According to a new study of other studies, yeah, we know how annoying that sounds, PMS just might be a MYTH. A team out of the University of Otago in New Zealand reviewed 47 different scientific studies on PMS.  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)