I got this in an email newsletter. Since it is a topic we frequently discuss, I figured I'd post it :)
An article in the American Journal of Gastroenterology noted that female patients with bowel disease commonly report worsening symptoms in relation to the menstrual cycle. Their aim was to determine the nature of gastrointestinal symptoms correlating with the menstrual cycle in women with inflammatory and irritable bowel disease.
All disease groups (IBD and IBS) had a cyclical pattern to their bowel habits significantly more than controls, including diarrhea, abdominal pain, and constipation. The study concluded that the prevalence of menstrually related symptoms is high, and appears to affect bowel patterns. The physiological and clinical effects of the menstrual cycle should be taken into consideration when assessing for disease activity. Go here for more information about this study...