This is one of THOSE THINGS. I doubt you can nail the female cycle down to any one or even two or three things. As far as I can make out the main thing is your oestrogen / prooestrogen levels and how they interact with several other endochrinic systems within your body. These, in turn, are affected by 'loadsastuff' ( technical term there) externally. Circadian/ diurnal rythmns, men or their lack, food/ vitamins,essential minerals, how physical your usual routine is, aparantly how many pregnant women you are in regular contact with ( 'en famille' , for some reason it doesn't seem to work with women who work in obstetrics ) has an affect. It would not be untruthful to say that the medical profession is still trying to retro-engineer this one. Much to my disadvantage, typical, huh!
TTFN M'Lady P.