Kathryn Clancy explains the science behind a "normal" but not so common 28-day menstrual cycle and how it's influenced by the amount of food consumed, exercise and even psychological stress:
Most young girls just getting their period take years to achieve regular cycles like the one I described above: this is the main characteristic of the reproductive functioning of adolescents. Yet this normal variation sets us up for a lifetime of checking our cycle, counting days, doubting that we’re like other girls, and feeling bad about ourselves, because of that early information that we should be achieving twenty-eight-day cycles regularly. ...
If responding to our environment is adaptive, then variation is adaptive, and variation is the real norm. The better we understand how our lifestyle and environment impact our cycles, the better we can forgive a little variation, and thank our bodies for knowing what to do, sometimes better than we do.
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