The surprising truth: you share your birthday with 20 million other people!

From the top of your head, can you think of someone you know who has the same date of birth? On your birthday, did you find yourself sending good wishes to many friends on Facebook? We bet you asked Google about famous people who share your date of birth. One thing is certain: although your birthday is your special day, you're not the only one.

This may seem relatively unlikely, but for some birthdays more often than others, it's just the opposite. If you've ever wondered how many people share your date of birth, don't look too much.

How many living "Christmas friends" do you have around the world?

When we discuss this on a basic level, the probability of sharing your birthday with anyone you meet—if your birthday falls on any day other than February 29—is about 1/365 in any population (0.274%). Since the world's population is estimated at more than seven and a half billion, in theory, you should share your birthday with more than 20 million people (~20,438,356).

However, if you were born on the leap day February 29, you should share your date of birth with only 1/1461 of the population, as 366 + 365 + 365 + 365 equals 1461. As this day only comes once every four years, only 0.068% of people worldwide consider it their birthday – that means only 5,072,800 people!

Some days are more popular than others

Although the probability of the day of birth on a given date logically sounds like it should be around one in 365.25, birth rates don't follow a regular distribution – there are a lot of things that affect when babies are born. In the American tradition, for example, a high percentage of marriages occur in June, and this results in many children being born between February and March.

It also seems likely that people have children when they are in a state of rest and relaxation and/or when entertainment options are very limited. Random natural and abnormal events such as power outages, blizzards, and floods tend to keep people indoors, and thus increase load rates. Holidays known to evoke warm emotions, such as Valentine's Day and Thanksgiving, are also known for high pregnancy rates. In addition, maternal health greatly affects her fertility, so it makes sense that environmental pressures make pregnancy less likely.

Since the nineties, several scientific studies have shown that there are seasonal fluctuations in pregnancy rates. For example, birth rates in the Northern Hemisphere typically peak between March and May, and are lowest between October and December. Of course, these figures vary greatly depending on age, education, socioeconomic status and marital status of parents.

Grinding numbers

In 2006, the New York Times published a spreadsheet titled "How Common Is Your Birthday?" that provided data collected by Harvard's Amitabh Chandra who gave data on how many babies are born in the United States every day from Jan. 1 to Dec. 31. According to this article, babies are much more likely to be born in summer than in any other season, followed by autumn, spring and winter respectively. The period from the beginning to the middle of September is marked by the most common birthdays, although the most popular day changes slightly from year to year. This day at the moment is September 9.

It's no surprise that February 29 is the least common birthday or one of the least common – and likely will always be like that. With the exception of this rare day, the ten most popular days reported in this study were the holidays: the Fourth of July, late November (Thanksgiving and nearby days), the period around Christmas (December 24-26), or New Year (December 29 and January 1-3) in particular.

Some may suggest that low-popularity birthdays mean that mothers have a say in when their babies will be born and prefer not to give birth on holidays. Since this study, more recent data has emerged confirming that holidays maintain the lowest birth rates and that the first ten days of September are the highest.