This year I was updating the data to show 2012 and 2013 births, and discovered that...
August edged out September for the most popular birth month of the year!
There is a long-standing and clear pattern of "birth seasonality" resulting from a "seasonal cycle in fecundability" documented in the scientific literature. Holidays and long winter nights are partly to blame for more birthdays in the late summer and early fall, but human biology is at work as well.
In short, in the northern hemisphere, women are more likely to get pregnant in late fall and early winter than at other times of the year. As a result more births occur late summer and early autumn, a trend displayed clearly in the chart below. (In the southern hemisphere, the seasonal peak occurs about 6 months earlier.)
What's new in the past three years is that the peak seems to have shifted a few weeks from September into August.
In the chart below, blue represents lower numbers and red highlights peaks. The left panel is total births in a given month, and the right (as explained below) shows births per day in each month.
To correct for this, we can estimate the average number of births per day of the month. With this adjustment the seasonal pattern becomes even more pronounced. (See panel to the right, above.)
And the August trend continued in 2012 and 2013! In fact, the August peak in 2012 is quite pronounced - more than 300 births per day higher than September.
The lower number of births in September 2011 may, or may not, be a consequence of a massive blizzard that shut down transportation along much of the east coast in December 2010... but the continuation of the pattern bears watching.