Although now I am no longer having periods (thank god that is finished - and saving a ton on not purchasing tampons) - it never had bearing on therapy that I can see. But I always knew when my period was coming because of the sore/tender breasts and the fact I craved chocolate and potato chips (two things I really care nothing about on a daily basis) and wanted to weep at sentimental commercials (again - not my base position). Seriously - I never bothered keeping track of 28 days - but I could always tell.