My previous post sparked an issue which i thought was worth a bit more investigation and here's what i came up with:
The McClintock Effect
or sometimes known as 'Menstrual Synchrony' (also sometimes known as the 'Dormitory Effect')
So yes, as the name suggests (and i've sort of got wind of such a phenomenon prior to this) is the hypothesis that the menstrual cycles of women living together for example in prisons, convents, dormitories and the like tend to become synchronized over time.
This 'phenomenon' which sometimes has been referred to as the 'social regulation of ovulation' has been postulated to have a pheromone basis by a psychologist named Martha McClintock.
Having said all that however, apparently there have been deficiencies in the theory and study as propagated by McClintock and generally speaking the scientific community has sort of discounted the 'phenomenon' as invalid.
*most of the information is what i made sense out of an article in Wikipedia.. like i said i just did a 'bit' of investigating ;) *