.A team out of the University of Otago in New Zealand reviewed 47 different scientific studies on PMS.  Please don't kill the messenger. I'm just reporting the story, ladies.

And only 15% of the studies found that "classic" PMS is real.  Meaning a woman's mood got worse as her time of the month approached, then got better once it started.

Another 38% of the studies found that premenstrual syndrome is really just menstrual syndrome...so, a woman's mood does get worse, but it doesn't go away as soon as her period starts.

Of the rest of the studies, another 38% found there's no association between a woman's mood and her cycle and 9% found a woman's worst moods happen OUTSIDE of the traditional PMS period.

So if 85% of studies have found that classic PMS doesn't really exist, why is it still such a widespread idea?  The researchers think it may be psychological.  If you BELIEVE your mood swings badly right before your period, it's more likely to happen.

(Time)