Childbirth: Why do you think babies are born early during monsoon season?

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A couple of years ago I worked in a hospital, and I couldn't help but notice that, whenever there was a full moon.... there was an unusual number of pregnant women who would show up to deliver babies. Now, I'm listening to the news and I hear that pregnant women are having a tendency to "go into labor" during the monsoon season. This is an interesting phenomenon that I'd like to understand, if possible. Why, in your opinion, does this happen?