History

Which days Americans are born

21 years of US birth records on one calendar, with the weekday effect removed. Christmas runs 41% below an ordinary day, the 13th of every month dips, and mid-September peaks. Deliveries are scheduled now, and the calendar shows it.

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Data & source

Source: US births by day, CDC/NCHS and SSA via FiveThirtyEight (CC BY 4.0), 1994–2014, fetched —. Each date's figure is its observed births divided by what its particular mix of weekdays predicts, so 1.00 is an ordinary day. Feb 29 is left out — it has a quarter of the evidence. Scheduled inductions and caesareans are what make the calendar visible at all: the pattern is about when doctors and parents choose, not when labour naturally starts.