It's not caused by an event. There have been quite a few discussions about the phenomenon over the years with each of the VV games, but the reason for it lies in the way the game calculates what 'would have happened' when it is started after being off for some period of time. As long as a female is at least 18 years old when the game is started, it's possible that the game will 'decide' that she could be a mom if there are any males who might choose to mate during that time. Unfortunately, the game doesn't 'consider the math' involved, and will occasionally produce this, and other, equally unlikely situations.
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Barbara
Unicorn
Last Day of Work