Making Babies is Not a Totally Random Process

If you have ever thought about how many people shared your birthday (month and year), you probably came to the obvious answer--1 out of every 365 people on the planet (give or take a few, since we really should account for the leap year babies). That assumption, however, is based on the idea that the probability of conception (and the eventual live birth) is the same on every day of the year. It turns out that this is not quite the case. Demographers have known about the seasonality of births for a long time, but it is more fun when that is discovered by a lay person such as Matt Stiles, a journalist at NPR, who has a blog called thedailyviz. Recently he ran across data from the National Center for Health Statistics showing births by month in the US, which reminded us all that August has more births than any other month, suggesting that "deep in December" is when more babies are conceived than at any other time. He did a nuance to the calculation by dividing each month by its number of days, showing that September was the winner by this count, but it will refer to very early in September, so the overall conclusion about the power of winter conceptions isn't changed. In the end, though, he offers this caution:

But notice there isn’t much difference between months in the distribution of the births.

Just enough for us to notice it and comment on it...