In time, the female menstrual cycle evolves to happen much less frequently, causing far fewer births. A twenty-second century witch-hunt begins when billions of people convince themselves that this is an ungodly occurrence.

I will agree with Mike and guide you towards the formula, as well. Not because we should all be members of a Formula-worshipping cult, but because this formula will really help you straighten out the spine of your story and write an effective log line.

This isn't really a logline. It's simply setting the stage. We need an individual story within this world to follow ? our protagonist's story.

Check out the formula or give the generator a go ? links at the top. Both of these are really useful resources when it comes to creating a good logline.

Hope this helps.

UPDATE 1:

The human female reproductive system evolves so that menstrual cycles happen only once a year, causing far fewer births. A twenty-second century witch-hunt begins, forcing women to revert back to primitive eating habits.