Jordyn and Olivia Campbell of Dawson Creek were both born on a leap day

Olivia and Jordyn Campbell like to joke that they are turning three and four years old on Thursday — despite being born four years apart.

That’s because the sisters, who are actually 12 and 16, share the same birthday: Feb. 29, a leap day that only comes around every four years.

“A lot of people are surprised when I tell them [my birthday],” Jordyn said from her home in Dawson Creek — a city in B.C.’s Peace River region around 740 kilometres north of Vancouver —  where her family was celebrating with a trip to the local rock climbing wall.

And when she tells them about her younger sister?

“They’re even more surprised,” she said.

The sisters — who have four other siblings — generally mark their birthday on Feb. 28 or March 1. While some “leaplings,” as those born on Feb. 29 are known, make a big deal of the quadrennial event with major blowouts or taking part in a leapling cruise, the Campbells take it in stride, getting ice cream cakes like they do every year.

“It’s kind of nice to have an official date, I guess,” Jordyn said.

One reason it may seem less impressive is that Jordyn and Olivia’s mother, Jennifer, also shares a birthday with her sister, born two years after her, so it’s a phenomenon that seems to run in the family.

But the Campbells were a little more impressed after being told the chances of two siblings sharing a Feb. 29 birthday.

What are the chances?

According to a Statistics Canada post from 2020 — the last leap year — there were close to 25,000 Canadians who were born on a Feb. 29 as of July 2019. That amounts to 0.066 per cent of the 37.6 million population at the time. CBC News has emailed the federal statistics agency for updated numbers.

According to the Associated Press, there are an estimated five million people worldwide born on Feb. 29.

Not surprisingly, it’s less likely to be born on Feb. 29 than any other date simply because of how seldom it comes around.

According to Pranesh Kumar, a statistics professor at the University of Northern British Columbia, it hinges on simple math: assuming the chances of being born on any individual date are even, he said, then the probability of having a Feb. 28 birthday or one that falls on March 1 is one in 365. But because leap years only occur every four years, the probability changes to one in 1,461 (365 multiplied by four plus one leap day).

The problem, Kumar said, is that not every day is created equal. Statistics Canada data, for example, shows the last five years have seen 22,318 more babies born in July than December, despite both months having 31 days.

“So there’s a lot more data we would need,” he explained.

Questions about the chances of two people sharing the same birthday are a popular topic in online forums for sharing math questions. The birthday problem, for example, is a theory that says in a random group of 23 people, the chances of two of those people sharing the same birthday is greater than 50 per cent.

But that applies to a random group of people, rather than relatives. A user on the r/askmath forum on Reddit asked about the chances of two siblings both being born on Feb. 29, years apart, and received many different answers with variations based on real-world statistics and questions about whether it is planned or not.

However, even the most conservative estimate placed the probability at no more than one in two million, with even lower probabilities coming in from other users.

“Wow,” said Olivia when relayed that information. “That’s amazing.”

But long odds don’t mean something isn’t possible, or even probable, given enough people, as one Reddit user explained.

“But when you look at a whole lot of families, this unlikely phenomenon is statistically guaranteed (just like how you winning the lottery is very unlikely, but it’s very likely that someone will),” the user wrote.

And it has happened before. In 2020, Lindsay and Dame Demchak of New York made international headlines when Lindsay gave birth to a child on Feb. 29 for the second time. The same happened to Michigan parents in 2016.

Jordyn and younger sister Olivia take the extra attention they get in stride, focusing instead on the bond they share as sisters.

“I do think it’s pretty cool that I got Olivia for my birthday,” says Jordyn.