Given that there are 365 days in a year, it would be logical to assume the odds of being born on any given day are 1 in 365. Of course, that would mean that birthdays are distributed evenly throughout the year. However, birthdays would appear to fly in the face of the laws of statistics, with one particular day of the year being the “most common birthday” in the U.S. In fact, 9 of the top 10 most common birthdays in the U.S. fall within that very month.

You might think that month is naturally the most common birthday month. Except it’s not. Depending on the year being considered, the month with the most common birthdays is only the third or fourth most common birthday month. Confused? Read on below. We list the most common birthdays, the most common birthday month and the rarest birthdays too.

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What are the most common birthdays?

The dates below were determined by actual birthdate data from the U.S. Centers for Disease Control and Prevention’s National Center for Health Statistics (NCHS) and the Social Security Administration, as crunched by statisticians at FiveThirtyEight fame and The Daily Viz. The top 10 most common birthdays are:

  1. September 9
  2. September 19
  3. September 12
  4. September 17
  5. September 19
  6. July 7
  7. September 20
  8. Septemeber 15
  9. September 16
  10. September 18

We asked Tod Furniss, an independent healthcare statistician, to help make sense of the seemingly random dates. First, Furniss points out that, like any data set, this one is tied to a specific time period, in this case, the 20 years between 1994 and 2014. That is the latest data analysis that is available, but it may not represent current trends, exactly, he says.

“Plus, it’s important to be aware that the difference between the most common birthday and other popular birth dates is often small,” Furniss says. For example, September 19th, the second most common birthday of the year, has an average birth rate of 12,229 babies; Meanwhile, November 24th, the 356th most common birthday, has an average birth rate of 10,015 newborns. That could help explain how, despite including nine of the most common birthdays, September is not actually the most common birthday month. In fact, September doesn’t even make it into the top three.

What is the most common birth month?

These Are The Most Common Birthdays In The U.SYEARS/GETTY IMAGES

The most common birth month in the U.S. is August, according to FiveThirtyEight’s data analysis. The trend would appear to be continuing. According to more recent info from the NCHS, as of 2022, August is in the lead, followed by July and December. September shows up in fourth place, followed by March, November, October, June, May, January, April, and February, in that order.

What are the least common birthdays?

Although September is fourth place for the most common birthday month, no September dates even make a showing in the top 10 least common birthdays in the U.S., based on FiveThrityEight’s data analysis. The least common birthdays are:

  1. December 25
  2. January 1
  3. December 24
  4. July 4
  5. January 2
  6. December 26
  7. November 27
  8. November 23
  9. November 25
  10. October 31

What is the rarest birthday?

If you look closely at the list of rarest birthdays in the U.S. you may notice that all either fall on, or near to major holidays. To wit, the rarest U.S. birthday of them all is Christmas Day. Then comes New Year’s Day, followed by Christmas Eve and then Independence Day. Then it’s the day after New Year’s Day, followed by the day after Christmas, three dates floating around Thanksgiving, and finally, Halloween.

So, what gives? Is it that babies don’t like competing for attention with major holidays? Or that moms don’t want to saddle their children with holiday birthdays and somehow manage to time it accordingly? Actually, it’s more likely that these rarest of birthdays are the days on which obstetricians choose not to schedule inductions and C-sections, and that, itself, is enough to tip the scales.

What factors contribute to the most common birthdays?

It’s impossible to know exactly why things shake out as they do in terms of the least and most common birthdays and birthday months because not all pregnancies last exactly nine months, not all pregnancies are planned, and not all months have the same number of days (not for nothing, the shortest month, February, also records the least number of births). But Furniss shared with Reader’s Digest some of what may factor into both August being the most popular birth month and the fact that nine of the most popular birthdays fall between September 9th and September 20th.

  1. Less time spent outdoors as the weather grows colder. Assuming a nine-month pregnancy, births between August and late September mean conception between November and late December. And those are the months when the weather just happens to be getting cold. That, as Furniss notes, “may lead to more intimate time spent together,” thereby increasing the likelihood of conception.
  2. More days off from work make for more opportunities for conception. Starting in mid-November and continuing through early January, Americans tend to schedule time off from work. Time off can mean more time spent together, which could lead to more opportunities for baby-making. In addition, as Furniss notes, “this relaxed environment and increased togetherness can lead to higher rates of conception.”

  3. Biological and evolutionary factors. “Some theories suggest that humans may have a slight seasonal pattern in fertility, with higher conception rates in winter months,” Furniss tells Reader’s Digest. “This could be linked to historical survival strategies, where giving birth in late summer or early fall would ensure that infants are born at a time when food is more abundant and the weather is milder.”

  4. Cultural and planning preferences. Although family planning isn’t foolproof, some couples make efforts to plan pregnancies to align with certain times of the year, notes Furniss. This could reflect any number of personal preferences. For example, some industries slow down a bit in August, making it easier to take time off. And a baby born in August or September may be among the oldest in their grade, which some believe could lead to greater academic success. Indeed, some studies suggest that September babies end up being more successful.

About the expert

  • Todd Furniss is the author of the new book The 60% Solution: Rethinking Healthcare, which offers insights to improve the availability and affordability of healthcare. He is CEO of gTC Group and has over 30 years experience in private equity, consulting, and operations as a senior-level operating executive

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Sources:

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Lauren Cahn
Lauren has covered knowledge, history, the British royal family, true crime and riddles for Reader's Digest since 2017. She sharpened her research skills as a lawyer in the 1990s, became an early HuffPost blogger in the 2000s, then began reporting local news in the 2010s. She's been writing news, features and celebrity interviews for a variety of p...
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