If somebody asked you to guess their birthday, no extra information given, you might find yourself at a bit of a loss. After all, the chances are one in 365 that you're going to get it with a random guess, right? And who’d take odds like that?
Well, it turns out you might be able to game the system a little bit. There is one day of the year when people are way less likely to be born than any other: December 25, also known as Christmas Day.
This isn't a joke – and it's not a probabilistic paradox like the birthday problem, either. It's a genuine statistical fact, found originally for a 1999 paper exploring a quirk of the US tax system.
“Dec. 25 is the least popular day in the U.S., Australia, and New Zealand to give birth. In England, Wales and Ireland, it’s the second-least popular, behind Dec. 26, when Brits celebrate Boxing Day,” explained economist Jay Zagorsky in a 2019 article for The Conversation.
Using information from US natality records spanning from 1978 to 1992, researchers Stacy Dickert‐Conlin and Amitabh Chandra were able to list every single day of the year in order of how likely a birth was to fall on it – and in 2006, that data was collected into a list published in the New York Times.
Discounting February 29 – as it only turns up once every four years, it kind of has an advantage in this respect – there was a clear winner in terms of popularity. Or rather, a clear loser: out of every day of the year available, none was less likely to be a birthday than December 25.
But that's weird, right? If it were a random date – one with no particular cultural significance – then it wouldn't really be that interesting. But Christmas day is different. So, is it just a peculiar luck of the draw that has made this traditional holiday the one day of the year that people don't feel like giving birth? Is there some biological variable coming into play? Or is it something entirely different?
Well, researchers have found biological reasons for the yearly ebb and flow of birth rates. Late summer and fall babies are more common than any other birthday for example, and that is thought to be due to a range of factors that link our bodies to the changing world around us.
“Hypotheses include deterioration of sperm quality during summer, seasonal differences in anterior pituitary-ovarian function caused by changes in the daylight length, and variation in quality of the ovum or endometrial receptivity,” explains a 2001 paper in the journal Obstetrics and Gynecology.
“Increased sexual activity associated with end-of-year holiday festivities has also been postulated as a possible behavioral explanation for the December peak in conceptions,” the paper adds, although “the exact reasons remain unknown.”
But while that may explain why there are so many September babies out there, it doesn’t really account for the lack of Christmas birthdays. After all, go back nine months from the holiday season, and you’re at late March – hardly the sperm-crushingly hot midpoint of summer. So, what gives?
In fact, the reason you're especially unlikely to give birth on Christmas Day is almost disappointingly practical. Far from being a wild coincidence, the reason that people aren't born on holidays is, well, because they’re holidays.
“All of the least-favored days in the U.S. are tied to holidays, whether it’s Christmas, New Year’s, Fourth of July, or Thanksgiving,” Zagorsky wrote. But “depending on the year and place, between 30 percent and 40 percent fewer babies are born on Dec. 25 than on the peak day of the year.”
While you may think of your date of birth as something entirely out of human control, in this modern world the truth is that it’s often far more scheduled than we tend to realize. “Almost no cesarean births are scheduled by doctors to happen on public holidays or weekends,” Zagorsky pointed out. “About one in three American babies are born this way.”
Even among babies born vaginally, more than one-quarter of births in the US are medically induced. That, too, is less likely to happen when doctors would rather be celebrating the holidays with family than at work – or, for that matter, when the pregnant people themselves would rather be receiving presents than episiotomies.
So, if you're one of the relatively few people who have had to spend their life giving everybody else presents on your birthday, you can at least take solace in the knowledge that you are a rare breed. Up to 40 percent rarer, in fact, than these ten-a-penny September births.
And the fact that your special birthday was likely only due to your mom’s OB/GYN being sozzled on eggnog? Well, we won’t tell if you don’t.