Is August now the best month for births?

For the past few years, I've been updating data about birth seasonality and noting that September is the most common birth month of the year.

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.

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In the chart above, you may notice that March 2010 births appear high, but consider that February is a short month, and March is a 31-day month. August also has more days than neighboring month, September.

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.)

Correcting for the number of days per month shows September as the clear leader for births in 2008, 2009, and 2010, as it is in most years. But 2011 shows a slightly different pattern - August leads September by about 75 births per day.

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.