Happy birthday! How did I know? Because humans are pretty predictable when it comes to having babies (also, data).
In 2006, The New York Times published a simple table graph ranking every day of the year by how many babies were born on a particular day. The dataset included children born in the United States between 1973 and 1999, a fairly huge sample size that was originally compiled by the Harvard economist Amitabh Chandra.
The set showed that September 16th is the most common birthday, while February 29th was the least common, for obvious reasons. Beyond leap year birthdays, January 1 was the least common.
Why do more conceptions happen in the winter (leading to more births in early autumn)? The authors of a study in Obstetrics & Gynecology that looked at seasonal conception trends say we don't really know, but provide some hypotheses:
The year-end peak found in all age groups studied is consistent with previous studies in the United States and elsewhere. This finding suggests that biologic processes or common behaviours may account for the seasonal variation.
Biologic hypotheses include deterioration of sperm quality during summer, seasonal differences in anterior pituitary-ovarian function caused by changes in the daylight length, and variation in quality of the ovum or endometrial receptivity. Increased sexual activity associated with end-of-year holiday festivities has also been postulated as a possible behavioural explanation for the December peak in conceptions. The exact reasons remain unknown.
As MIT pointed out on Twitter today, that table was turned into a nice little visualisation a few years ago by The Daily Viz's Matt Stiles. One thing to consider about his birthday heat map, as he later pointed out, that the most and least popular birthdays are still fairly close together in terms of actual births. It's their rankings that differ a lot.
Meanwhile, the most common birth month is August, but September wins again when you account for the extra day in August. "But notice there isn't much difference between months in the distribution of the births," Stiles concluded. "Alas, all our birthdays are probably pretty normal."