Why is September a common birthday month?

Note: This post is adapted from last year's post and includes new data and analysis...

Do you feel like you're spending a small fortune on birthday cards every September? Is your Facebook feed alight with birthday reminders this month? Have you noticed more birthday party invitations arriving in the mail in recent weeks?

If you have noticed a spike, you are not alone. There is a 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 September, 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 with birth data from the United States in 2010 and 2011. (Note: In the southern hemisphere, the seasonal peak occurs about 6 months earlier.)

Source: U.S. Centers for Disease Control and Prevention and author's calculations

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.

Source: U.S. Centers for Disease Control and Prevention and author's calculations

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

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...