by James Hunt
At some point in their lives, everyone counts back from their birthday and tries to figure out what anniversary, special occasion, or other excuse might have happened to their parents nine months before they were born. To make this backtracking exercise easier—and give us the chance to do it for a much larger population—data journalist Matt Stiles created an interactive “heat map” showing the most common birthdays in the United States for individuals born between 1994 and 2014.
Click on the map and you’ll quickly notice that July, August, and September are by far the most common birth months. It’s no surprise that nine months prior you’ll find the dark and rainy period of October, November, and December when—to put it delicately—people have to make their own entertainment.
According to Stiles, “People generally seem to have time for baby-making during their time off. Several of the most common birth dates, in September, correspond with average conception periods around Christmas. September 9 is most common in this dataset, though other days in that month are close. September 19 is second. Following a customary gestation period, many of these babies would, in theory, have been conceived on December 17 and December 27, respectively.”
But that’s not all we can tell from the chart. When you take into account the fact that some people get to choose their child’s birthday because of induced and elective births, they tend to want to stay away from the hospital during understaffed holiday periods.
“The least-common birthdays in this dataset were Christmas Eve, Christmas [Day], and New Year’s Day,” Stiles concluded. “Dates around Thanksgiving aren’t as common. July 4 is also at the bottom of the list. Conversely, Valentine’s Day ranks relatively high, as you can see in the graphic, as are the days just before a new tax year begins.”
Amazingly, though it only comes around every four years, Leap Year babies aren't as uncommon as you might think: February 29 ranked 347th out of 366 on the list.
You can play around with the interactive graphic, and see the full ranking of birthdays here. And when you’re done there, check out the map that will tell you how long it will take for that birthday card your grandparents sent to arrive.
A version of this story ran in 2018; it has been updated for 2023.