What’s the Most Common Birthday? (And What’s the Least Common?)

If your child is in preschool or elementary school and the birthday e-invitations have started flooding your inbox, you may have noticed there are a lot of parties in the late summer and early fall months—but far fewer in the winter. What gives?

It’s true—certain days (and even entire months) are more common for birthdays than others. According to the now-shuttered national statistics and polling organization FiveThirtyEight, the most recent data collection on births in the United States is reflective of a few distinct trends. August, for example, may be the month with the highest number of births, but September tops the list when it comes to most common birthdates: From 1994 to 2014, 9 of the 10 most common birthdays in the US fell in September. The least common month to have a baby? February, aka the shortest month of the year. So what is the most common birthday, and which dates are the rarest? Keep reading to find out what the numbers reveal.

Key Takeaways

• September is the most common birthday month: Nine of the 10 most common birthdates in the US fall in September, with September 9 ranking as the most common birthday.
• August has the highest overall number of births: While September dominates the list of most common birthdays, August typically sees the greatest total volume of live births nationwide.
• Holidays are among the least common birthdays: Outside of Leap Day (February 29), the rarest birthdays tend to fall on major holidays like Christmas, New Year’s Day and the Fourth of July, likely because scheduled inductions and C-sections are avoided on those dates.
• Late summer birthdays reflect winter conceptions: The prevalence of August and September birthdays suggests many babies are conceived in late fall or winter, particularly around the holiday season, when people (and couples) may spend more time indoors.
• February is the least common birth month: With fewer days than other months—and generally lower birth totals—February consistently ranks as the rarest month for birthdays overall.

What is the most common birthday?

September 9 consistently ranks as the most common birthday in the US. According to FiveThirtyEight, on average, more than 12,000 babies are born on this date each year.

For Michael L., a teaching assistant in Westchester, New York, the statistic came as a surprise. Learning that he was born on the most common birthday in the US, suddenly made things click in his mind. He shares his September 9 birthday with a relative, two of his students and even some of his favorite celebrities. “One of my all-time favorite NBA players and my favorite comedian, Adam Sandler, also have my birthday,” he says. These days, he’s happy to share the spotlight at home, too: "I share my birthday with my 5-year-old niece too, so it’s her day now,” he adds.

10 Most Common Birthdays

Remember when we said 9 of the 10 most common birthdays fall in September? We weren’t exaggerating. In fact, only one date breaks the September streak—and it’s July 7.

According to US birth data collected between 1994 and 2014, the 10 most common birthdays are:

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

In other words, if it feels like your September baby shares a birthday with half their class, it’s probably not your imagination.

When Jenna from Allentown, Pennsylvania, was pregnant with her oldest child, she discovered she was due within two weeks of two close friends. The trio formed an unofficial September pregnancy support group.

Some families even notice the trend within their own households. Arrabella from California says her two children share the same mid-September birthday—two years apart. Even more surprising? She had the same due date for both pregnancies, and both siblings were born 13 days early by pure coincidence.

What Month Has the Most Live Births?

Even though so many shared birthdays happen in September, it’s not the month with the highest number of births overall. That record is held by August. It’s followed by July in second place and either September or October in third, depending on the data you use.

Though FiveThirtyEight gives September the edge from 1994 to 2014, the most recent information from the Centers for Disease Control and Prevention (CDC) puts October ahead of September, at least for 2023:

1. August: 322,000 births
2. July: 312,000 births
3. October: 309,000 births
4. September: 305,000 births

What Is the Least Common Birthday?

Raise your hand if you’re a leap-year baby! Unsurprisingly, the least common birthday in the US is February 29; considering this date only officially rolls around once every four years, it makes sense that the fewest number of babies are born on this day. In fact, you only have a one in 1,461 chance of being born on February 29 (instead of the typical one in 365 chance).

If you have a leap-year baby, they probably won’t share their birthday with many others, and you’ll need to decide when to celebrate in non-leap years. You might even choose to honor both, because when your “real” birthday only rolls around every four years, why not stretch out the celebration?

For Missouri mom-of-four Natalie, who has a 12-year-old born on February 29, figuring out whether to celebrate on February 28 or March 1 each year has been a bigger deal than she expected. But, otherwise, she says her daughter loves the reactions she gets when she tells people about her unusual birthday: “She was born on a unique day, and is her own unique self, so it’s been mostly fun,” she says.

10 Least Common Birthdays

After February 29, many of the least common birthdays fall on or near major holidays like Thanksgiving, Christmas and New Year’s. This isn’t a coincidence: conception rates are slightly lower in the spring, notes Dallas Reed, MD, FACOG, principal medical advisor at Myriad Genetics, leading to fewer winter births. But the biggest reason these holidays aren’t common for birthdays simply comes down to scheduling.

Reed says that healthcare providers and expectant parents typically want to avoid booking an induction or planned C-section on holidays. Obviously, emergencies happen, but given the choice, providers and families aren’t scheduling births on these celebratory dates.

According to FiveThirtyEight, the 10 least common birthdays are:

1. February 29
2. December 25
3. January 1
4. December 24
5. July 4
6. January 2
7. December 26
8. November 27
9. November 23
10. November 25

What Is the Month With the Least Live Births?

Because it has far fewer days than any other month, February is the least common month for live births. In 2023, only 271,000 babies were born, according to the CDC. April, with only 30 dates, is often the second-least common, followed by December. In 2023, April had 279,000 births, while December had 297,000.

Many people with these uncommon birthdays love the added excitement of celebrating their special day during a holiday season, so don’t stress if you’re expecting a baby and due right before Thanksgiving or Christmas.

Reasons for Seasonal Birthday Trends

However you crunch the numbers, it’s clear that summer into early fall is the most popular time to have a baby. If you count backwards by nine months, that means late fall and early winter (November and December) are the most common times to conceive.

Seasonal changes

It may be freezing outside but it’s warm and cozy inside, making many couples feel more amorous between Thanksgiving and New Year’s. “These periods often involve more family time, celebrations and relaxation, which can encourage intimacy and lead to higher conception rates,” says Reed.

Danielle, mom of one from Florida, shares her experience conceiving the baby she would eventually deliver on September 7 the following year: “We went up to Calgary for Christmas the year my /[September] baby was conceived; it was -40 degrees outside, so needless to say we were finding ways to keep warm and busy inside for that whole week.”

Fertility changes

Two factors can increase your chances of conceiving during the winter versus the summer: average temperatures and changes in daylight hours.

According to Banafsheh Kashani, MD, FACOG, an ob-gyn, reproductive endocrinologist and infertility specialist practicing in California, some research suggests that sperm quality is lower in the summer and higher in the winter, possibly because sperm like cooler environments (so the extreme heat of the summer reduces overall sperm quality, including quantity and motility).

Meanwhile, daylight saving time ushers in changes to daylight hours that may help stabilize female circadian rhythms. Reed says shorter fall and winter days can increase melatonin levels (i.e. help you get more sleep within your natural circadian rhythm), which might indirectly support higher levels of fertility by stabilizing other hormones like prolactin and luteinizing hormone (LH).

The bottom line? Colder weather likely means increased sperm quality and higher fertility, making it easier to conceive.

Practical concerns

If you’re lucky enough to plan your child’s conception for a time of year that’s nine months before you would ideally like to have a newborn, you’ll probably try to aim for a summer birth. There are no major holidays on the calendar and no seasonal illnesses circulating.

Still, we may not even be fully conscious of the desire to conceive in winter and give birth in the summer. “Evolutionary theories suggest that late-summer births might be advantageous in certain climates; babies born at this time historically avoided extreme winter conditions during infancy, which could have impacted survival rates in pre-modern societies,” adds Reed.

IVF transfers

Janene Oleaga, Esq., a fertility attorney and reproductive rights advocate, says she sees this trend unfold among her own clients too. Many fertility clinics close at the end of December or into January for the holidays, which can push embryo transfers or egg retrievals into the new year. “Many of those resulting babies arrive in August and September,” Oleaga says.

For the most part, you can simply take the inverse of all the reasons why August, July and October are such popular months to have a baby and apply them to the least popular months: There’s less canoodling going on in the heat and humidity of the summer, fertility and sperm quality may be lower and anyone intentionally conceiving with a mind toward having a baby in nine months probably isn’t thinking, “Wouldn’t it be nice to bring a new baby home in January?”

How Common Is My Birthday?

If you have a head for data, you can scour FiveThirtyEight or the CDC, to do some sleuthing, but it’s not necessarily the most accessible information. (Just know that if you’re an August or September baby, you’re in good company!)

Chart of the Most and Least Common Birthdays

To Sum It Up

The data shows clear patterns: August and September lead in total births, September 9 consistently ranks as the most common birthday and holidays are among the rarest birthdays. But as experts point out, even with modern planning and scheduling, there’s always a degree of unpredictability to account for. Common or rare, every birthday tells its own story.