What Is the Most Popular Birthday in the U.S.?

man getting covered in silly string on birthday — Happy Birthday! Certain months are far more popular for births than others. Shannon Fagan/Getty Images

It may seem as if you're buying a lot more birthday gifts at certain times of the year. Why is this? Is it your imagination or are there more babies born in specific months?

The career website Zippia used statistics compiled by the data website fivethirtyeight looking at American births between 1994 and 2014. The birth data set came from the Centers for Disease Control and Prevention's National Center for Health Statistics and the Social Security Administration.

The final result? Sept. 9 was the most common birthday in the U.S. An average of 12,301 babies were born on that particular day of the year. So, it's very likely that you know a lot of 9/9 babies. Here is a list of the top 10 most common birthdays each year:

Sept. 9
Sept. 19
Sept. 12
Sept. 17
Sept. 10
July 7
Sept. 20
Sept. 15
Sept. 16
Sept. 18

Notice that all except one in the top 10 is in September. The outlier is July 7.

Why Are So Many Babies Born in September?

Little girl celebrates Birthday Party with rose decor in beautiful garden — Nine of the 10 most popular birth dates in the U.S. are in September. Natalia Kirichenko/Shutterstock

Although no one knows the exact reason why September is such busy month in the U.S., there are some pretty credible theories. First, people are feeling extra celebratory during the Christmas/New Year season, which is probably why so many births happen nine months later. There are a lot of parties going on, which often come hand-in-hand with alcohol consumption, tending to lower inhibition and increase sexual drive. Plus, people usually have time off from work over the holidays. A study has shown that people have more sex on the weekends and over holiday periods.

However, one review of data from a health tracking app combined with birth records noted that the end of year sees a spike in fertility for a lot of women so that could be the true reason for the September birth spike. In addition, men produce sperm of better quality in the colder months, which could contribute to the September baby boom. It's also possible that seasonal differences like daylight length are a contributing factor, as people are spending less time outside in the winter months and more time canoodling and thus baby-making.

Interestingly, birth seasonality has been observed around the world — it seems that the further north you get from the equator, the earlier in the year the birth peak is. For instance, in Finland, the peak birth time is in late April, while in the Caribbean, it is in November.

Which Dates Are Least Common for Birthdays?

Woman Blowing Out Candles On Birthday Cake — Many of the least common birthdays fall on holidays like Christmas and New Year. Monkey Business Images/Shutterstock

The least common birth date is Dec. 25, which you probably know better as Christmas Day. Chalk this one up to the sheer number of prescheduled C-sections and inductions! Not too many doctors, midwives or impending parents really want to spend this holiday in the hospital unless it's absolutely necessary. On average, 6,574 babies are born each year on this date, making it the last-place finisher in the fivethirtyeight list. The least common birth month is February.

Here is the list of the bottom 10 days for baby births in the U.S. As you can see, the list is heavy on holidays and holiday-adjacent days, like Christmas Eve, New Year, Thanksgiving and even July 4.

Dec. 25
Jan. 1
Dec. 24
July 4
Jan. 2
Dec. 26
Nov. 27
Nov. 23
Nov. 25
Oct. 31

If your birthday happens to fall on one of those days, don't worry — it just shows how unique you are!