Are More Babies Born In September Because It's Nine Months After Freezing Winter?

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If I were to guess which month holds the superlative of "most births," I would guess February. That's because it seems like every person I know or am related to is born between February 4 and February 8. (I went to four birthday dinners last week.) My guess, however, would be wrong. My own 10 friends are not exactly a proper data set. According to actual data, the most popular birth month in the U.S. is September. But why are more babies born in September than any other month?

Well, my first thought was: "What's nine months before September?" That would be December, the month that reminds us all that winter is coming and it's going to be cold. December is also a month full of boozy holiday parties and a week of vacation. Perhaps inebriated couples feel the impulse to cozy up and keep each other warm in December, which results in a myriad of babies being born in early September.

After considering how wrong my guess on which month has the most births was, I decided to look at the actual numbers. I spoke to Matt Stiles, a data journalist who covers Korea for the Los Angeles Times. Stiles is the creator of a visualization on The Daily Viz that uses data on birth dates from 1994 through 2014 to identify which birth dates are the most popular in the United States. According to this data, Sept. 9 is crowned "most popular birthday."

Stiles has not done his own study on why September begets the most babies, but his visualization has over 800,000 views, and I thought he'd have an interesting opinion on why September is such a hot month for welcoming infants into the world. Here's what he had to say:

The Daily Viz / Matt Stiles

Why Do You Think September Is The Most Popular Month For Birthdays?

"I don't have any hard evidence for why September is the most popular month — I haven't done my own study — but it's pretty clear that it's related to the holidays in November and December, when people have time away from work to, shall we say, focus on their families," says Stiles.

Perhaps it's not the chilly temperatures so much as the time of year in general. In the U.S., most people are busy with festive celebrations from Thanksgiving all the way through the new year. Often times, these holidays include lots of family time as well, so maybe the desire to procreate is heightened.

"If you look at the average gestational period for humans, the timing makes sense," explains Stiles. "The second most-popular birthday, for example, is Sept. 19 — placing the estimated conception date around Dec. 27, two days after Christmas. All the most-popular days fall around this general period. " I guess the proof is in the Christmas pudding.

What Else Can We Take Away From The Data?

The fact that September is a popular month for births suggests more than simply what month people are most likely to get busy during. "What's also clear from the data is that many Americans schedule their births through C-sections or inductions," explains Stiles. "Why else would July 4 be one of the least-common birthdays? Births on Christmas and Thanksgiving also decline."

Having a baby on Christmas has always seems like a lose-lose situation to me. The mother and father miss out on hanging out in their PJs opening presents all day, and the baby forever gets fewer birthday gifts than other children with their birthdays on say, Oct. 10th. It makes sense that if one were to plan a C-section, Christmas wouldn't be the most popular day to choose.

"On other seeming unpopular months, such as February, births spiked on Valentine's Day," adds Stiles. "And just before the end of the year, as the tax deduction deadline looms, births increased on average. These things are not coincidences."

It seems that the simple fact of the weather getting frightful in early December is probably less likely the reason for the spike in September births than the fact that it is a season ripe with holidays. The data suggests that many Americans are planners, even when it comes to deciding when to give birth. Humans never cease to fascinate me.

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