Most Common Birthdays in America – Ranked by Month

The most common birthday in the United States is September 9, with September standing as the single most common birth month overall. Nine of the top 10 most common U.S. birthdays fall in September, and conceptions for those births cluster heavily around the last two weeks of December, pointing to a clear seasonal driver.

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Why September Dominates the U.S. Birthday Calendar

September produces more American births than any other month, and the reason traces directly to conception timing. Babies born in September were conceived roughly nine months earlier, meaning December, specifically the period between Christmas and New Year’s Eve, is the peak conception window in the United States. Colder weather, holiday downtime, and increased time spent indoors together as couples all contribute to this pattern.

The data behind this claim is substantial. Analysis of more than 20 years of U.S. birth records from the National Center for Health Statistics (NCHS), a federal agency that collects and publishes vital statistics including birth counts by date, consistently shows September outperforming every other month. Researchers at Harvard University and the Social Science Research Network have independently confirmed the September concentration across multiple study periods.

This is not a recent development. The September birthday peak has appeared in American birth data going back to at least the 1970s, suggesting it reflects deeply ingrained seasonal behavior rather than any temporary social trend.

The 25 Most Common Birthdays in the United States

September 9 is the single most common U.S. birthday, and September dates claim 14 of the top 25 spots on the list. The rankings below are drawn from analyses of CDC vital statistics (the Centers for Disease Control and Prevention, which publishes annual U.S. birth totals) and widely cited birthday frequency research covering birth records from 1994 through 2014.

October and August each contribute a handful of entries, reflecting a broader late-summer and early-fall peak that spans roughly August 20 through October 10.

Month-by-Month Breakdown of U.S. Birth Frequency

September is the highest-volume birth month in the United States, and February is the lowest, with all 12 calendar months falling in a consistent ranked order that has held steady for decades. The table below ranks each month from highest to lowest average birth volume, based on CDC and NCHS historical data.

The summer-to-fall cluster covering July, August, September, and October consistently produces the highest birth totals. The winter months of January and February are reliably among the quietest on the calendar for U.S. births.

The Rarest Birthdays in the United States

The least common birthday in the United States is February 29, the leap day that appears only once every four years, meaning people born on that date are sometimes called “leaplings.” Because it occurs so infrequently, the total pool of Americans with a February 29 birthday is a small fraction of the population compared to any fixed date.

Among fixed dates that exist every year, the five rarest birthdays are:

1. December 25 (Christmas Day) because hospitals actively avoid scheduling elective inductions and C-sections on the holiday
2. January 1 (New Year’s Day) for the same reason as Christmas
3. December 24 (Christmas Eve) as a near-holiday with reduced elective procedures
4. July 4 (Independence Day) reflecting both holiday scheduling avoidance and a mid-summer dip in natural births
5. December 26 (the day after Christmas) as hospital scheduling rebounds slowly

The influence of hospital scheduling practices on rare birthdays is significant and often overlooked. Elective cesarean sections (C-sections, meaning surgically assisted deliveries planned in advance rather than emergency procedures) and labor inductions allow delivery dates to be chosen by medical providers and families. Providers consistently steer away from major federal holidays, which compresses births away from those dates and inflates surrounding dates slightly.

How Hospital Scheduling Shapes Birthday Distribution

Modern obstetric practices have measurably shifted which days record the most and fewest births. Approximately 32 percent of all U.S. births are now delivered by cesarean section, and a large share of those are scheduled in advance. Labor inductions, where medication is used to start labor before it begins naturally, account for roughly 25 percent of U.S. births.

Together, these two practices mean that nearly half of all U.S. births involve some degree of scheduling, giving families and physicians real influence over birth dates. The practical result is a visible suppression of births on weekends and holidays, and a concentration of births on Tuesday, Wednesday, and Thursday, which are the three most common birth days of the week in the United States.

Sunday is the least common birth day of the week for the same reasons: fewer scheduled procedures and reduced elective labor inductions. The gap between the busiest and least busy days of the week is large enough to appear clearly in national data, not just in individual hospital statistics.

Regional Variation in Common Birthdays

Birth timing patterns vary across U.S. states, though the September peak appears in virtually every region. States in the Deep South, including Mississippi, Alabama, and Louisiana, show slightly elevated birth rates in late summer compared to the national average, which some researchers link to regional cultural and climatic factors affecting conception timing.

Alaska shows a notably stronger winter birth peak than the lower 48 states. Researchers have attributed this partly to the extreme length and darkness of Alaskan winters, which may amplify the seasonal indoor-activity effect that drives holiday-period conceptions nationally.

Western states such as California, Nevada, and Arizona show a somewhat flatter birthday distribution across months than the national average. This may reflect the milder winters in those regions, which reduce the behavioral isolation effect that concentrates conceptions in December in colder states.

Despite these regional differences, no U.S. state shows a reversal of the national pattern. September consistently ranks among the top two or three birth months in every state for which data is publicly available.

How Birthdays Are Distributed Across the Week

Tuesday is the most common birth day of the week in the United States, and Sunday is the least common, a gap driven almost entirely by hospital scheduling rather than biology. The table below shows the full day-of-week ranking.

The difference between Tuesday and Sunday birth totals is approximately 30 to 35 percent when measured across multi-year national datasets. This is one of the clearest non-seasonal patterns in U.S. birth data and reflects scheduling behavior almost entirely, since there is no known biological reason for births to be rarer on Sundays.

The Role of Conception Timing Research

Conception timing research confirms that the December holiday window is the primary driver of the U.S. September birth peak, using a back-calculation method called the conception-to-birth lag, meaning the approximately 280-day average duration of a full-term pregnancy measured from the last menstrual period. Applying this method to national datasets allows researchers to map which weeks of the year produce the most conceptions.

The December conception peak that produces September births is the most prominent feature in this analysis. A secondary peak appears in late March and early April, which produces a smaller but visible cluster of December and January births. Researchers have linked this secondary peak to another period of increased indoor time following the end of summer outdoor activity, though it is far less pronounced than the December effect.

Demographic and socioeconomic variables also influence conception timing. Research published in the journal Demography found that women with higher levels of formal education showed a weaker seasonal conception pattern, possibly because professional schedules and family planning behaviors reduce the influence of seasonal factors. Women in lower-income brackets showed a stronger seasonal conception signal in the data.

Birthday Frequency and Population-Level Implications

Birthday frequency patterns directly affect actuarial modeling, educational outcomes, and lifetime labor earnings, making them relevant well beyond trivia. Actuarial science, the discipline that calculates financial risk for insurance companies and pension funds, uses birth date distribution data when modeling large population cohorts. A cohort with a disproportionate number of September birthdays will reach each age milestone at a slightly different time than a uniformly distributed cohort.

In education, the relative-age effect (also called the Redshirt Effect in American contexts, a term borrowed from athletic practice referring to holding a child back a year before kindergarten enrollment) is influenced directly by birthday distribution. Children born in September in states with a September 1 school enrollment cutoff are the oldest in their grade, which has been associated with measurable academic and athletic advantages documented in multiple longitudinal studies.

Labor market research has found small but statistically significant differences in lifetime earnings linked to birth month, operating primarily through the relative-age effect in early education. A child born in August in a state with a September 1 cutoff is the youngest in the grade and may enter school at a slight developmental disadvantage compared to the September-born child seated across the room. The long-run earnings effect of this gap has been estimated at between 2 and 4 percent over a lifetime in some studies.

Zodiac Signs and the Birthday Distribution

The uneven distribution of birthdays across the calendar means that Virgo (August 23 to September 22) and Libra (September 23 to October 22) are statistically the most common zodiac signs in the United States by birth frequency. Virgo in particular benefits directly from the September birthday peak.

Capricorn (December 22 to January 19) and Aquarius (January 20 to February 18) are among the rarest zodiac signs by birth frequency, consistent with the low birth rates seen in December, January, and February.

These rankings shift slightly from year to year depending on annual birth totals, but the broad ordering has remained stable for decades.

Historical Shifts in U.S. Birthday Patterns

The September birthday peak is not a permanent fixture of all historical periods. In the early 20th century, before widespread hospital births and modern obstetric scheduling, U.S. birth patterns showed a stronger winter birth peak, particularly in January and February, consistent with spring conceptions in agricultural communities where summer was peak labor season.

The transition from home births to hospital births accelerated through the 1940s and 1950s, and birth patterns began shifting toward the September-dominant pattern recognizable today. By the 1970s, the September peak was firmly established in national data, and it has remained the dominant feature of the U.S. birthday calendar ever since.

Baby boom years from 1946 through 1964 showed somewhat elevated birth rates across all months compared to adjacent decades, but the seasonal distribution within those years still reflected the same September tendency. The total volume of births rose sharply without disrupting the relative ranking of birth months.

More recently, the gradual decline in the U.S. total fertility rate (the average number of children born per woman, a standard demographic measure) from 2.1 in 2007 to approximately 1.6 in 2023 has reduced absolute birth counts across all months but has not meaningfully altered the relative seasonal distribution. September remains the peak month at lower total volumes.

Twin and Multiple Births and Their Effect on Date Clustering

Multiple births produce a small but measurable dispersing effect on birthday clustering because pregnancies conceived through assisted reproductive technology (ART), a category of fertility treatments that includes in vitro fertilization (IVF), are planned around deliberate timing windows rather than seasonal behavior. The U.S. twin birth rate is approximately 32 per 1,000 births, a figure that has nearly doubled since the 1980s partly due to increased ART use.

ART pregnancies are more likely to be planned around specific timing windows, which means ART-conceived births show a slightly more dispersed seasonal pattern than naturally conceived births. This effect is not large enough to significantly alter national birthday rankings but is detectable in state-level data from states with high ART utilization such as Massachusetts and Connecticut, which have state-level insurance mandates covering fertility treatments.

Multiple-birth deliveries are more likely to be delivered by cesarean section than singleton births, reinforcing the scheduling effect described earlier. This means twins and triplets are even more concentrated on weekdays and away from major holidays than singletons.

How to Interpret Birthday Frequency Data

Birthday frequency data in the United States comes from several overlapping sources, and it is worth understanding what each measures. The National Center for Health Statistics publishes the gold-standard dataset: actual birth certificates filed by state vital records offices and aggregated federally. This source covers every registered birth in the country and is updated annually.

Academic analyses, including the widely shared work of Matt Stiles at the Los Angeles Times data desk and the FiveThirtyEight statistical journalism outlet, have visualized NCHS data as heat maps showing relative birthday frequency. These visualizations consistently confirm the September dominance and holiday suppression effects described above.

It is important to note that birthday frequency data reflects registered U.S. births and does not capture birthdays of U.S. residents born abroad. Approximately 13 percent of the U.S. population is foreign-born, and those individuals bring their home-country birthday distributions with them, which may differ from the U.S. pattern. National birthday frequency rankings apply specifically to U.S.-born individuals.

Practical Uses of Birthday Frequency Knowledge

The knowledge that September birthdays are most common drives real decisions in retail marketing, event pricing, public health program design, and youth sports administration. Retailers and e-commerce platforms use birth date distributions to time birthday-related promotional campaigns. Concentrating email marketing around late August and early September captures a larger share of the birthday audience than equivalent spending spread across slower months.

Event planners and venue operators report higher demand for birthday party reservations in September and October, consistent with the birthday frequency data. Pricing strategies that account for this demand cycle can meaningfully affect revenue.

Public health researchers use birth date distributions to design age-stratified vaccination campaigns and population screening programs. When a target age group is reached in September-heavy cohorts, the sample pool available in September months is larger than in January months.

Sports organizations at the youth and collegiate level use birthday cutoff dates to define eligibility years, and the relative-age effect means that coaches, scouts, and administrators interpreting youth performance data should account for the fact that September and October birthdays are overrepresented among the oldest players in any given age group.

What the Data Reveals About American Seasonal Behavior

The September birthday peak is ultimately a mirror of American social behavior in December. The concentration of conceptions during the Christmas and New Year’s holiday period reflects increased leisure time, reduced work obligations, emotional warmth associated with family gatherings, and the physiological effects of colder weather driving people indoors.

Research published in PLOS ONE, a peer-reviewed scientific journal, found that internet search activity related to romantic and sexual content peaks globally during Christmas and Eid al-Fitr, two of the world’s most widely observed holidays, with Christmas producing a larger spike in countries with predominantly Christian populations, including the United States. This behavioral signal aligns precisely with the nine-month lag that produces the September birth peak.

The December conception peak and the September birth peak together form one of the most well-documented and durable patterns in American demographic data, robust across decades of records, consistent across almost all demographic subgroups, and visible in every region of the country.

FAQs

What is the most common birthday in the United States?

The most common birthday in the United States is September 9. September as a whole is the most common birth month, with nine of the top 10 most common U.S. birthdays falling within that month. Conceptions for September babies most often occur during the December holiday period.

What is the rarest birthday in the United States?

The rarest birthday is February 29, the leap day that occurs only once every four years. Among dates that exist every year, December 25 (Christmas Day) and January 1 (New Year’s Day) are the rarest, because hospitals avoid scheduling elective deliveries on major holidays.

Why are so many birthdays in September?

September birthdays are common because they correspond to conceptions that occur approximately nine months earlier, during the Christmas and New Year’s holiday period in December. Colder weather, increased time at home, and the emotional and social atmosphere of the holidays are the primary drivers of this peak conception window.

What is the most common birth month in the US?

September is the most common birth month in the United States, followed by August and July. The late-summer and early-fall period from roughly July through October consistently produces the highest birth volumes, while January and February produce the fewest births.

What day of the week are the most babies born in the US?

Tuesday is the most common birth day of the week in the United States, followed closely by Wednesday and Thursday. Sunday is the least common birth day. This pattern is driven almost entirely by hospital scheduling practices, since elective cesarean sections and labor inductions are concentrated on weekdays.

How does school cutoff date relate to common birthdays?

In states with a September 1 kindergarten enrollment cutoff, children born in September are the oldest in their grade, while children born in August are the youngest. Research shows that the oldest children in a grade tend to show stronger early academic performance, a phenomenon called the relative-age effect. Because September is already the most common birth month, these states have a large pool of kindergarteners who are relatively advanced for their grade year.

What zodiac sign is most common in the United States?

Virgo (August 23 to September 22) is the most common zodiac sign in the United States by birth frequency, followed by Libra (September 23 to October 22). Both signs benefit directly from the September birthday peak. Capricorn and Aquarius are among the rarest, consistent with the low birth rates in December, January, and February.

Has the most common birthday in the US changed over time?

The September birthday peak has been a stable feature of U.S. birth data since at least the 1970s. Before widespread hospital births in the mid-20th century, winter birth months showed a stronger relative presence. The shift toward September dominance coincided with the rise of hospital deliveries, scheduled C-sections, and labor inductions, all of which allow birth timing to be influenced by seasonal conception patterns without the natural variation that home births introduced.

Do all U.S. states share the same most common birthday?

Broadly yes. September ranks among the top two or three birth months in every U.S. state with available public data. Some regional variation exists, with Alaska showing a stronger winter birth peak and Western states showing a slightly flatter seasonal curve, but no state reverses the national pattern of September dominance.

How many Americans share the most common birthday, September 9?

Exact counts are not published annually, but applying the average daily birth rate to the U.S. population suggests that approximately 10,000 to 12,000 Americans are born on September 9 in a typical year, making it the single most populated birth date in the country on an annual basis. Over the total living U.S. population, the number of people with a September 9 birthday likely exceeds 800,000.