The Most Common Birth Month

Which month has the most US births per day?

September, by a clear margin. Across the 1994–2014 series of 85,386,227 U.S. births, an average September day produced about 11,764 babies — the highest of any month. August (11,668) and July (11,522) are next, with the rest of the year clustering below.

The gap matters: September is about 10% higher per day than the January floor (10,678 per day). The whole Jul–Sep block sits above 11,500/day, and everything from October through May sits between roughly 10,800 and 11,200 — a calm middle.

This per-day measure is the fair answer to "which month is the most common to be born in?" — it isolates the rate from the calendar artifact of month length. The single most common day of the entire year is September 9, and 17 of the 30 most common dates overall fall in September.

Which month has the most US births in total?

August, at 8.90% of all U.S. births over the period. July is second (8.78%) and September third (8.68%) — even though September dominates on a per-day basis.

The reason is calendar arithmetic, not biology. August has 31 days, September has 30. Multiply September's higher per-day rate by 30 days and August's slightly lower per-day rate by 31 days, and August comes out a hair ahead in raw count.

Seven months have 31 days (Jan, Mar, May, Jul, Aug, Oct, Dec), four have 30 (Apr, Jun, Sep, Nov), and February has 28 or 29 — the day-count alone redistributes a percentage point or two of total share between months that would otherwise rank differently.

That's why "most common birth month" has two correct answers, and why per-day is the measure people usually intend when they ask. By total count, the leaderboard tells you something about the calendar; by per-day, it tells you something about U.S. natality patterns.

Which month has the fewest US births?

By per day, January — about 10,678 births on an average January day, the lowest of any month. The whole winter trough (Jan, Feb, Mar, Apr) sits closely packed: February (10,940) and March (10,940) are tied a step above January, with April (10,836) just behind them.

By total count, the answer flips: February is the lowest at 7.60% of all births. It's the most-quoted "rarest birth month" figure you'll see in trivia roundups — and it's a misleading framing.

February isn't a low-birth month; it's a short month. Per day it sits mid-pack (essentially tied with March). Its total share is small only because it has two or three fewer days than the months around it.

In other words: the popular "February is the rarest birth month" is true on the technicality of total count, but false on the substance. The honest answer to "which month has the fewest births?" depends on whether you mean per day (January) or in total (February — calendar artifact).

Within-month detail: top dates and lows in each month

The monthly average smooths over real day-to-day structure. Some months have a tight ±3% spread between their highest and lowest day (August, March), while others swing wildly because the calendar has holiday dips that cluster on specific dates (December, November, January, July). The within-month spread metric below is (highest day's rate − lowest day's rate) / lowest — the same shape as the page's "September is ~10% above January" framing.

Feb 29 is excluded from per-month rankings, consistent with how the rarity page handles it.

Top 5 dates in each month

Sixty data points, twelve cards. The September card is the global top 5 of the entire year — every member is also in the top 5 nationally. Holiday months (December, November) don't show their within-month structure here because the top 5 sit comfortably above the holiday cluster; for that shape, see the extrema column above.

Each card lists the five most-common dates in that month by average births per year, 1994–2014. The September card is also the year's overall top 5 — Sep 9 leads the entire calendar, and four of the next four are all in September.

Why does late summer peak?

The most widely cited explanation for the late-summer peak is conception around the winter-holiday period roughly nine months earlier — a tidy story that lines up with the calendar, but one that's an explanation rather than a proven cause. Whatever the mechanism, the seasonal pattern is real, it's large, and it persists across the entire 21-year window.

At the daily resolution the pattern is even sharper: mid-September days dominate the rarity ranking, and the single most common birthday — September 9 — is consistent with this seasonal hump (rarity page · monthly view). The seasonal curve below shows the full Jan→Dec shape clearly: a broad late-summer ridge, a steady descent into late autumn, a winter floor.

The pattern isn't unique to the United States — Northern Hemisphere natality records broadly show late-summer or early-autumn peaks. But we keep this page tight to the U.S. figures the dataset actually supports and don't editorialize beyond it.

The curve makes the calm middle (Mar–Jun, Oct–Dec) visible: months in that range fall within a narrow band around 10,800–11,200 births per day. The two outliers — September high, January low — are what move the dial.

Working back nine months: conception months

This section infers conception months from the MONTHLY birth aggregate only. Each conception month is mapped to the birth month roughly nine months later. Gestation is actually 38–40 weeks and varies, so the shift is an illustration of the monthly pattern, not a precise alignment.

The within-day holiday dips visible in the rarity data — Christmas (Dec 25), Thanksgiving (Nov 23–27), July 4, New Year's (Jan 1) — are scheduling artifacts: elective deliveries (induced labor, scheduled C-sections) avoid holidays. They carry no conception-side meaning at all. The seasonal pattern in monthly averages is one signal; the day-specific holiday troughs are a completely separate signal layered on top, with a completely different cause. Don't conflate them.

The most widely cited explanation for the late-summer peak is conception around the winter-holiday period roughly nine months earlier — a tidy story that lines up with the calendar, but one that's an explanation rather than a proven cause. We use the same framing here as on the rarity page and birth-rate trends: commonly cited, not proven.

Has the late-summer peak changed over 21 years?

For each year from 1994 to 2014, the Sept-vs-Jan premium is computed as (Sept avg/day − Jan avg/day) / Jan avg/day. The 21-year mean is 10.20%, with a year-to-year range from 5.60% (2001) to 13.86% (2006).

Splitting the window into halves: the first seven years (1994–2000) averaged 10.36% premium; the last seven years (2008–2014) averaged 9.95%. That's a difference of -0.40 pp — well inside the year-to-year noise band of about ±3 pp. No trend line is drawn on the chart for exactly this reason.

The finding is worth stating plainly: the absolute count fell, the seasonal shape didn't. U.S. fertility per 1,000 population declined across this window — the headline figure in Birth-Rate Trends — but the proportional gap between September and January held roughly constant. Whatever's driving the seasonal hump has stayed similarly strong from 1994 through 2014 even as the absolute birth count changed.