birthday trafficWe shift birth counts backward using the day range above. This estimates seasonal patterns. How it works ↗
Average U.S. births each time a date occurred.
SAME BIRTHDAY. DIFFERENT CROWD.
Each card uses its own country and years. Rarity percentiles compare calendar dates using total births. Tap a country to explore its full records.
THE BIGGER PICTURE
THE LEADERBOARD
DENMARK / CURRENT POPULATION
People living in Denmark with a September 9 birthday:
—This comes from the resident register, including immigrants. It counts people alive in the country.
Administrative birthday assignments inflate January 1 and July 1. Statistics Denmark documents the assignments.
U.S. / TIME OF BIRTH
Opening the records…
Recorded birth times. Delivery groups include hospital scheduling.
Tell me which dataset and how you would use it.
| Dataset | Coverage | What it covers | Publisher |
|---|---|---|---|
| Famous birthdaysGAME COLLECTION | All 366 dates | Arts, science and sport entries born in 1800–2008. A separate collection of people. | English Wikipedia, CC BY-SA 4.0 |
| England & Wales: daily birthsNEW / OFFICIAL | 1995–2025 | 20,231,846 births by date of occurrence. | Office for National Statistics |
| France: daily birthsOFFICIAL | 2025–July 2026 | Daily counts from individual birth records. 2025 final, 2026 provisional. Regional, sex and maternal-age counts were also collected. | INSEE |
| Denmark’s newbornsOFFICIAL | 2007–2025 | 1,145,696 births by date. Every annual total checked against the publisher’s own figures. | Statistics Denmark |
| Denmark: current residentsOFFICIAL | January 1, 2026 | 6,025,603 residents by birthday, with birth-year detail and 19 population snapshots of the same population. | Statistics Denmark |
| U.S. birth recordsOFFICIAL | U.S., 2024 | 3,638,436 birth records by month, weekday, hour, gestational weeks and delivery. Includes 9,502 foreign-resident births, separately labeled. | CDC/NCHS |
| Brazil: regional birth recordsMIRROR | Rondônia, Brazil, 2019 | Regional records with exact birth date, gestational weeks and delivery fields. Teaching mirror. | SINASC, Brazil |
| U.S. family and pregnancy surveySURVEY | U.S. NSFG, 2022–2023 | Public-use survey files. Pregnancy dates are year-only, gestation is grouped, and survey weights apply. | CDC/NCHS |
| Historical birth countsHISTORY | France 1968–2023; U.S. 42 observed years | 56 years of metropolitan French daily births and 42 observed U.S. years. | INSEE; CDC/NCHS |
The exact-date clinical join is in the Brazilian regional mirror; the U.S. clinical file runs at month, weekday and hour.
METHODS AND SOURCES
Birth counts are recorded. Conception dates are estimated.
The default is the U.S. series, 85.7 million births on 7,670 dates, 1994 to 2014, from CDC/NCHS and the Social Security Administration. The selector also offers England and Wales, Denmark, France and Malaysia. Birth events, living-population stocks and surveys are kept separate.
Two daily U.S. birth datasets compiled by FiveThirtyEight: CDC/NCHS for 1994–2003 and the Social Security Administration for 2000–2014. The U.S. view uses CDC/NCHS 1994–1999, then SSA 2000–2014: 85,712,738 recorded births on 7,670 dates. The overlapping 2000–2003 years are counted once.
The sources have different administrative coverage and totals. Use the source selector to check whether a pattern survives that choice. ONS birth-registration tables can include late-registration effects.
FiveThirtyEight data: CC BY 4.0. This app adds aggregation and modeling.
Average per occurrence divides a date’s total births by the number of times that date appears in the selected data. In the combined view, September 9 appears 21 times; February 29 appears five times. This makes seasonal comparisons fair.
Total births in sample keeps the actual count. February 29 is far rarer by this measure. Ranks follow your chosen measure, use unrounded values, and give equal ranks to exact ties. Dates absent from a selected year are marked as missing.
Card colors use fixed labels: Ultra rare (top 1%), Very rare (top 5%), Rare (top 10%), Uncommon (the remainder of the rarer half), Very common (top 25% most common), and Common (the remaining common half). A top-25% rarity badge is displayed only when its exact threshold is met. Exact ties are kept together; thresholds compare observed calendar dates.
Share-card rarity always uses total births: #1 is the rarest date. The percentile is the proportion of other observed calendar dates with strictly more births. Ties receive the same rank; a top-X% badge includes the full tied group. This compares calendar dates. Unobserved dates are excluded.
Month bars always show births per observed calendar day. “1 in N” divides total births in the sample by births on the selected date, rounded to the nearest whole number. The birthday picker asks for month and day only. Use “Recorded year” to filter the birth records.
The default is a conventional approximation: 40 weeks minus an assumed two weeks before conception = 266 days. The NIH’s pregnancy overview dates the usual 40 weeks from the last menstrual period, a clock that starts about two weeks before conception.
A 2013 study by Jukic and colleagues followed 125 naturally conceived singleton pregnancies and reported a median of 268 days from ovulation to birth, with a 37-day range even after excluding preterm births. It was a small, specific cohort.
Our ±14-day default is an illustrative assumption. Each lag from 252 to 280 days gets equal weight. Change the center and spread, including a literal single-lag rewind, to see how fragile precise dates are. Real pregnancies also run shorter and longer than this window.
For each real calendar date c, average the recorded births at dates c + lag − spread through c + lag + spread. Retain only dates whose entire window has birth data. Group these modeled daily values by month and day, and average across their observed occurrences.
Divide each date’s modeled average by the overall modeled daily mean and multiply by 100. 100 = an average modeled day. An index of 110 means 10% above that mean. Dates use UTC arithmetic, including real leap days; incomplete windows at the dataset edges are excluded.
This is a shifted, smoothed birth-season pattern. Birth scheduling, premature delivery, pregnancy losses, fertility treatment, and changing population size are all inside it; clinical records are analyzed separately. Read the calculation code ↗
The calendar’s birth files contain dates of birth. The CDC’s 2013 birth-timing analysis shows that delivery timing differs by interventions and setting. A birth calendar carries hospital scheduling as well as biology.
The app explores seasonality among births that were recorded.
Additional data: the research includes birth date × gestation × delivery for a 2019 Brazilian regional mirror, and recent clinical detail from the full 2024 U.S. file at month/weekday/hour resolution. A newer nationwide exact-date clinical file remains the main acquisition gap. The current NSFG public pregnancy file groups gestation and releases pregnancy dates as years. Source notes and additional detail are available by request.