September birthdays

Are your Facebook birthday reminders lighting up this month? Have you ever noticed that you receive more birthday party invitations and have more cards to send in September than in any other month of the year?

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.

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. 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

You may note that the March births appear high, but consider that February is a short month, and March is a 31-day month. Similarly, the number of August births is nearly equal to that in September, but August is a longer month than September.

To correct for this, we can estimate the average number of births per day of the month.

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

With this adjustment the seasonal pattern becomes even more pronounced.