Giving Birth on the 13th Is Less Popular Than Ever

Here’s a fascinating little chart from the good folks at 538:

This whole thing, of course, is an artifact of scheduling births instead of just letting Mother Nature take her course. “The effect for the 13th,” says Carl Bialik, “was twice as high as it had been in the 1970s and 1980s, as we’d expect since births were scheduled less often then.”

But there’s one odd anomaly: The 13th of the month is an unpopular day for giving birth except in September. What’s up with that?