TL;DR - Planned C-Sections. There's also a drop on weekends for the same reason as fewer individuals plan for a delivery on the weekends as hospitals are staffed with fewer people.
When I saw the headline, I immediately thought "it's because of C-Sections." A few pretty graphs and thousands of filler words later, I discovered that — <i>SURPRISE</i> – it's due to C-Sections!<p>Data visualization of the bleeding obvious that anyone who gave it a moment's thought would have been able to guess.
There's a similar demographic wiggle in France around May 1: very few born on the 1st more born on April 30th. Why? Everybody takes may 1 holiday and the hospital will induce to get the kid out before the holiday.<p>(My kid's due date was 1 May so we were advised to make plans. As it happens we were in a different country when he arrived so didn't have to see this process in action, but the predictions were accurate).