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?
According to data collected by 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 birth dates: 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.
Here’s what these trends tell us about conception and birthing trends in the US, as well as how to figure out how common your child’s birthday is.
Per data compiled by FiveThirtyEight, the most common birthday shared by Americans is September 9. If this is your birthday, you’re in good company: Adam Sandler, Michael Bublé, Leo Tolstoy, and Otis Redding were all born on this day. On average, more than 12,000 babies are born on September 9 each year.
Remember when we said nine of the 10 most common birthdays fall in the month of September? We meant it! Only the sixth most popular date, July 7, breaks the September hot streak. According to data collected from 1994 to 2014, the 10 most common birthdays are:
- September 9
- September 19
- September 12
- September 17
- September 10
- July 7
- September 20
- September 15
- September 16
- September 18
In other words, if it seems like your September baby shares a birthday with half of their classmates, it’s not your imagination. When Jenna from Allentown, Pennsylvania was pregnant with the oldest of her four children, she was due within two weeks of two of her friends, allowing them to form an unofficial pregnancy support group (Jenna’s daughter was born on September 30).
Sometimes, parents even notice the shared birthday trend among their own kids: Arbella from California says her two children have the same birthday in mid-September, two years apart—but the really weird thing is that she had the same due date for both pregnancies—and then both siblings were born 13 days early, completely by coincidence.
Even though so many shared birthdays happen in September, it’s not the month with the highest number of births overall. That record is set by August, which has repeatedly stood out as the most common month to have a baby. It’s followed by July in second place and either September or October in third, depending on which data you use.
Though FiveThirtyEight gives September the edge from 1994 to 2014, the most recent data from the Centers for Disease Control and Prevention (CDC) puts October ahead of September, at least for 2023:
- August: 322,000 births
- July: 312,000 births
- October: 309,000 births
- September: 305,000 births
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—and there are a few reasons why.
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 Dallas Reed, MD, FACOG, principal medical advisor for women’s health at Myriad Genetics.
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 savings 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, no seasonal illnesses circulating and, depending on your job (looking at you, teachers!), you may even have some of the summer off already, making it easier to take parental leave.
We may not even be fully conscious of the desire to conceive in winter and give birth in the summer, says Reed: “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.”
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 would be 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’re not likely to find many other people who share their birthday, and you’ll have to decide when to actually celebrate in non-leap years.
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 it to be. 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.”
After February 29, many of the least common birthdays take place 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 Reed, leading to fewer winter births, but the biggest reason these holidays aren’t synonymous with birthdays is primarily because of 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 Christmas or New Year’s Day.
According to FiveThirtyEight, the 10 least common birthdays are:
- February 29
- December 25
- January 1
- December 24
- July 4
- January 2
- December 26
- November 27
- November 23
- November 25
Because it has far fewer days than any other month, February is the least common birth month. 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.
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?”
Still, many people with these uncommon birthdays love the added excitement of celebrating their special day during the holiday season, so don’t stress if you’re expecting a baby and due right before Thanksgiving or Christmas.
If you have a head for data, you can scour the pFiveThirtyEight 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!)
Plus, more from The Bump:
Banafsheh Kashani, MD, FACOG, is an ob-gyn, reproductive endocrinologist and infertility specialist practicing in California. She earned her medical degree from University of South Alabama, College of Medicine where she graduated in the top of her class.
Dallas Reed, MD, FACOG, is the principal medical advisor for women’s health at Myriad Genetics. He earned his medical degree from the Boston University School of Medicine.
FiveThirtyEight, [US Births 2000 - 2014)(https://github.com/fivethirtyeight/data/blob/master/births/US_births_2000-2014_SSA.csv))
National Center for Health Statistics, Monthly and 12 month-ending number of live births, deaths and infant deaths: United States
Journal of Human Reproductive Sciences, Impact of seasonal variations on semen parameters: a retrospective analysis of data from subjects attending a tertiary care fertility centre, 2023
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