal temperature variations both may play a role in the timing of these peaks.

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