Movies would have you believe that birth is random and unpredictable. (And if you haven’t been part of the birth process, you’d be surprised by how slow it actually is.) While uncertainty is always in play, there’s a certain cycle to it all. Zan Armstrong and Nadieh Bremer for Scientific American, using 2014 data from the Centers for Disease Control and Prevention, examined the regularity and the reasons for the spikes.
I like the greater/lesser than average split for contrast. The circle time series layout doesn’t always fit the data, but in this case the metaphor fits the cyclical aspect.